Perl: Map Function to List

By Xah Lee. Date: . Last updated: .

Map Function to List

Use map to apply a function to a list.

map { expression } list
Evaluates expression for each element of list. Any $_ in expression is replaced by a element in list. Returns the new list. (In scalar context, returns the list length.)
map { f $_ } list
Use function f on the elements.
# -*- coding: utf-8 -*-
# perl

use Data::Dumper; $Data::Dumper::Indent=0;

sub ff {return $_[0] + 1;};

print Dumper [ map { ff $_ } (1,2,3,4)]; # [2,3,4,5];

# ff written as embedded expression
print Dumper [ map { $_ + 1 } (1,2,3,4)]; # [2,3,4,5];
$_
a builtin variable that represent a argument given to a subroutine. $_ is also the default input for regex to match, and in general $_ represents a default argument.
@_
the entire arguments as array.
$_[0]
The first argument.

The (0..10) generate a list from 0 to 10.

The Data::Dumper module is to import the “Dumper” function for printing list.

Removing Elements in a List

Use grep to remove elements in a list.

grep {f $_} list
where function f's result is intrepreted as true/false. items which f returns false are removed.
grep {expression ;} list
using a expression

Example:

# -*- coding: utf-8 -*-
# perl

use Data::Dumper;

sub ff {return $_[0] % 2 == 0}; # return true if divisible by 2

print Dumper[ grep {ff $_} (0..10)]; # [ 0, 2, 4, 6, 8, 10 ]

Here's the same thing, where the function is embedded as a expression:

# -*- coding: utf-8 -*-
# perl

use Data::Dumper;

print Dumper[ grep {$_[0] % 2 == 0} (0..10) ]; # [ 0, 2, 4, 6, 8, 10 ]

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