====== Perl Cheat Sheet ======

[[http://www.perl.org/ | Perl]] is a highly capable, feature-rich programming language with over 25 years of development. These notes follow the path of the [[http://www.amazon.com/Learning-Perl-Randal-L-Schwartz/dp/1449303587 | Learning Perl]] book.

  * First line should be #!/usr/bin/perl or #!/usr/bin/env perl
  * Perl compiles to byte-code before executing
  * Do not use an extension for scripts.
  * To specify a minimum version use "use 5.010;"
  * Semicolons (;) required to separate statements
  * "perl -w" turns on warnings for entire program. For just the current file use "use warnings;"
  * "use diagnostics;" adds explanations to warnings
  * "use strict;" requires variable declaration
  * Comments start with # (no block comments)
  * Parenthesis are optional unless part of syntax.

===== Numbers =====

  * 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)
  * 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 \\).
  * Double-quoted strings can contain control-characters starting with a backslash and are variable interpolated e/g/ "${var}s $var\n".
  * . is for concatenation, x for string repetition.
  * 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 =====

  * if-elsif-else <code perl>if ($a == $b) {
  a = 0;
} elsif ($a > $b) {
  a = 1;
} else {
  a = -1;
}</code>
  * while loop <code perl>$count=0;
while ($count < 10) {
    $count += 2;
}</code>
  * foreach loop <code perl>foreach $rock (@rocks) {
  # modifications to $rock modify the list element
  # $_ is used if loop variable is omitted
}</code>

===== Input/Output =====

  * 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> chmop(@lines = <STDIN>);</code>