Perl-Python Tutorial: Applying a Function to a List

2005-01-17

Python

Removing Elements in a List

To remove elements in a list that satisfies some criterion, use the function “filter(testFunction,list)”. The “testFunction” will be applied to each element in the list. If testFunction(element) returns False, then that element will not be in the resulting list.

def even(n): return n % 2 == 0
print filter( even, range(11))

Applying a Function to a List

The “map” function applies a function to all elements of a list. Example:

def square(n): return n*n
print map(square, range(11))

Reference: Python Doc↗.

Perl

Removing Elements in a List

Use “grep” to remove elements in a list. The form is “grep {testFunction $_} myList”. Example:

use Data::Dumper;
sub even {return $_[0]%2==0};
print Dumper[ grep {even $_} (0..10)];

In perl in general, “$_” means the entire argument(s) given to a subroutine. “$_[0]” means the first argument. The “(0..10) generate a list from 0 to 10”.

Applying a Function to a List

Use “map” to apply a function to a list. The basic form is “map {myFunction($_)} myList”.

use Data::Dumper;
sub square {return ($_[0])**2;};
print Dumper [ map {square($_)} (0..10)];

The “Data::Dumper” module is to import the “Dumper” function for printing lists. The “%” is the operator for computing remainder. The “**” is the exponential operator.

Reference: perldoc Data::Dumper↗.

Reference: perldoc -f grep↗.

Reference: perldoc -f map↗.

See also:

• Lambda in Python 3000
• one-liner loop in Functional Style
• Sorting

Page created: 2005-01.
© 2005 by Xah Lee.