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--- notes:perl_cheat_sheet [2013/08/31 23:37]
smthng [Input/Output]
+++ notes:perl_cheat_sheet [2013/09/22 19:28] (current)
smthng [Hashes]
@@ 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, $! will contain any system error message. Without \n at the end, perl will append line number to error message.
+===== Numbers =====
+  * All numbers are stored in the same format e.g.-6.5e24, 037 (octal), 0x2a13b (hex), 0b11010 (binary)
+  * Underscores for readability are ignored e.g. 0x1377_0B77
+  * Arithmetic operators are * / + - % (modulus) %%**%% (exponentiation)
+  * Comparison operators are < <= == >= > !=
+===== Strings =====
+  * To use unicode in source code - "use utf8;"
+  * Single-quoted literals '⅚∞☃☠' can contain any character except single-quote (use \') and backslash (use \\).
+  * Double-quoted strings can contain control-characters starting with a backslash and are variable interpolated e/g/ "${var}s $var\n".
+  * . is for concatenation, x for string repetition.
+  * To insert a character by code - chr(0x05d0). To convert a character to its code - ord('א')
+  *  String comparison operators are lt, le, eq, ge, gt, and ne
 ===== Scalar Variables =====
@@ Line 44: / Line 61: @@
   * Expressions parsed in either a scalar context or a list context. Scalars are promoted to single-element lists in list context.
   * List functions may return different scalars - array variables return number of elements. The ''scalar'' function forces a scalar context e.g. for print function.
-===== Numbers =====
-  * All numbers are stored in the same format e.g.-6.5e24, 037 (octal), 0x2a13b (hex), 0b11010 (binary)
+===== Hashes =====
-  * Underscores for readability are ignored e.g. 0x1377_0B77
-  * Arithmetic operators are * / + - % (modulus) ** (exponentiation)
-  * Comparison operators are < <= == >= > !=
-===== Strings =====
+  * A hash is a list indexed by a string (key) - a collection of key-value pairs. ''%hash'' has its own namespace.
-  * To use unicode in source code - "use utf8;"
+  * Uses scalable, efficient algorithms. Used to be called associative arrays.
-  * Single-quoted literals '⅚∞☃☠' can contain any character except single-quote (use \') and backslash (use \\).
+  * ''$hash{$some_key}'' to access a value (curly-braces instead of square braces).
-  * Double-quoted strings can contain control-characters starting with a backslash and are variable interpolated e/g/ "${var}s $var\n".
+  * Assigning a hash to an array unwinds (flattens) it.
-  * . is for concatenation, x for string repetition.
+  * To initialize a hash :<code perl>%some_hash = ('a', 1, 'b', 2, 'c', 3);
-  * To insert a character by code - chr(0x05d0). To convert a character to its code - ord('א')
+%some_hash = ( 'a' => 1, 'b'=>2, 'c'=>3,);</code>
-  *  String comparison operators are lt, le, eq, ge, gt, and ne
+  * 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 72: / Line 92: @@
     $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
   # $_ is used if loop variable is omitted
@@ Line 81: / Line 101: @@
   * Read a line of input - ''$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.