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--- notes:perl_cheat_sheet [2026/09/15 08:46]
216.73.216.243 old revision restored (2026/09/11 18:08)
+++ notes:perl_cheat_sheet [2026/09/16 12:27] (current)
47.79.206.7 old revision restored (2026/06/27 05:22)
@@ Line 14: / Line 14: @@
   * Parenthesis are optional unless part of syntax.
   * ''@ARGV'' contains arguments, $0 contains program name.
-  * ''die'' can be used to exit early $! will contain any system error message. Without \n at the end, perl will append line number to error message.
+  * ''die''/''warn'' can be used to exit/warn, $! will contain any system error message. Without \n at the end, perl will append line number to error message.
 ===== Numbers =====
@@ Line 61: / Line 61: @@
   * Expressions parsed in either a scalar context or a list context. Scalars are promoted to single-element lists in list context.
   * List functions may return different scalars - array variables return number of elements. The ''scalar'' function forces a scalar context e.g. for print function.
+===== Hashes =====
+  * A hash is a list indexed by a string (key) - a collection of key-value pairs. ''%hash'' has its own namespace.
+  * Uses scalable, efficient algorithms. Used to be called associative arrays.
+  * ''$hash{$some_key}'' to access a value (curly-braces instead of square braces).
+  * Assigning a hash to an array unwinds (flattens) it.
+  * To initialize a hash :<code perl>%some_hash = ('a', 1, 'b', 2, 'c', 3);
+%some_hash = ( 'a' => 1, 'b'=>2, 'c'=>3,);</code>
+  * When using a big arrow (a fat-comma) or when accessing a value, simple keys don't have to be quoted (barewords) e.g. <code perl>%some_hash = ( a => 1}; $some_hash{a};</code>
+  * ''%revhash = reverse %hash'' to reverse a hash (for non-unique values, last one wins).
+  * ''keys %hash;'' and ''values %hash;'' return a list of keys or value in same order (or # of keys/values in scalar context).
+  *  '' %hash' is true only if hash has at least one key-value pair.
+  * To iterate over hash : <code perl>while ( ($key, $value) = each %hash ) {
+  print "$key => $value\n"; }</code> or in order of keys <code perl>foreach $key (sort keys %hash) {
+  print "$key => $hash{$key}\n"; } </code>
+  * %ENV hash holds environment variables.
 ===== Control Structures =====
@@ Line 91: / Line 108: @@
   * ''<>'' 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";
+    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'
@@ Line 103: / Line 120: @@
 open BEDROCK, '>fred' || die "Cannot open fred: $!";
 open LOG, '>>:encoding(UTF-8)','logfile'; # for perl >= 5.6
-open BEDROCK, '>:crlf', $file_name; # DOS-formatted output
+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 =====

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