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 The Elements
The rest of these notes will refer to the Perl 5 Reference Guide, highlighting and expanding on important points. So get your Reference Guide and turn to Section 2, Literals.
Perl's Three Data Structures
* Scalars can be numeric or character as determined by context:
123 12.4 5E-10 0xff (hex) 0377 (octal)
'What you $see is (almost) what \n you get' 'Don\'t Walk'
"How are you?" "Substitute values of $x and \n in \" quotes."
`date` `uptime -u` `du -sk $filespec | sort -n`
$x $list_of_things[5] $lookup{'key'}
Single-quotes ' ' allow no substitution except for \\ and \'. Double-quotes " " allow substitution of variables like $x and control codes like \n (newline). Back-quotes ` ` also allow substitution, then try to execute the result as a system command, returning as the final value whatever the system command outputs.
* Arrays of scalars (also called lists) are sequentially-arranged scalars:
('Sunday', 'Monday', 'Tuesday', 'Wednesday',
'Thursday', 'Friday', 'Saturday')
(13,14,15,16,17,18,19) equivalent to (13..19)
(13,14,15,16,17,18,19)[2..4] equivalent to (15,16,17)
@whole_list
* Associative arrays (also called hashes) help you remember things:
$DaysInMonth{'January'} = 31; $enrolled{'Joe College'} = 1;
$StudentName{654321} = 'Joe College';
$score{$studentno,$examno} = 89;
%whole_hash
Perl 5 allows combinations of these, such as lists of lists and associative arrays of lists.
Name Conventions
Scalar variables start with '$', even when referring to an array element. The variable name reference for a whole list starts with '@', and the variable name reference for a whole associative array starts with '%'.
Lists are indexed with square brackets enclosing a number, normally starting with [0]. In Perl 5, negative subscripts count from the end. Thus, $things[5] is the 6th element of array @things, and
('Sun','Mon','Tue','Wed','Thu','Fri','Sat')[1]
equals 'Mon'.
Associative arrays are indexed with curly brackets enclosing a string. $whatever, @whatever, and %whatever are three different variables.
@days = (31,28,31,30,31,30,31,31,30,31,30,31);
# A list with 12 elements.
$#days # Last index of @days; 11 for above list
$#days = 7; # shortens or lengthens list @days to 8 elements
@days # ($days[0], $days[1],... )
@days[3,4,5] # = (30,31,30)
@days{'a','c'} # same as ($days{'a'},$days{'c'})
%days # (key1, value1, key2, value2, ...)
Case is significant--"$FOO", "$Foo" and "$foo" are all different variables. If a letter or underscore is the first character after the $, @, or %, the rest of the name may also contain digits and underscores. If this character is a digit, the rest must be digits. Perl has several dozen special variables whose second character is non-alphanumeric. For example, $/ is the input record separator, newline "\n" by default. An uninitialized variable has a special "undefined" value which can be detected by the function defined(). Undefined values convert depending on context to 0, null, or false.
The variable "$_" Perl presumes when needed variables are not specified. Thus:
<STDIN>; assigns a record from filehandle STDIN to $_
print; prints the curent value of $_
chop; removes the last character from $_
@things = split; parses $_ into white-space delimited
words, which become successive
elements of list @things.
$_, $1, $2, $3, and other implicit variables contribute to Perl Paradox Number Two: What you don't see can help you or hurt you. See Quick Reference Guide Section 25, Special Variables.
Subroutines and functions are referenced with an initial '&', which is optional if reference is obviously a subroutine or function such as following the sub, do, and sort directives:
sub square { return $_[0] ** 2; }
print "5 squared is ", &square(5);
Filehandles don't start with a special character, and so as to not conflict with reserved words are most reliably specified as uppercase names: INPUT, OUTPUT, STDIN, STDOUT, STDERR, etc.
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