Perl: List/Array
Create a List
- List is enclosed by parenthesis “()”.
- To assign a list to a variable, the variable must have a “@” sign in front.
- To print a list, use the “Dumper” function in the package “Data::Dumper”.
# -*- coding: utf-8 -*- # perl @a = (0, 1, 2, 'three', 4, 5, 6, 7); # assigns a list to @a. use Data::Dumper; # loads the list-printing module print '@a is:', Dumper(\@a);
The backslash in front of @a is necessary. It returns the “reference” of the array @a, and the argument to Dumper must be a reference. Once a list is assigned to a variable, it's called array.
Perl's concept of “list” and “array” is a bit complex. Basically, when a list is assigned to a variable, it's a array. For detail, see: Perl: Difference Between List and Array
Counting Elements
To find the number of elements in a list, use “scalar”.
# -*- coding: utf-8 -*- # perl @a = (4, 5, 6); # a list print scalar(@a); # prints 3. The length. print @a + 0; # prints 3. The 「+」 forces a scalar context.
Perl has a “list context” and “scalar context”. How things are evaluated depends on whether the thing is in list or scalar context. “Context” basically means what's adjacent.
When a list/array is in a scalar context, it returns its length. The function scalar forces things in a scalar context.
Adding Elements
To add a element, or join two lists, use push(array, new_item).
# -*- coding: utf-8 -*- # perl use Data::Dumper; @b = (1, 9); push(@b, 3); # add a element to @b, at the end print Dumper(\@b); # [1, 9, 3]
# -*- coding: utf-8 -*- # perl use Data::Dumper; @a = (1, 9); @b = (3, 4); @c = (); push(@c, @a, @b); # @c is the joined list of @a and @b print Dumper(\@c); # [1, 9, 3, 4]
Nested List
Perl automatically flatten array/lists, even if the new item added to list is a list. To force creating a nested list, you have to use square brackets, like this:
# -*- coding: utf-8 -*- # perl use Data::Dumper; @a = (1, 9); @b = (5, 6); # create nested list push(@a, [@b]); print Dumper(\@a); # [1, 9, [3, 4]]
Square brackets actually creates a reference to a array.
# -*- coding: utf-8 -*- # perl use Data::Dumper; @a = (1, 8); # array $b = [1, 8]; # reference to array print Dumper(\@a); # [1, 8] print Dumper($b); # [1, 8]
Technically, the backslash forces the list into a reference.
Getting Elements
To extract list element, use @array[index]. The index can be several integers separated by comma, for getting multiple elements.
# -*- coding: utf-8 -*- # perl use Data::Dumper; @a = (0, 1, 2, 'three', 4, 5, 6, 7); @b = @a[3, 1, 5]; # ['three', 1, 5] $c = @a[2]; # gets 2 print Dumper \@b; # ['three', 1, 5] print $c, "\n"; # 2
Here a example of extracting sublist (aka slice).
# -*- coding: utf-8 -*- # perl use Data::Dumper; @a = (0, 1, 2, 'three', 4, 5, 6, 7); @b = @a[1..4]; # the 1..4 creates a range print Dumper \@b; # [1, 2, 'three', 4]
Changing Elements
To replace parts, just assign them. For example, $myarray[3] = 'rabbit';.
# -*- coding: utf-8 -*- # perl use Data::Dumper; @a = (0, 1, 2, 'three', 4); $a[3] = 'new'; print Dumper(\@a); # [ 0, 1, 2, 'new', 4 ]
Note the dollar sign $ above. This tells Perl that this data is a “scalar” as opposed to a “multiple”.
In Perl, a variable of “scalar” type (such as numbers and strings) starts with a $. A variable for array (aka list) starts with the sign @. A variable for harshes/dictionaries starts with %. All Perl variables must start with one of {$ @ %}. (this is a simplified story)
Nested List
To create a nested list, use square brackets for the inner list.
# -*- coding: utf-8 -*- # perl use Data::Dumper; @b = (4, 5, [1, 2, [9, 8]], 7); # nested list print Dumper \@b; # [ 4, 5, [ 1, 2, [ 9, 8 ] ], 7 ]
You can embed a array as a nested list into another array. For example, @b = (4, 5, \@myarray, 7).
# -*- coding: utf-8 -*- # perl use Data::Dumper; @a=(1, 2, 3); @b = (4, 5, \@a, 7); # embed @a as sublist. print '@b', Dumper \@b;# [ 4, 5, [ 1, 2, 3 ], 7 ]
To extract element from nested list, use the form $array[index1]->[index2]->[index3]….
# -*- coding: utf-8 -*- # perl use Data::Dumper; @b = (1, 2, ['x', 'y'], 3); $c = $b[2]->[1]; print $c; # 'y' @b = (1, 2, ['x', [4, 5], 7], 3); $c = $b[2]->[1]->[1]; print $c; # 5
See also: Perl: Difference Between List and Array