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--- notes:perl_cheat_sheet [2026/11/01 20:59]
114.119.151.209 old revision restored (2026/10/15 17:45)
+++ notes:perl_cheat_sheet [2026/11/03 04:42] (current)
104.252.46.124 old revision restored (2026/08/27 01:06)
@@ Line 12: / Line 12: @@
   * "use strict;" requires variable declaration
   * Comments start with # (no block comments)
+  * Parenthesis are optional unless part of syntax.
+  * ''@ARGV'' contains arguments, $0 contains program name.
-===== Scalar Variables =====
-  * Scalars are numbers or strings (or undefined or references), used interchangeably.
-  * Scalar variables start with $ and a letter or underscore, but can contain letters (including unicode), numbers or underscores.
-  * Scalar assignment looks like  $a = 3; or $a .= "suffix"; or $a %%**%%= 3;
-  * Recommended to use all lowercase variable names with underscores for word separation e.g. $var_name
-  * print takes a single scalar or a list of scalars (separated by commas)
-  * When a scalar is interpreted as a boolean, the following are //false// : 0, '0', %%''%%, undefined .
-  * !! is a handy shortcut to convert any scalar to 0 or 1 (to represent a boolean)
 ===== Numbers =====
@@ Line 28: / Line 19: @@
   * All numbers are stored in the same format e.g.-6.5e24, 037 (octal), 0x2a13b (hex), 0b11010 (binary)
   * Underscores for readability are ignored e.g. 0x1377_0B77
-  * Arithmetic operators are * / + - % (modulus) ** (exponentiation)
+  * Arithmetic operators are * / + - % (modulus) %%**%% (exponentiation)
   * Comparison operators are < <= == >= > !=
 ===== Strings =====
   * To use unicode in source code - "use utf8;"
   * Single-quoted literals '⅚∞☃☠' can contain any character except single-quote (use \') and backslash (use \\).
@@ Line 38: / Line 30: @@
   * To insert a character by code - chr(0x05d0). To convert a character to its code - ord('א')
   *  String comparison operators are lt, le, eq, ge, gt, and ne
+===== Scalar Variables =====
+  * Scalars are numbers or strings (or undefined or references), used interchangeably.
+  * $calar variables start with $ and a letter or underscore, but can contain letters (including unicode), numbers or underscores.
+  * Scalar assignment looks like  $a = 3; or $a .= "suffix"; or $a %%**%%= 3;
+  * Recommended to use all lowercase variable names with underscores for word separation e.g. $var_name
+  * print takes a single scalar or a list of scalars (separated by commas)
+  * When a scalar is interpreted as a boolean, the following are //false// : 0, '0', %%''%%, undefined .
+  * !! is a handy shortcut to convert any scalar to 0 or 1 (to represent a boolean)
+  * Variables have value ''undef'' before being assigned. ''undef'' acts as a 0 or "" as needed, but will throw a warning if printed.
+  * ''defined()'' checks if a variable has been defined. Can also set a variable to ''undef''.
+===== Lists and Arrays =====
+  * A list is an ordered set of elements (each of which is a scalar value), starting with position 0.
+  * @rrays are variables that store a list (separate namespace from scalar variables). @x refers to entire array x.
+  * Undefined arrays start out as ''()'', the empty list and not ''undef''.
+  * To access an element of an array - ''$myarr[0]'' - ''undef'' if never set.
+  * Index of last element is in ''$#arr''
+  * Negative array indices wrap around (only once) so -1 refers to last element.
+  * Literal lists - ''('abc','def')'' or ''(1..6)'' (integers 1 to 6) or ''qw(abc def)'' or ''qw#abc def#'' (quoted by whitespace).
+  * Can assign list values to variables e.g. <code perl>($fred, $barney, $dino) = ("flintstone", "rubble", undef);
+($fred, $barney) = ($barney, $fred); # swap those values</code>
+  * ''push'' adds element(s) to **end** of array. ''pop'' removes a single element and returns it.
+  * ''unshift'' adds element(s) to **start** of array. ''shift'' removes a single element and returns it.
+  * ''splice'' removes elements from middle of array, returns them and optionally replaces them <code perl>splice @arr, start, len, @newelems</code>
+  * ''sort'' and ''reverse'' functions return the modified list (can be saved to original array variable).
+  * Expressions parsed in either a scalar context or a list context. Scalars are promoted to single-element lists in list context.
+  * List functions may return different scalars - array variables return number of elements. The ''scalar'' function forces a scalar context e.g. for print function.
 ===== Control Structures =====
@@ Line 51: / Line 73: @@
 while ($count < 10) {
     $count += 2;
+}</code>
+  * foreach loop <code perl>foreach my $rock (@rocks) {
+  # modifications to $rock modify the list element
+  # $_ is used if loop variable is omitted
 }</code>
@@ Line 57: / Line 83: @@
   * Read a line of input - ''$line = <STDIN>;''
   * ''chomp'' removes a newline e.g. ''chomp($line=<STDIN>);''
+  * Assigning <STDIN> to a list reads all input up till EOF e.g. <code perl> chomp(@lines = <STDIN>);</code>
+  * Loop over input ($_ can only be used in this specific case) <code perl>while (<STDIN>) {
+    print "I saw $_";
+}</code>
+  * ''foreach'' can also be used but uses more memory since it reads all input into memory first.
+  * ''<>'' iterates over all files in @ARGV (or STDIN if no args) like e.g. cat/sed/awk. <code perl>while (<>) {
+    chomp;
+    print "It was $_ that I saw!\n";
+}</code>
+===== User Subroutines =====
+  * Subroutines are in their own namespace and are typically called using ''&mysub'' (sometimes the ampersand can be omitted).
+  * Subroutines can be defined anywhere e.g. <code perl> sub mysub {
+  my($m, $n) = @_
+  $m + $n;
+} </code>
+  * The value of the last expression evaluated is returned. But ''return $a;'' can be used to return immediately.
+  * A list can be returned if the subroutine is called in a list context (''wantarray'' can be used to detect list or scalar context).
+  * Arguments are in the list @_ . If ''&mysub;'' is used, the parent's argument list is inherited.
+  * Lexical scoped variables can be used in any block by prefixing with ''my''.
+  * In Perl 5.10 and up ''state $x;'' can be used to declare a private variable that keep state between calls.
+  * Ampersand is optional if subroutine has previously been declared or if parenthesis are used. Ampersand is not optional if built-in subroutine is being overridden.

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