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--- notes:perl_cheat_sheet [2026/07/17 03:01]
47.79.201.132 old revision restored (2026/06/16 01:57)
+++ notes:perl_cheat_sheet [2026/07/24 13:03] (current)
47.79.201.51 old revision restored (2026/06/15 21:28)
@@ Line 13: / Line 13: @@
   * Comments start with # (no block comments)
   * Parenthesis are optional unless part of syntax.
+  * ''@ARGV'' contains arguments, $0 contains program name.
+  * ''die''/''warn'' can be used to exit/warn, $! will contain any system error message. Without \n at the end, perl will append line number to error message.
 ===== Numbers =====
@@ Line 73: / Line 75: @@
     $count += 2;
 }</code>
-  * foreach loop <code perl>foreach $rock (@rocks) {
+  * foreach loop <code perl>foreach my $rock (@rocks) {
   # modifications to $rock modify the list element
   # $_ is used if loop variable is omitted
@@ Line 82: / Line 84: @@
   * Read a line of input - ''$line = <STDIN>;''
   * ''chomp'' removes a newline e.g. ''chomp($line=<STDIN>);''
-  * Assigning <STDIN> to a list reads all input up till EOF e.g. <code perl> chmop(@lines = <STDIN>);</code>
+  * Assigning <STDIN> to a list reads all input up till EOF e.g. <code perl> chomp(@lines = <STDIN>);</code>
+  * Loop over input ($_ 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 LOGFILE "It was $_ that I saw!\n";
+}</code>
+  * ''print'' takes list of items and sends all to STDOUT (unseparated). ''print @array;'' vs ''print "@array";'' <code perl>print <>;          # source code for 'cat'
+print sort <>;     # source code for 'sort'</code>
+  * C-like printf function %g for number auto-format,%10s, %-10d etc.<code perl>my @items = qw( wilma dino pebbles );
+my $format = "The items are:\n" . ("%10s\n" x @items);
+printf $format, @items;
+printf "The items are:\n".("%10s\n" x @items), @items;
+</code>
+  * Filehandles can be barewords (upper-cased) or variables. Special filehandles are : STDIN, STDOUT, STDERR, DATA, ARGV, and ARGVOUT .<code perl>open CONFIG, '<dino';  # < is optional
+open BEDROCK, '>fred' || die "Cannot open fred: $!";
+open LOG, '>>:encoding(UTF-8)','logfile'; # for perl >= 5.6
+open my $bedrock, '>:crlf', $file_name; # DOS-formatted output
+binmode STDOUT, ':encoding(UTF-8)';</code>
+  * ''select'' can be used to change default output filehandle. ''$| = 1;'' unbuffers currently selected output.
+===== 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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