perl invalid initialization by assignment

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Perl Initialize Array

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Table of Contents

Introduction

Initializing arrays in perl, tips and tricks, common errors and how to avoid them.

• Direct Assignment:
• Assigning Range:
• Initializing with qw Operator:
• Understanding the Context:
• Using Array Functions:
• Off-by-One Errors:
• Incorrect Context:
• Not Initializing Properly:

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My Favorite Warnings: uninitialized

This warning was touched on in A Belated Introduction , but I thought it deserved its own entry.

When a Perl scalar comes into being, be it an actual scalar variable or an array or hash entry, its value is undef . Now, the results of operating on an undef value are perfectly well-defined: in a nuneric context it is 0 , in a string context it is '' , and in a Boolean context it is false.

The thing is, if you actually operate on such a value, did you mean to do it, or did you forget to initialize something, or initialize the wrong thing, or operate on the wrong thing? Because of the latter possibilities Perl will warn about such operations if the uninitialized warning is enabled.

If you really intended to do this, no warnings 'uninitialized'; will suppress the error.

Note that this warning has nothing to do with the fatal error Can't call method "%s" on an undefined value that turns up every so often on various Perl outlets. This generally results from an instantiator or other factory method returning undef on failure rather than throwing an exception, and the programmer not checking the value before using it. Because this error is fatal, it can not be turned off with the no warnings pragma.

Previous entries in this series:

• A Belated Introduction
• redundant and missing

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misleading warning: Use of uninitialized value in list assignment #9894

p5pRT commented Oct 4, 2009

Migrated from (status was 'resolved') Searchable as RT69560$

The following program generates a perplexing warning​:   % perl -w   my %h = map {   my(@​x) = (undef, undef);   } 1;   Use of uninitialized value in list assignment at - line 2. What's wrong with the list assigment at line 2? Nothing! It's really complaining about the hash assigment at line 1. Tell me what's really upsetting you, perl, don't blame something else! I hope there's a very cool optimization behind this misdirection. :-) Andrew

Sorry, something went wrong.

p5pRT commented Nov 11, 2009

- Status changed from 'new' to 'open'

p5pRT commented Jan 1, 2010

On Sat, Oct 03, 2009 at 11​:02​:56PM -0700, andrew@​pimlott.net (via RT) wrote​: What's wrong with the list assigment at line 2? Nothing! It's really complaining about the hash assigment at line 1. Tell me what's really upsetting you, perl, don't blame something else! Ah, no! There are only two types of assignment​: scalar and list, and the warning is correct. You can see the same warning in the simplified code​:   $ perl -we 'my %h =(undef,undef)'   Use of uninitialized value in list assignment at -e line 1. -- Fire extinguisher (n) a device for holding open fire doors.

- Status changed from 'open' to 'resolved'

On Fri, Jan 1, 2010 at 2​:49 PM, Dave Mitchell <davem@​iabyn.com> wrote​: What's wrong with the list assigment at line 2? Nothing! It's really complaining about the hash assigment at line 1. Tell me what's really upsetting you, perl, don't blame something else! Ah, no! There are only two types of assignment​: scalar and list, and the warning is correct. You can see the same warning in the simplified code​: $ perl -we 'my %h =(undef,undef)' Use of uninitialized value in list assignment at -e line 1. I agree that "list assignment" is correct, but why does the warning say line 2?

p5pRT commented Jan 2, 2010

On Fri, Jan 01, 2010 at 05​:42​:57PM -0500, Eric Brine wrote​: Because perl currently only records line numbers per statement, not per op; e.g.​: $ cat -n /tmp/p   1 #!/usr/bin/perl -w   2   3 my $x =   4 1   5 + 2   6 + 3   7 + 4   8 + undef;   9 $ perl /tmp/p Use of uninitialized value in addition (+) at /tmp/p line 3. $ -- O Unicef Clearasil! Gibberish and Drivel!   -- "Bored of the Rings"

On Fri, Jan 1, 2010 at 7​:15 PM, Dave Mitchell <davem@​iabyn.com> wrote​: Because perl currently only records line numbers per statement And it records the line on which the statement started. In this case, that was line one. My question stands unanswered.

On Fri, Jan 1, 2010 at 7​:15 PM, Dave Mitchell <davem@​iabyn.com> wrote​: Because perl currently only records line numbers per statement, not per op I realize that, but it doesn't explain the difference between   (undef, undef) ); ^Z Use of uninitialized value in list assignment at - line 1. The assignment didn't move, so why did the warning?

On Fri, Jan 1, 2010 at 7​:59 PM, Eric Brine <ikegami@​adaelis.com> wrote​: Because perl currently only records line numbers per statement, not per op I realize that, but it doesn't explain the difference between my %h = map { my $x; (undef, undef) } ^Z syntax error at - line 4, at EOF Execution of - aborted due to compilation errors. (undef, undef) ); ^Z Use of uninitialized value in list assignment at - line 1. The assignment didn't move, so why did the warning? Can you tell I'm tired? I recreated the test I ran the test earlier a "little" too fast. Here's what I should have said​: I realize that, but it doesn't explain the difference between and   (undef, undef) ); ^Z Use of uninitialized value in list assignment at - line 1. The assignment didn't move, so why did the warning?

On Fri, Jan 01, 2010 at 08​:02​:19PM -0500, Eric Brine wrote​: and (undef, undef) ); ^Z Use of uninitialized value in list assignment at - line 1. The assignment didn't move, so why did the warning? Because perl's line number reporting is totally borked and needs ripping out and rewriting from scratch. The line number that is reported is based on what is stored in what happens to have been last nextstate op to have been executed, and there isn't even a guarantee that the value in that op corresponds to the line number where the statement boundary occurred. -- Standards (n). Battle insignia or tribal totems.

On Fri, Jan 01, 2010 at 08​:53​:49PM -0800, Andrew Pimlott wrote​: I'm sure this is strictly true, but ... $ perl -we 'my %h =(undef,undef)' Use of uninitialized value in list assignment at -e line 1. ... at some level perl knows it's not a "normal" list assignment, otherwise it wouldn't complain about the uninitialized value. When the warning is generated, perhaps something could be added about why undef is not allowed. Would be better. Even though it remains somewhat confusing to talk about a hash key in a list assigment, at least there's an indication of what the real problem is. The undefined value warning was raised while perl was executing the list assignment operator, and so that's what perl reports. To start special-casing this ubiquitous and highly generic warning would open a large can of worms. -- It's not that I'm afraid to die, I just don't want to be there when it happens.   -- Woody Allen

2010/1/2 Dave Mitchell <davem@​iabyn.com>​: and    (undef, undef) ); ^Z Use of uninitialized value in list assignment at - line 1. The assignment didn't move, so why did the warning? Because perl's line number reporting is totally borked and needs ripping out and rewriting from scratch. The line number that is reported is based on what is stored in what happens to have been last nextstate op to have been executed, and there isn't even a guarantee that the value in that op corresponds to the line number where the statement boundary occurred. That's especially problematic in loops. See for example bug #60954 <http​://rt.perl.org/rt3//Public/Bug/Display.html?id=60954>

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How to Initialize a Variable in Perl

In Perl, variables are automatically initialized with the value of undef until they are assigned a defined value. However, it is good practice to explicitly initialize variables to avoid any confusion or errors.

In the given code snippet, the variable $count is not initialized, and this may result in warnings or unpredictable behavior. To initialize $count to a specific value, you can simply add an assignment statement when declaring the variable, like this:

By assigning 0 to $count , you ensure that it has a valid initial value before being used in the code.

It is important to note that initializing variables is not mandatory in Perl, but it is considered good programming practice. Explicitly initializing variables helps improve code readability and reduces the chances of bugs or unexpected behavior.

In the given code snippet, initializing $count to 0 is recommended because it is used to count the number of records created. Without initialization, the value of $count would be undef , and this may cause issues when the code attempts to increment it.

Resolving ‘use Of Uninitialized Value’ In Perl Scripts

Resolving use of uninitialized value Warnings in Perl Scripts

The “ use of uninitialized value ” warning in Perl occurs when a variable is used without being assigned a value. This can lead to runtime errors or unexpected behavior, especially when debugging scripts. To resolve this issue, you need to ensure that all variables are initialized with a value before they are used.

Check for Missing Variable Declarations: Ensure that all variables are properly declared using my $variable_name; or my %variable_name; syntax.

Assign Initial Values: Assign a default value to variables when declaring them, such as my $value = 0; or my $name = 'John Doe'; .

Use Conditional Statements and Default Values: Use if statements to check if a variable has been initialized and assign a default value if not. For example:

Suppress Specific Warnings: In certain cases, you may need to suppress the “ use of uninitialized value ” warning for specific variables. Use the no warnings 'uninitialized' pragma to temporarily disable the warning for those variables.

Executive Summary

Uninitialized values in Perl scripts can lead to runtime errors and unpredictable program behavior. To ensure code stability and accuracy, it is crucial to address and resolve these errors effectively. This article provides a comprehensive guide, exploring the causes and solutions of ‘use of uninitialized value’ in Perl scripts. It covers key concepts, best practices, and practical tips to help developers identify, debug, and prevent these issues, ultimately enhancing code quality and improving script performance.

Introduction

Perl’s dynamic typing nature allows variables to be used without explicit declaration or initialization. However, attempting to use a variable that has not been assigned a value results in the ‘use of uninitialized value’ error. Understanding the root causes of this error and implementing effective countermeasures is essential for developing robust and reliable Perl scripts.

Understanding the Root Causes

Avoiding uninitialized value errors, analyzing code and identifying issues.

Addressing ‘use of uninitialized value’ errors is crucial for developing robust and error-free Perl scripts. By understanding the root causes of these errors, implementing proper initialization techniques, and utilizing effective debugging strategies, developers can ensure that their scripts run smoothly and produce accurate results. Embracing best practices and incorporating comprehensive testing methodologies will enhance code quality, increase script reliability, and ultimately lead to successful software development projects.

Keyword Phrase Tags

This is a bug that has been in Perl for decades, and it’s still not fixed

The error message ‘use of uninitialized value’ is a syntax error that occurs when a variable is used without being declared. This can happen when a variable is misspelled, or when a variable is declared but not assigned a value.

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laziness, impatience, and hubris

by (Acolyte)

) NODE.title = determine the variable causing the error: Use of uninitialized value NODE.owner = 907257 N.title = monktitlebar sitedoclet N.owner = 17342 -->

( = : , )

has asked for the wisdom of the Perl Monks concerning the following question: It happens to me frequently that I got the error: when I'm printing several values together using printf. Obviously, if I debug the application I can determine the uninitialized variable, but that's usually a lengthy and boring process that I'd like to avoid if possible. Is there any way to know which variable is the culprit without debugging? In other words, is it possible somehow for perl to specify in the output error something like: Thanks in advance! NOTICE Just to be clear, I know how to avoid these type of errors either by a) initializing the variables with a default value and/or b) checking whether or not they are defined. I don't want to add any kind of extra checking code to avoid the errors, I just want to know, when the error happens, which variable is not initialized correctly. determine the variable causing the error: Use of uninitialized value or Code

Replies are listed 'Best First'.
by (Archbishop) on Apr 13, 2017 at 18:29 UTC
Could you show some code that demonstrates the problem ( )? Also, which version of Perl are you using? Just to be clear, by "the problem" I mean the variable name not showing up in the warning message. )
by (Archbishop) on Apr 13, 2017 at 18:55 UTC
: This is an important question. The boon you seek was added with Perl version 5.10:
by (Acolyte) on Apr 13, 2017 at 19:23 UTC
it's correct what you say, but it only work with scalars, it doesn't work with more complex structures like hashes (or references to hashes): EXAMPLE: h"=>1, "i"=>2); print $x, $y{x}; printf "%s", $x;' OUTPUT:
by (Saint) on Apr 13, 2017 at 19:35 UTC
by (Saint) on Apr 15, 2017 at 01:25 UTC
by (Acolyte) on Apr 16, 2017 at 02:37 UTC
have not been shown here
by (Saint) on Apr 13, 2017 at 18:41 UTC
is a quick fix if you receive the variable from an uncontrolled source. Otherwise try initializing strings, like From a general perspective: This warning is probably the most disputed, if you just want to silence it in your scope, try HTH! :) !
by (Acolyte) on Apr 13, 2017 at 19:34 UTC
by (Archbishop) on Apr 14, 2017 at 00:04 UTC
The point raised by is critical: you're really dealing with input data validation, not a way to flag the use of an value that might be present if your input data was invalid. In other words, don't bother trying to lock the barn door after the horse has bolted. The ideal time to deal with bad data is when you get it, not after it's drifted downstream and had a chance to gum up other processes. The only way I can think of to validate a data structure is to write a validation function specific to that structure and invoke it as soon as possible after the structure is captured: Obviously, the bigger your data, the bigger your validation job, but that's life. (You may even decide that certain missing data fields can, in fact, be fixed up with a zero, an empty string or whatever; if so, the function is the ideal place to do this patching.) The other variation on this theme is when you read your input data on a line-by-line or block-by-block basis. In this case, you must figure out a way to validate each line or block as it is read — at least, that's what I would do. Anyway, you get the idea...
by (Acolyte) on Apr 14, 2017 at 03:08 UTC
by (Archbishop) on Apr 14, 2017 at 04:14 UTC
by (Abbot) on Apr 14, 2017 at 14:19 UTC
by (Archbishop) on Apr 13, 2017 at 19:52 UTC
If it's s in a Perl data structure that you seek...
by (Saint) on Apr 13, 2017 at 20:30 UTC
t /home/lanx/pm/warn_undef.pl line 13. hash at /home/lanx/pm/warn_undef.pl line 13. Use of uninitialized value in concatenation (.) or string at /home/lan x/pm/warn_undef.pl line 14. hoa at /home/lanx/pm/warn_undef.pl line 14. Use of uninitialized value in concatenation (.) or string at /home/lan x/pm/warn_undef.pl line 16. hashref at /home/lanx/pm/warn_undef.pl line 16. Use of uninitialized value in concatenation (.) or string at /home/lan x/pm/warn_undef.pl line 17. hoa ref at /home/lanx/pm/warn_undef.pl line 17. Use of uninitialized value $array[1] in concatenation (.) or string at /home/lanx/pm/warn_undef.pl line 19. array at /home/lanx/pm/warn_undef.pl line 19. Use of uninitialized value in concatenation (.) or string at /home/lan x/pm/warn_undef.pl line 20. aoh at /home/lanx/pm/warn_undef.pl line 20. Use of uninitialized value in concatenation (.) or string at /home/lan x/pm/warn_undef.pl line 22. array ref at /home/lanx/pm/warn_undef.pl line 22. Use of uninitialized value in concatenation (.) or string at /home/lan x/pm/warn_undef.pl line 23. aoh ref at /home/lanx/pm/warn_undef.pl line 23. :) !
by (Saint) on Apr 13, 2017 at 21:14 UTC
by (Abbot) on Apr 14, 2017 at 15:13 UTC
(Thomas Mann) Furthermore I consider that Donald Trump must be impeached as soon as possible
by (Archbishop) on Apr 14, 2017 at 10:38 UTC
Please clarify. How big and deep are your data structures? Did you try running the code I posted on your data, and how long did it take? Also, how are you accessing your data structure? Directly, as in , do you copy subsections into temporary variables, do you walk the structure recursively, etc.? Again, please see , if you could show us something that's actually representative of your code, we can give better advice. I believe that ( now even a post by who it himself!) conclusively that in many cases, this isn't built into Perl, so the answer to your question is . You'll have to add the checks yourself, even if you :-P You could check the arguments: led at - line 7 Use of uninitialized value in printf at - line 5. Use of uninitialized value in printf at - line 5. quz--baz- Or you could check for s when accessing the data: "[3]") called at - line 22 Use of uninitialized value in printf at - line 22. <> undef at: {this}{is}{an} at - line 10. main::dive(HASH(0x9e47a4), "this", "is", "an", "example") called a t - line 23 Use of uninitialized value in printf at - line 23. <>
by (Priest) on Apr 14, 2017 at 03:44 UTC
not detect this as an error? Original command placed in script and executed: Produces, as advertised: So I tried to capture the error in an block: But this produced: What is it about that I'm not understanding properly here?
by (Archbishop) on Apr 14, 2017 at 04:28 UTC
is a warning, not an error (unless it's escalated to -ity); no exception is thrown.
by (Priest) on Apr 14, 2017 at 04:42 UTC
So -- Since you didn't mention one, I presume there isn't an equivalent which catch warnings?
by (Archbishop) on Apr 14, 2017 at 05:22 UTC
by (Abbot) on Apr 14, 2017 at 13:02 UTC
by (Saint) on Apr 14, 2017 at 13:44 UTC
:) !
by (Saint) on Apr 23, 2017 at 10:58 UTC
by (Abbot) on Apr 14, 2017 at 14:56 UTC
Probably it happens when you access the data and then it's too late ;-) Kidding aside: Can you please provide some example data to ease the discussion? Most of this thread is based on assumptions about your data. Regards, Karl Furthermore I consider that Donald Trump must be impeached as soon as possible
by on Apr 13, 2017 at 19:36 UTC
$o , $p , $q; }

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Home » Perl Variables

Perl Variables

Summary :  in this tutorial, you’ll learn about Perl variables , variable scopes, and variable interpolation.

To manipulate data in your program, you use variables.

Perl provides three types of variables: scalars, lists, and hashes to help you manipulate the corresponding data types including scalars, lists, and hashes .

You’ll focus on the scalar variable in this tutorial.

Naming variables

You use scalar variables to manipulate scalar data such as numbers and strings .

A scalar variable starts with a dollar sign ( $ ), followed by a letter or underscore, after that, any combination of numbers, letters, and underscores. The name of a variable can be up to 255 characters.

Perl is case-sensitive. The $variable and $Variable are different variables.

Perl uses the dollar sign ( $ ) as a prefix for the scalar variables because of the $  looks like the character S in the scalar. You use this tip to remember when you want to declare a scalar variable.

The following example illustrates valid variables:

However, the following variables are invalid in Perl.

Declaring variables

Perl doesn’t require you to declare a variable before using it.

For example, you can introduce a variable in your program and use it right away as follows:

In some cases, using a variable without declaring it explicitly may lead to problems. Let’s take a look at the following example:

The expected output was Your favorite color is red .  

However, in this case, you got Your favorite color is , because the $color and $colour are different variables. The mistake was made because of the different variable names.

To prevent such cases, Perl provides a pragma called strict that requires you to declare variable explicitly before using it. 

In this case, if you use the  my keyword to declare a variable and try to run the script, Perl will issue an error message indicating that a compilation error occurred due to the  $colour variable must be declared explicitly.

A variable declared with the  my keyword is a lexically scoped variable.

It means the variable is only accessible inside the enclosing block or all blocks nested inside the enclosing block. In other words, the variable is local to the enclosing block.

Now, you’ll learn a very important concept in programming called variable scopes.

Perl variable scopes

Let’s take a look at the following example:

In the example above:

• First, declared a global variable named  $color .
• Then, displayed the favorite color by referring to the $color variable. As expected, we get the red color in this case.
• Next, created a new block and declared a variable with the same name $color using the my keyword. The  $color variable is lexical. It is a local variable and only visible inside the enclosing block.
• After that, inside the block, we displayed the favorite color and we got the blue color. The local variable takes priority in this case.
• Finally, following the block, we referred to the $color variable and Perl referred to the  $color global variable.

If you want to declare global variables that are visible throughout your program or from external packages, you can use our   keyword as shown in the following code:

Perl variable interpolation

Perl interpolates variables in double-quoted strings. It means if you place a variable inside a double-quoted string, you’ll get the value of the variable instead of its name.

Perl interpolates the value of $amount into the string which is 20.

Note that Perl only interpolates scalar variables and arrays , not hashes . In addition, the interpolation is only applied to the double-quoted string, but not the single-quoted string.

In this tutorial, you have learned about Perl variables including naming and declaring scalar variables. You’ve also learned about variable scopes and variable interpolation.

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"Use of initialized value in scalar assignment"

• Thread starter kj
• Start date Jan 2, 2008
• Jan 2, 2008

The following innocent-looking one-liner: % perl -we '$ENV{ FOO } = undef' produces the warning: Use of uninitialized value in scalar assignment at -e line 1 Is this a bug, or is there a good reason for this? FWIW, I've seen this behavior only with %ENV. I got the above using v5.8.8 on Linux. TIA! kj  

John W. Krahn

kj said: The following innocent-looking one-liner: % perl -we '$ENV{ FOO } = undef' produces the warning: Use of uninitialized value in scalar assignment at -e line 1 Is this a bug, or is there a good reason for this? FWIW, I've seen this behavior only with %ENV. I got the above using v5.8.8 on Linux. Click to expand...

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• Variable names
• Identifier parsing
• Scalar values
• Demarcated variable names using braces
• Special floating point: infinity (Inf) and not-a-number (NaN)
• Version Strings
• Special Literals
• Array Interpolation
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perldata - Perl data types

# DESCRIPTION

# variable names.

Perl has three built-in data types: scalars, arrays of scalars, and associative arrays of scalars, known as "hashes". A scalar is a single string (of any size, limited only by the available memory), number, or a reference to something (which will be discussed in perlref ). Normal arrays are ordered lists of scalars indexed by number, starting with 0. Hashes are unordered collections of scalar values indexed by their associated string key.

Values are usually referred to by name, or through a named reference. The first character of the name tells you to what sort of data structure it refers. The rest of the name tells you the particular value to which it refers. Usually this name is a single identifier , that is, a string beginning with a letter or underscore, and containing letters, underscores, and digits. In some cases, it may be a chain of identifiers, separated by :: (or by the deprecated ' ); all but the last are interpreted as names of packages, to locate the namespace in which to look up the final identifier (see "Packages" in perlmod for details). For a more in-depth discussion on identifiers, see "Identifier parsing" . It's possible to substitute for a simple identifier, an expression that produces a reference to the value at runtime. This is described in more detail below and in perlref .

Perl also has its own built-in variables whose names don't follow these rules. They have strange names so they don't accidentally collide with one of your normal variables. Strings that match parenthesized parts of a regular expression are saved under names containing only digits after the $ (see perlop and perlre ). In addition, several special variables that provide windows into the inner working of Perl have names containing punctuation characters. These are documented in perlvar .

Scalar values are always named with '$', even when referring to a scalar that is part of an array or a hash. The '$' symbol works semantically like the English word "the" in that it indicates a single value is expected.

Entire arrays (and slices of arrays and hashes) are denoted by '@', which works much as the word "these" or "those" does in English, in that it indicates multiple values are expected.

Entire hashes are denoted by '%':

In addition, subroutines are named with an initial '&', though this is optional when unambiguous, just as the word "do" is often redundant in English. Symbol table entries can be named with an initial '*', but you don't really care about that yet (if ever :-).

Every variable type has its own namespace, as do several non-variable identifiers. This means that you can, without fear of conflict, use the same name for a scalar variable, an array, or a hash--or, for that matter, for a filehandle, a directory handle, a subroutine name, a format name, or a label. This means that $foo and @foo are two different variables. It also means that $foo[1] is a part of @foo, not a part of $foo. This may seem a bit weird, but that's okay, because it is weird.

Because variable references always start with '$', '@', or '%', the "reserved" words aren't in fact reserved with respect to variable names. They are reserved with respect to labels and filehandles, however, which don't have an initial special character. You can't have a filehandle named "log", for instance. Hint: you could say open(LOG,'logfile') rather than open(log,'logfile') . Using uppercase filehandles also improves readability and protects you from conflict with future reserved words. Case is significant--"FOO", "Foo", and "foo" are all different names. Names that start with a letter or underscore may also contain digits and underscores.

It is possible to replace such an alphanumeric name with an expression that returns a reference to the appropriate type. For a description of this, see perlref .

Names that start with a digit may contain only more digits. Names that do not start with a letter, underscore, digit or a caret are limited to one character, e.g., $% or $$ . (Most of these one character names have a predefined significance to Perl. For instance, $$ is the current process id. And all such names are reserved for Perl's possible use.)

# Identifier parsing

Up until Perl 5.18, the actual rules of what a valid identifier was were a bit fuzzy. However, in general, anything defined here should work on previous versions of Perl, while the opposite -- edge cases that work in previous versions, but aren't defined here -- probably won't work on newer versions. As an important side note, please note that the following only applies to bareword identifiers as found in Perl source code, not identifiers introduced through symbolic references, which have much fewer restrictions. If working under the effect of the use utf8; pragma, the following rules apply:

That is, a "start" character followed by any number of "continue" characters. Perl requires every character in an identifier to also match \w (this prevents some problematic cases); and Perl additionally accepts identifier names beginning with an underscore.

If not under use utf8 , the source is treated as ASCII + 128 extra generic characters, and identifiers should match

That is, any word character in the ASCII range, as long as the first character is not a digit.

There are two package separators in Perl: A double colon ( :: ) and a single quote ( ' ). Use of ' as the package separator is deprecated and will be removed in Perl 5.40. Normal identifiers can start or end with a double colon, and can contain several parts delimited by double colons. Single quotes have similar rules, but with the exception that they are not legal at the end of an identifier: That is, $'foo and $foo'bar are legal, but $foo'bar' is not.

Additionally, if the identifier is preceded by a sigil -- that is, if the identifier is part of a variable name -- it may optionally be enclosed in braces.

While you can mix double colons with singles quotes, the quotes must come after the colons: $::::'foo and $foo::'bar are legal, but $::'::foo and $foo'::bar are not.

Put together, a grammar to match a basic identifier becomes

Meanwhile, special identifiers don't follow the above rules; For the most part, all of the identifiers in this category have a special meaning given by Perl. Because they have special parsing rules, these generally can't be fully-qualified. They come in six forms (but don't use forms 5 and 6):

A sigil, followed solely by digits matching \p{POSIX_Digit} , like $0 , $1 , or $10000 .

A sigil followed by a single character matching the \p{POSIX_Punct} property, like $! or %+ , except the character "{" doesn't work.

A sigil, followed by a caret and any one of the characters [][A-Z^_?\] , like $^V or $^] .

Similar to the above, a sigil, followed by bareword text in braces, where the first character is a caret. The next character is any one of the characters [][A-Z^_?\] , followed by ASCII word characters. An example is ${^GLOBAL_PHASE} .

A sigil, followed by any single character in the range [\xA1-\xAC\xAE-\xFF] when not under "use utf8" . (Under "use utf8" , the normal identifier rules given earlier in this section apply.) Use of non-graphic characters (the C1 controls, the NO-BREAK SPACE, and the SOFT HYPHEN) has been disallowed since v5.26.0. The use of the other characters is unwise, as these are all reserved to have special meaning to Perl, and none of them currently do have special meaning, though this could change without notice.

Note that an implication of this form is that there are identifiers only legal under "use utf8" , and vice-versa, for example the identifier $état is legal under "use utf8" , but is otherwise considered to be the single character variable $é followed by the bareword "tat" , the combination of which is a syntax error.

This is a combination of the previous two forms. It is valid only when not under "use utf8" (normal identifier rules apply when under "use utf8" ). The form is a sigil, followed by text in braces, where the first character is any one of the characters in the range [\x80-\xFF] followed by ASCII word characters up to the trailing brace.

The same caveats as the previous form apply: The non-graphic characters are no longer allowed with "use utf8" , it is unwise to use this form at all, and utf8ness makes a big difference.

Prior to Perl v5.24, non-graphical ASCII control characters were also allowed in some situations; this had been deprecated since v5.20.

The interpretation of operations and values in Perl sometimes depends on the requirements of the context around the operation or value. There are two major contexts: list and scalar. Certain operations return list values in contexts wanting a list, and scalar values otherwise. If this is true of an operation it will be mentioned in the documentation for that operation. In other words, Perl overloads certain operations based on whether the expected return value is singular or plural. Some words in English work this way, like "fish" and "sheep".

In a reciprocal fashion, an operation provides either a scalar or a list context to each of its arguments. For example, if you say

the integer operation provides scalar context for the <> operator, which responds by reading one line from STDIN and passing it back to the integer operation, which will then find the integer value of that line and return that. If, on the other hand, you say

then the sort operation provides list context for <>, which will proceed to read every line available up to the end of file, and pass that list of lines back to the sort routine, which will then sort those lines and return them as a list to whatever the context of the sort was.

Assignment is a little bit special in that it uses its left argument to determine the context for the right argument. Assignment to a scalar evaluates the right-hand side in scalar context, while assignment to an array or hash evaluates the righthand side in list context. Assignment to a list (or slice, which is just a list anyway) also evaluates the right-hand side in list context.

When you use the use warnings pragma or Perl's -w command-line option, you may see warnings about useless uses of constants or functions in "void context". Void context just means the value has been discarded, such as a statement containing only "fred"; or getpwuid(0); . It still counts as scalar context for functions that care whether or not they're being called in list context.

User-defined subroutines may choose to care whether they are being called in a void, scalar, or list context. Most subroutines do not need to bother, though. That's because both scalars and lists are automatically interpolated into lists. See "wantarray" in perlfunc for how you would dynamically discern your function's calling context.

# Scalar values

All data in Perl is a scalar, an array of scalars, or a hash of scalars. A scalar may contain one single value in any of three different flavors: a number, a string, or a reference. In general, conversion from one form to another is transparent. Although a scalar may not directly hold multiple values, it may contain a reference to an array or hash which in turn contains multiple values.

Scalars aren't necessarily one thing or another. There's no place to declare a scalar variable to be of type "string", type "number", type "reference", or anything else. Because of the automatic conversion of scalars, operations that return scalars don't need to care (and in fact, cannot care) whether their caller is looking for a string, a number, or a reference. Perl is a contextually polymorphic language whose scalars can be strings, numbers, or references (which includes objects). Although strings and numbers are considered pretty much the same thing for nearly all purposes, references are strongly-typed, uncastable pointers with builtin reference-counting and destructor invocation.

A scalar value is interpreted as FALSE in the Boolean sense if it is undefined, the null string or the number 0 (or its string equivalent, "0"), and TRUE if it is anything else. The Boolean context is just a special kind of scalar context where no conversion to a string or a number is ever performed. Negation of a true value by ! or not returns a special false value. When evaluated as a string it is treated as "" , but as a number, it is treated as 0. Most Perl operators that return true or false behave this way.

There are actually two varieties of null strings (sometimes referred to as "empty" strings), a defined one and an undefined one. The defined version is just a string of length zero, such as "" . The undefined version is the value that indicates that there is no real value for something, such as when there was an error, or at end of file, or when you refer to an uninitialized variable or element of an array or hash. Although in early versions of Perl, an undefined scalar could become defined when first used in a place expecting a defined value, this no longer happens except for rare cases of autovivification as explained in perlref . You can use the defined() operator to determine whether a scalar value is defined (this has no meaning on arrays or hashes), and the undef() operator to produce an undefined value.

To find out whether a given string is a valid non-zero number, it's sometimes enough to test it against both numeric 0 and also lexical "0" (although this will cause noises if warnings are on). That's because strings that aren't numbers count as 0, just as they do in awk :

That method may be best because otherwise you won't treat IEEE notations like NaN or Infinity properly. At other times, you might prefer to determine whether string data can be used numerically by calling the POSIX::strtod() function or by inspecting your string with a regular expression (as documented in perlre ).

The length of an array is a scalar value. You may find the length of array @days by evaluating $#days , as in csh . However, this isn't the length of the array; it's the subscript of the last element, which is a different value since there is ordinarily a 0th element. Assigning to $#days actually changes the length of the array. Shortening an array this way destroys intervening values. Lengthening an array that was previously shortened does not recover values that were in those elements.

You can also gain some minuscule measure of efficiency by pre-extending an array that is going to get big. You can also extend an array by assigning to an element that is off the end of the array. You can truncate an array down to nothing by assigning the null list () to it. The following are equivalent:

If you evaluate an array in scalar context, it returns the length of the array. (Note that this is not true of lists, which return the last value, like the C comma operator, nor of built-in functions, which return whatever they feel like returning.) The following is always true:

Some programmers choose to use an explicit conversion so as to leave nothing to doubt:

If you evaluate a hash in scalar context, it returns a false value if the hash is empty. If there are any key/value pairs, it returns a true value. A more precise definition is version dependent.

Prior to Perl 5.25 the value returned was a string consisting of the number of used buckets and the number of allocated buckets, separated by a slash. This is pretty much useful only to find out whether Perl's internal hashing algorithm is performing poorly on your data set. For example, you stick 10,000 things in a hash, but evaluating %HASH in scalar context reveals "1/16" , which means only one out of sixteen buckets has been touched, and presumably contains all 10,000 of your items. This isn't supposed to happen.

As of Perl 5.25 the return was changed to be the count of keys in the hash. If you need access to the old behavior you can use Hash::Util::bucket_ratio() instead.

If a tied hash is evaluated in scalar context, the SCALAR method is called (with a fallback to FIRSTKEY ).

You can preallocate space for a hash by assigning to the keys() function. This rounds up the allocated buckets to the next power of two:

# Scalar value constructors

Numeric literals are specified in any of the following floating point or integer formats:

You are allowed to use underscores (underbars) in numeric literals between digits for legibility (but not multiple underscores in a row: 23__500 is not legal; 23_500 is). You could, for example, group binary digits by threes (as for a Unix-style mode argument such as 0b110_100_100) or by fours (to represent nibbles, as in 0b1010_0110) or in other groups.

String literals are usually delimited by either single or double quotes. They work much like quotes in the standard Unix shells: double-quoted string literals are subject to backslash and variable substitution; single-quoted strings are not (except for \' and \\ ). The usual C-style backslash rules apply for making characters such as newline, tab, etc., as well as some more exotic forms. See "Quote and Quote-like Operators" in perlop for a list.

Hexadecimal, octal, or binary, representations in string literals (e.g. '0xff') are not automatically converted to their integer representation. The hex() and oct() functions make these conversions for you. See "hex" in perlfunc and "oct" in perlfunc for more details.

Hexadecimal floating point can start just like a hexadecimal literal, and it can be followed by an optional fractional hexadecimal part, but it must be followed by p , an optional sign, and a power of two. The format is useful for accurately presenting floating point values, avoiding conversions to or from decimal floating point, and therefore avoiding possible loss in precision. Notice that while most current platforms use the 64-bit IEEE 754 floating point, not all do. Another potential source of (low-order) differences are the floating point rounding modes, which can differ between CPUs, operating systems, and compilers, and which Perl doesn't control.

You can also embed newlines directly in your strings, i.e., they can end on a different line than they begin. This is nice, but if you forget your trailing quote, the error will not be reported until Perl finds another line containing the quote character, which may be much further on in the script. Variable substitution inside strings is limited to scalar variables, arrays, and array or hash slices. (In other words, names beginning with $ or @, followed by an optional bracketed expression as a subscript.) The following code segment prints out "The price is $100."

There is no double interpolation in Perl, so the $100 is left as is.

By default floating point numbers substituted inside strings use the dot (".") as the decimal separator. If use locale is in effect, and POSIX::setlocale() has been called, the character used for the decimal separator is affected by the LC_NUMERIC locale. See perllocale and POSIX .

# Demarcated variable names using braces

As in some shells, you can enclose the variable name in braces as a demarcator to disambiguate it from following alphanumerics and underscores or other text. You must also do this when interpolating a variable into a string to separate the variable name from a following double-colon or an apostrophe since these would be otherwise treated as a package separator:

Without the braces, Perl would have looked for a $whospeak, a $who::0 , and a $who's variable. The last two would be the $0 and the $s variables in the (presumably) non-existent package who .

In fact, a simple identifier within such curly braces is forced to be a string, and likewise within a hash subscript. Neither need quoting. Our earlier example, $days{'Feb'} can be written as $days{Feb} and the quotes will be assumed automatically. But anything more complicated in the subscript will be interpreted as an expression. This means for example that $version{2.0}++ is equivalent to $version{2}++ , not to $version{'2.0'}++ .

There is a similar problem with interpolation with text that looks like array or hash access notation. Placing a simple variable like $who immediately in front of text like "[1]" or "{foo}" would cause the variable to be interpolated as accessing an element of @who or a value stored in %who :

would attempt to access index 1 of an array named @who . Again, using braces will prevent this from happening:

will be treated the same as

This notation also applies to more complex variable descriptions, such as array or hash access with subscripts. For instance

Without the braces the above example would be parsed as a two level array subscript in the @name array, and under use strict would likely produce a fatal exception, as it would be parsed like this:

and not as the intended:

A similar result may be derived by using a backslash on the first character of the subscript or package notation that is not part of the variable you want to access. Thus the above example could also be written:

however for some special variables (multi character caret variables) the demarcated form using curly braces is the only way you can reference the variable at all, and the only way you can access a subscript of the variable via interpolation.

Consider the magic array @{^CAPTURE} which is populated by the regex engine with the contents of all of the capture buffers in a pattern (see perlvar and perlre ). The only way you can access one of these members inside of a string is via the braced (demarcated) form:

is equivalent to

Saying @^CAPTURE is a syntax error, so it must be referenced as @{^CAPTURE} , and to access one of its elements in normal code you would write ${^CAPTURE}[1] . However when interpolating in a string "${^CAPTURE}[1]" would be equivalent to ${^CAPTURE} . "[1]" , which does not even refer to the same variable! Thus the subscripts must also be placed inside of the braces: "${^CAPTURE[1]}" .

The demarcated form using curly braces can be used with all the different types of variable access, including array and hash slices. For instance code like the following:

would output

# Special floating point: infinity (Inf) and not-a-number (NaN)

Floating point values include the special values Inf and NaN , for infinity and not-a-number. The infinity can be also negative.

The infinity is the result of certain math operations that overflow the floating point range, like 9**9**9. The not-a-number is the result when the result is undefined or unrepresentable. Though note that you cannot get NaN from some common "undefined" or "out-of-range" operations like dividing by zero, or square root of a negative number, since Perl generates fatal errors for those.

The infinity and not-a-number have their own special arithmetic rules. The general rule is that they are "contagious": Inf plus one is Inf , and NaN plus one is NaN . Where things get interesting is when you combine infinities and not-a-numbers: Inf minus Inf and Inf divided by Inf are NaN (while Inf plus Inf is Inf and Inf times Inf is Inf ). NaN is also curious in that it does not equal any number, including itself: NaN != NaN .

Perl doesn't understand Inf and NaN as numeric literals, but you can have them as strings, and Perl will convert them as needed: "Inf" + 1. (You can, however, import them from the POSIX extension; use POSIX qw(Inf NaN); and then use them as literals.)

Note that on input (string to number) Perl accepts Inf and NaN in many forms. Case is ignored, and the Win32-specific forms like 1.#INF are understood, but on output the values are normalized to Inf and NaN .

# Version Strings

A literal of the form v1.20.300.4000 is parsed as a string composed of characters with the specified ordinals. This form, known as v-strings, provides an alternative, more readable way to construct strings, rather than use the somewhat less readable interpolation form "\x{1}\x{14}\x{12c}\x{fa0}" . This is useful for representing Unicode strings, and for comparing version "numbers" using the string comparison operators, cmp , gt , lt etc. If there are two or more dots in the literal, the leading v may be omitted.

Such literals are accepted by both require and use for doing a version check. Note that using the v-strings for IPv4 addresses is not portable unless you also use the inet_aton()/inet_ntoa() routines of the Socket package.

Note that since Perl 5.8.1 the single-number v-strings (like v65 ) are not v-strings before the => operator (which is usually used to separate a hash key from a hash value); instead they are interpreted as literal strings ('v65'). They were v-strings from Perl 5.6.0 to Perl 5.8.0, but that caused more confusion and breakage than good. Multi-number v-strings like v65.66 and 65.66.67 continue to be v-strings always.

# Special Literals

The special literals __FILE__, __LINE__, and __PACKAGE__ represent the current filename, line number, and package name at that point in your program. __SUB__ gives a reference to the current subroutine. They may be used only as separate tokens; they will not be interpolated into strings. If there is no current package (due to an empty package; directive), __PACKAGE__ is the undefined value. (But the empty package; is no longer supported, as of version 5.10.) Outside of a subroutine, __SUB__ is the undefined value. __SUB__ is only available in 5.16 or higher, and only with a use v5.16 or use feature "current_sub" declaration.

The two control characters ^D and ^Z, and the tokens __END__ and __DATA__ may be used to indicate the logical end of the script before the actual end of file. Any following text is ignored by the interpreter unless read by the program as described below.

Text after __DATA__ may be read via the filehandle PACKNAME::DATA , where PACKNAME is the package that was current when the __DATA__ token was encountered. The filehandle is left open pointing to the line after __DATA__. The program should close DATA when it is done reading from it. (Leaving it open leaks filehandles if the module is reloaded for any reason, so it's a safer practice to close it.) For compatibility with older scripts written before __DATA__ was introduced, __END__ behaves like __DATA__ in the top level script (but not in files loaded with require or do ) and leaves the remaining contents of the file accessible via main::DATA .

The DATA file handle by default has whatever PerlIO layers were in place when Perl read the file to parse the source. Normally that means that the file is being read bytewise, as if it were encoded in Latin-1, but there are two major ways for it to be otherwise. Firstly, if the __END__ / __DATA__ token is in the scope of a use utf8 pragma then the DATA handle will be in UTF-8 mode. And secondly, if the source is being read from perl's standard input then the DATA file handle is actually aliased to the STDIN file handle, and may be in UTF-8 mode because of the PERL_UNICODE environment variable or perl's command-line switches.

See SelfLoader for more description of __DATA__, and an example of its use. Note that you cannot read from the DATA filehandle in a BEGIN block: the BEGIN block is executed as soon as it is seen (during compilation), at which point the corresponding __DATA__ (or __END__) token has not yet been seen.

# Barewords

A word that has no other interpretation in the grammar will be treated as if it were a quoted string. These are known as "barewords". As with filehandles and labels, a bareword that consists entirely of lowercase letters risks conflict with future reserved words, and if you use the use warnings pragma or the -w switch, Perl will warn you about any such words. Perl limits barewords (like identifiers) to about 250 characters. Future versions of Perl are likely to eliminate these arbitrary limitations.

Some people may wish to outlaw barewords entirely. If you say

then any bareword that would NOT be interpreted as a subroutine call produces a compile-time error instead. The restriction lasts to the end of the enclosing block. An inner block may countermand this by saying no strict 'subs' .

# Array Interpolation

Arrays and slices are interpolated into double-quoted strings by joining the elements with the delimiter specified in the $" variable ( $LIST_SEPARATOR if "use English;" is specified), space by default. The following are equivalent:

Within search patterns (which also undergo double-quotish substitution) there is an unfortunate ambiguity: Is /$foo[bar]/ to be interpreted as /${foo}[bar]/ (where [bar] is a character class for the regular expression) or as /${foo[bar]}/ (where [bar] is the subscript to array @foo)? If @foo doesn't otherwise exist, then it's obviously a character class. If @foo exists, Perl takes a good guess about [bar] , and is almost always right. If it does guess wrong, or if you're just plain paranoid, you can force the correct interpretation with curly braces as above.

If you're looking for the information on how to use here-documents, which used to be here, that's been moved to "Quote and Quote-like Operators" in perlop .

# List value constructors

List values are denoted by separating individual values by commas (and enclosing the list in parentheses where precedence requires it):

In a context not requiring a list value, the value of what appears to be a list literal is simply the value of the final element, as with the C comma operator. For example,

assigns the entire list value to array @foo, but

assigns the value of variable $bar to the scalar variable $foo. Note that the value of an actual array in scalar context is the length of the array; the following assigns the value 3 to $foo:

You may have an optional comma before the closing parenthesis of a list literal, so that you can say:

To use a here-document to assign an array, one line per element, you might use an approach like this:

LISTs do automatic interpolation of sublists. That is, when a LIST is evaluated, each element of the list is evaluated in list context, and the resulting list value is interpolated into LIST just as if each individual element were a member of LIST. Thus arrays and hashes lose their identity in a LIST--the list

contains all the elements of @foo followed by all the elements of @bar, followed by all the elements returned by the subroutine named SomeSub called in list context, followed by the key/value pairs of %glarch. To make a list reference that does NOT interpolate, see perlref .

The null list is represented by (). Interpolating it in a list has no effect. Thus ((),(),()) is equivalent to (). Similarly, interpolating an array with no elements is the same as if no array had been interpolated at that point.

This interpolation combines with the facts that the opening and closing parentheses are optional (except when necessary for precedence) and lists may end with an optional comma to mean that multiple commas within lists are legal syntax. The list 1,,3 is a concatenation of two lists, 1, and 3 , the first of which ends with that optional comma. 1,,3 is (1,),(3) is 1,3 (And similarly for 1,,,3 is (1,),(,),3 is 1,3 and so on.) Not that we'd advise you to use this obfuscation.

A list value may also be subscripted like a normal array. You must put the list in parentheses to avoid ambiguity. For example:

Lists may be assigned to only when each element of the list is itself legal to assign to:

An exception to this is that you may assign to undef in a list. This is useful for throwing away some of the return values of a function:

As of Perl 5.22, you can also use (undef)x2 instead of undef, undef . (You can also do ($x) x 2 , which is less useful, because it assigns to the same variable twice, clobbering the first value assigned.)

When you assign a list of scalars to an array, all previous values in that array are wiped out and the number of elements in the array will now be equal to the number of elements in the right-hand list -- the list from which assignment was made. The array will automatically resize itself to precisely accommodate each element in the right-hand list.

When, however, you assign a list of scalars to another list of scalars, the results differ according to whether the left-hand list -- the list being assigned to -- has the same, more or fewer elements than the right-hand list.

If the number of scalars in the left-hand list is less than that in the right-hand list, the "extra" scalars in the right-hand list will simply not be assigned.

If the number of scalars in the left-hand list is greater than that in the left-hand list, the "missing" scalars will become undefined.

List assignment in scalar context returns the number of elements produced by the expression on the right side of the assignment:

This is handy when you want to do a list assignment in a Boolean context, because most list functions return a null list when finished, which when assigned produces a 0, which is interpreted as FALSE.

It's also the source of a useful idiom for executing a function or performing an operation in list context and then counting the number of return values, by assigning to an empty list and then using that assignment in scalar context. For example, this code:

will place into $count the number of digit groups found in $string. This happens because the pattern match is in list context (since it is being assigned to the empty list), and will therefore return a list of all matching parts of the string. The list assignment in scalar context will translate that into the number of elements (here, the number of times the pattern matched) and assign that to $count. Note that simply using

would not have worked, since a pattern match in scalar context will only return true or false, rather than a count of matches.

The final element of a list assignment may be an array or a hash:

You can actually put an array or hash anywhere in the list, but the first one in the list will soak up all the values, and anything after it will become undefined. This may be useful in a my() or local().

A hash can be initialized using a literal list holding pairs of items to be interpreted as a key and a value:

While literal lists and named arrays are often interchangeable, that's not the case for hashes. Just because you can subscript a list value like a normal array does not mean that you can subscript a list value as a hash. Likewise, hashes included as parts of other lists (including parameters lists and return lists from functions) always flatten out into key/value pairs. That's why it's good to use references sometimes.

It is often more readable to use the => operator between key/value pairs. The => operator is mostly just a more visually distinctive synonym for a comma, but it also arranges for its left-hand operand to be interpreted as a string if it's a bareword that would be a legal simple identifier. => doesn't quote compound identifiers, that contain double colons. This makes it nice for initializing hashes:

or for initializing hash references to be used as records:

or for using call-by-named-parameter to complicated functions:

Note that just because a hash is initialized in that order doesn't mean that it comes out in that order. See "sort" in perlfunc for examples of how to arrange for an output ordering.

If a key appears more than once in the initializer list of a hash, the last occurrence wins:

This can be used to provide overridable configuration defaults:

# Subscripts

An array can be accessed one scalar at a time by specifying a dollar sign ( $ ), then the name of the array (without the leading @ ), then the subscript inside square brackets. For example:

The array indices start with 0. A negative subscript retrieves its value from the end. In our example, $myarray[-1] would have been 5000, and $myarray[-2] would have been 500.

Hash subscripts are similar, only instead of square brackets curly brackets are used. For example:

You can also subscript a list to get a single element from it:

# Multi-dimensional array emulation

Multidimensional arrays may be emulated by subscripting a hash with a list. The elements of the list are joined with the subscript separator (see "$;" in perlvar ).

The default subscript separator is "\034", the same as SUBSEP in awk .

A slice accesses several elements of a list, an array, or a hash simultaneously using a list of subscripts. It's more convenient than writing out the individual elements as a list of separate scalar values.

Since you can assign to a list of variables, you can also assign to an array or hash slice.

The previous assignments are exactly equivalent to

Since changing a slice changes the original array or hash that it's slicing, a foreach construct will alter some--or even all--of the values of the array or hash.

As a special exception, when you slice a list (but not an array or a hash), if the list evaluates to empty, then taking a slice of that empty list will always yield the empty list in turn. Thus:

This makes it easy to write loops that terminate when a null list is returned:

As noted earlier in this document, the scalar sense of list assignment is the number of elements on the right-hand side of the assignment. The null list contains no elements, so when the password file is exhausted, the result is 0, not 2.

Slices in scalar context return the last item of the slice.

If you're confused about why you use an '@' there on a hash slice instead of a '%', think of it like this. The type of bracket (square or curly) governs whether it's an array or a hash being looked at. On the other hand, the leading symbol ('$' or '@') on the array or hash indicates whether you are getting back a singular value (a scalar) or a plural one (a list).

# Key/Value Hash Slices

Starting in Perl 5.20, a hash slice operation with the % symbol is a variant of slice operation returning a list of key/value pairs rather than just values:

However, the result of such a slice cannot be localized or assigned to. These are otherwise very much consistent with hash slices using the @ symbol.

# Index/Value Array Slices

Similar to key/value hash slices (and also introduced in Perl 5.20), the % array slice syntax returns a list of index/value pairs:

Note that calling delete on array values is strongly discouraged.

# Typeglobs and Filehandles

Perl uses an internal type called a typeglob to hold an entire symbol table entry. The type prefix of a typeglob is a * , because it represents all types. This used to be the preferred way to pass arrays and hashes by reference into a function, but now that we have real references, this is seldom needed.

The main use of typeglobs in modern Perl is create symbol table aliases. This assignment:

makes $this an alias for $that, @this an alias for @that, %this an alias for %that, &this an alias for &that, etc. Much safer is to use a reference. This:

temporarily makes $Here::blue an alias for $There::green, but doesn't make @Here::blue an alias for @There::green, or %Here::blue an alias for %There::green, etc. See "Symbol Tables" in perlmod for more examples of this. Strange though this may seem, this is the basis for the whole module import/export system.

Another use for typeglobs is to pass filehandles into a function or to create new filehandles. If you need to use a typeglob to save away a filehandle, do it this way:

or perhaps as a real reference, like this:

See perlsub for examples of using these as indirect filehandles in functions.

Typeglobs are also a way to create a local filehandle using the local() operator. These last until their block is exited, but may be passed back. For example:

Now that we have the *foo{THING} notation, typeglobs aren't used as much for filehandle manipulations, although they're still needed to pass brand new file and directory handles into or out of functions. That's because *HANDLE{IO} only works if HANDLE has already been used as a handle. In other words, *FH must be used to create new symbol table entries; *foo{THING} cannot. When in doubt, use *FH .

All functions that are capable of creating filehandles (open(), opendir(), pipe(), socketpair(), sysopen(), socket(), and accept()) automatically create an anonymous filehandle if the handle passed to them is an uninitialized scalar variable. This allows the constructs such as open(my $fh, ...) and open(local $fh,...) to be used to create filehandles that will conveniently be closed automatically when the scope ends, provided there are no other references to them. This largely eliminates the need for typeglobs when opening filehandles that must be passed around, as in the following example:

Note that if an initialized scalar variable is used instead the result is different: my $fh='zzz'; open($fh, ...) is equivalent to open( *{'zzz'}, ...) . use strict 'refs' forbids such practice.

Another way to create anonymous filehandles is with the Symbol module or with the IO::Handle module and its ilk. These modules have the advantage of not hiding different types of the same name during the local(). See the bottom of "open" in perlfunc for an example.

See perlvar for a description of Perl's built-in variables and a discussion of legal variable names. See perlref , perlsub , and "Symbol Tables" in perlmod for more discussion on typeglobs and the *foo{THING} syntax.

Perldoc Browser is maintained by Dan Book ( DBOOK ). Please contact him via the GitHub issue tracker or email regarding any issues with the site itself, search, or rendering of documentation.

The Perl documentation is maintained by the Perl 5 Porters in the development of Perl. Please contact them via the Perl issue tracker , the mailing list , or IRC to report any issues with the contents or format of the documentation.

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Perl | Variables

Variables in Perl are used to store and manipulate data throughout the program. When a variable is created it occupies memory space. The data type of a variable helps the interpreter to allocate memory and decide what to be stored in the reserved memory. Therefore, variables can store integers, decimals, or strings with the assignment of different data types to the variables. 

A variable in Perl can be named anything with the use of a specific datatype. There are some rules to follow while naming a variable: 

• Variables in Perl are case-sensitive.   

Example:  

• It starts with $, @ or % as per the datatype required, followed by zero or more letters, underscores, and digits
• Variables in Perl cannot contain white spaces or any other special character except underscore. 

   Variable Declaration is done on the basis of the datatype used to define the variable. These variables can be of three different datatypes:   

• Scalar Variables: It contains a single string or numeric value. It starts with $ symbol.   

Syntax: $var_name = value;

• Array Variables: It contains a randomly ordered set of values. It starts with @ symbol. 

Syntax : @var_name = (val1, val2, val3, …..);

• Hash Variables: It contains (key, value) pair efficiently accessed per key. It starts with % symbol. 

Syntax : %var_name = ( key1=>val1, key2=>val2, key3=>val3, …..);

   Perl allows modifying its variable values anytime after the variable declaration is done. There are various ways for the modification of a variable:  

• A scalar variable can be modified simply by redefining its value. 
• An element of an array can be modified by passing the index of that element to the array and defining a new value to it. 
• A value in a hash can be modified by using its Key. 

   Perl provides various methods to define a String to a variable. This can be done with the use of single quotes, double quotes, using q-operator and double-q operator, etc.  Using single quotes and double quotes for writing strings is the same but there exists a slight difference between how they work. Strings that are written with the use of single quotes display the content written within it exactly as it is.   

The above code will print:

Whereas strings written within double quotes replace the variables with their value and then display the string. It even replaces the escape sequences with their real use. Example:  

The above code will print: 

Example Code: 

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Perl hash basics: create, update, loop, delete and sort

Jun 16, 2013 by David Farrell

Hashes are one of Perl’s core data types. This article describes the main functions and syntax rules for for working with hashes in Perl.

Declaration and initialization

A hash is an unsorted collection of key value pairs. Within a hash a key is a unique string that references a particular value. A hash can be modified once initialized. Because a hash is unsorted, if it’s contents are required in a particular order then they must be sorted on output. Perl uses the ‘%’ symbol as the variable sigil for hashes. This command will declare an empty hash :

Similar to the syntax for arrays, hashes can also be declared using a list of comma separated values:

In the code above, Perl takes the first entry in the list as a key (‘monday’), and the second entry as that key’s value (65). The third entry in the list (‘tuesday’) would then be declared as a key, and the fourth entry (68) as its value and so on.

The ‘fat comma’ operator looks like an arrow (‘=>’) and allows the declaration of key value pairs instead of using a comma. This makes for cleaner and more readable code. Additionally there is no need to quote strings for keys when using the fat comma. Using the fat comma, the same declaration of %weekly_temperature would look like this:

Access a value

To access the value of a key value pair, Perl requires the key encased in curly brackets.

Note that strings do not need to be quoted when placed between the curly brackets for hash keys and that the scalar sigil (‘$’) is used when accessing a single scalar value instead of (‘%’).

Take a slice of a hash

A slice is a list of values. In Perl a slice can be read into an array, assigned to individual scalars, or used as an input to a function or subroutine. Slices are useful when you only want to extract a handful of values from a hash. For example:

The code above declares the ‘weekly_temperature’ hash as usual. What’s unusual here is that to get the slice of values, the array sigil (‘@’) is used by pre-pending it to the hash variable name. With this change the has will then lookup a list of values.

Access all values with the values function

The values function returns a list of the values contained in a hash. It’s possible to loop through the list of returned values and perform operations on them (e.g. print). For example:

A couple more tips when working with key value pairs of a hash: the code is more readable if you vertically align the fat comma (‘=>’) operators and unlike C, Perl allows the last element to have a trailing comma, which makes it easier to add elements later without generating a compile error.

Access all keys with the keys function

The keys function returns a list of the keys contained in a hash. A common way to access all the key value pairs of a hash is to use loop through the list returned by the keys function. For example:

In the above code we used the keys function to return a list of keys, looped though the list with foreach, and then printed the key and the value of each pair. Note that the print order of the pairs is different from intialization - that’s because hashes store their pairs in a random internal order. Also we used an interpreted quoted string using speech marks (“). This allows us to mix variables with plain text and escape characters like newline (’\n’) for convenient printing.

Access all key value pairs with the each function

The each function returns all keys and values of a hash, one at a time:

Add a new key value pair

To add a new pair to a hash, use this syntax:

Delete a key value pair

To remove a key value pair from a hash use the delete function. Delete requires the key of the pair in order to delete it from the hash:

Update a value of a pair

To update the value of a pair, simply assign it a new value using the same syntax as to add a new key value pair. The difference here is that the key already exists in the hash:

Empty a hash

To empty a hash, re-declare it with no members:

increment / decrement a value

Quick answer: use the same syntax for assigning / updating a value with the increment or decrement operator:

Sort a hash alphabetically

Although the internal ordering of a hash is random, it is possible to sort the output from a hash into a more useful sequence. Perl provides the sort function to (obviously) sort lists of data. By default it sorts alphabetically:

Let’s review the code above. The compare block receives the hash keys using the keys function. It then compares the values of each key using $a and $b as special variables to lookup and compare the values of the two keys. This sorted list of keys is then passed to the foreach command and looped through as usual. Note how the order is printed in value order - however it is still alphabetical ordering.

Sort a hash numerically

Numerically sorting a hash requires using a compare block as in the previous example, but substituting the ‘cmp’ operator for the numerical comparison operator (’<=>’):

Get the hash size

To get the size of a hash, simply call the keys function in a scalar context. This can be done by assigning the return value of keys to a scalar variable:

This article was originally posted on PerlTricks.com .

David Farrell

David is a professional programmer who regularly tweets and blogs about code and the art of programming.

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• Installing and getting started with Perl
• The Hash-bang line, or how to make a Perl scripts executable on Linux
• Perl Editor
• How to get Help for Perl?
• Perl on the command line
• Core Perl documentation and CPAN module documentation
• POD - Plain Old Documentation
• Debugging Perl scripts
• Common Warnings and Error messages in Perl
• Prompt, read from STDIN, read from the keyboard in Perl
• Automatic string to number conversion or casting in Perl
• Conditional statements, using if, else, elsif in Perl
• Boolean values in Perl
• Numerical operators
• String operators: concatenation (.), repetition (x)
• undef, the initial value and the defined function of Perl
• Strings in Perl: quoted, interpolated and escaped
• Here documents, or how to create multi-line strings in Perl
• Scalar variables
• Comparing scalars in Perl
• String functions: length, lc, uc, index, substr
• Number Guessing game
• Scope of variables in Perl
• Short-circuit in boolean expressions
• How to exit from a Perl script?
• Standard output, standard error and command line redirection
• Warning when something goes wrong
• What does die do?
• Writing to files with Perl
• Appending to files
• Open and read from text files
• Don't Open Files in the old way
• Reading and writing binary files in Perl
• EOF - End of file in Perl
• tell how far have we read a file
• seek - move the position in the filehandle in Perl
• slurp mode - reading a file in one step
• Perl for loop explained with examples
• Perl Arrays
• Processing command line arguments - @ARGV in Perl
• How to process command line arguments in Perl using Getopt::Long
• Advanced usage of Getopt::Long for accepting command line arguments
• Perl split - to cut up a string into pieces
• How to read a CSV file using Perl?
• The year of 19100
• Scalar and List context in Perl, the size of an array
• Reading from a file in scalar and list context
• STDIN in scalar and list context
• Sorting arrays in Perl
• Sorting mixed strings
• Unique values in an array in Perl
• Manipulating Perl arrays: shift, unshift, push, pop
• Reverse Polish Calculator in Perl using a stack
• Using a queue in Perl
• Reverse an array, a string or a number
• The ternary operator in Perl
• Loop controls: next, last, continue, break
• min, max, sum in Perl using List::Util
• qw - quote word
• Subroutines and functions in Perl
• Passing multiple parameters to a function in Perl
• Variable number of parameters in Perl subroutines
• Returning multiple values or a list from a subroutine in Perl
• Understanding recursive subroutines - traversing a directory tree
• Hashes in Perl
• Creating a hash from an array in Perl
• Perl hash in scalar and list context
• exists - check if a key exists in a hash
• delete an element from a hash
• How to sort a hash in Perl?
• Count the frequency of words in text using Perl
• Introduction to Regexes in Perl 5
• Regex character classes
• Regex: special character classes
• Perl 5 Regex Quantifiers
• trim - removing leading and trailing white spaces with Perl
• Perl 5 Regex Cheat sheet
• What are -e, -z, -s, -M, -A, -C, -r, -w, -x, -o, -f, -d , -l in Perl?
• Current working directory in Perl (cwd, pwd)
• Running external programs from Perl with system
• qx or backticks - running external command and capturing the output
• How to remove, copy or rename a file with Perl
• Reading the content of a directory
• Traversing the filesystem - using a queue
• Download and install Perl
• Installing a Perl Module from CPAN on Windows, Linux and Mac OSX
• How to change @INC to find Perl modules in non-standard locations
• How to add a relative directory to @INC
• How to replace a string in a file with Perl
• How to read an Excel file in Perl
• How to create an Excel file with Perl?
• Sending HTML e-mail using Email::Stuffer
• Perl/CGI script with Apache2
• JSON in Perl
• Simple Database access using Perl DBI and SQL
• Reading from LDAP in Perl using Net::LDAP
• Global symbol requires explicit package name

Variable declaration in Perl

• Use of uninitialized value
• Barewords in Perl
• Name "main::x" used only once: possible typo at ...
• Unknown warnings category
• Can't use string (...) as an HASH ref while "strict refs" in use at ...
• Symbolic references in Perl
• Can't locate ... in @INC
• Scalar found where operator expected
• "my" variable masks earlier declaration in same scope
• Can't call method ... on unblessed reference
• Argument ... isn't numeric in numeric ...
• Can't locate object method "..." via package "1" (perhaps you forgot to load "1"?)
• Useless use of hash element in void context
• Useless use of private variable in void context
• readline() on closed filehandle in Perl
• Possible precedence issue with control flow operator
• Scalar value ... better written as ...
• substr outside of string at ...
• Have exceeded the maximum number of attempts (1000) to open temp file/dir
• Use of implicit split to @_ is deprecated ...
• Multi dimensional arrays in Perl
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• Minimal requirement to build a sane CPAN package
• Statement modifiers: reversed if statements
• What is autovivification?
• Formatted printing in Perl using printf and sprintf
• declaration

The trouble

The exceptions, the danger of the explicit package name, use warnings, always use strict.

Published on 2013-07-23

Author: Gabor Szabo

"Compilation error: assigning to an array from an initializer list" When changing values of an array

I am writing a script where I initialize an array, and then later enter a new set of values into it.

The color sensor calibration has a default set of values I am using that I then want to edit if I choose to recalibrate the sensor. The problem flags at the end of the script where I enter new values into these arrays giving the error in the title. I have done some research and I think the problem is that the compiler thinks I am trying to re-initialize the array, but I guess you can't do that? How do I resolve this issue?

Assign new values to the array's elements with individual assignment statements.

is there a more concise way to do that then writing 18 lines?

Your guess is correct

You need to initialise the array elements individually. However, there are other problems with the sketch such as these variables not being declared

I stopped looking but there may be other problems

They're initialized in another part of the project. This is a project with hundreds of lines of code so I didn't post the entire thing.

Not seeing the whole sketch makes giving anything but general help

I know, however, what I needed was an understanding of how an aspect of the language works so I could resolve an error, so general advice. Two of you have said I need to change each entry in the array individually. This is a lot of manual and hard to follow code to write. Is there not a more concise way to do so? - Other than a for() loop because I am uncertain how I would do this with a for() loop given how each entry is a uniquely created value that can't be iterated though.

I was going to suggest using memcpy() but you would need to put the source values into an array first so you might just as well put them in the target array in the first place

You can define your array as class and than define a custom assignment method for it.

By the way, why all this variables are float?

Because using integer division would be a problem. I would like to convert all those values to integers from their floats after the calculations have been done but in my research it seems like that is a lot more trouble than its worth.

As suggested above, you can make your own data structures, e.g.,

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VMware vCenter Server 7.0 Update 3s | 17 SEP 2024 | ISO Build 24201990 Check for additions and updates to these release notes.

This release resolves CVE-2024-38812 and CVE-2024-38813. For more information on these vulnerabilities and their impact on VMware by Broadcom products, see VMSA-2024-0019 .

Earlier Releases of vCenter Server 7.0

Features, resolved and known issues of vCenter Server are described in the release notes for each release. Release notes for earlier releases of vCenter Server 7.0 are:

VMware vCenter Server 7.0 Update 3r Release Notes

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VMware vCenter Server 7.0.0b Release Notes

VMware vCenter Server 7.0.0a Release Notes

For internationalization, compatibility, installation, upgrade, open source components and product support notices, see the VMware vSphere 7.0 Release Notes .

For more information on vCenter Server supported upgrade and migration paths, please refer to VMware knowledge base article 67077 .

Patches Contained in This Release

Patch for vmware vcenter server appliance 7.0 update 3s, download and installation.

	VMware-vCenter-Server-Appliance-7.0.3.02100-24201990-patch-FP.iso
	24201990
	6789.2 MB
	2cb420786b18aa97faf29bac78c437b5
	04ceab872c4278eaaf3ebffd4d28e421662d7c0c7e49c2f88ebe8bc99093c8a9
	N/A
	CVE-2024-38812, CVE-2024-38813

Log in to the Broadcom Support Portal to download this patch .

For download instructions for earlier releases, see Download Broadcom products and software .

For more information on using the vCenter Server shells, see VMware knowledge base article 2100508 .

For more information on patching vCenter Server, see Patching the vCenter Server Appliance .

For more information on staging patches, see Stage Patches to vCenter Server Appliance .

For more information on installing patches, see Install vCenter Server Appliance Patches .

For more information on patching using the Appliance Management Interface, see Patching the vCenter Server by Using the Appliance Management Interface .

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Warning: Use of uninitialized value substitution

Why does Perl warn in this case

• substitution
• initialization

2 Answers 2

$string is not involved in the substitution. It's on the RHS of an assignment, and having an undefined value on the RHS of an assignment shouldn't trigger a warning.

If it's ok for $new to be undefined, you could use

There is no substitution for $string , so it does not warn. This code copies undef value from $string into $new and then do substitution on $new .

If you want to remove this warning, you can change to this:

• 1 The changes undef to an empty string. –  ikegami Commented Dec 25, 2012 at 11:21

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