Python, Ruby, Perl: Loop Thru a List

Xah Lee, 2005-01-18, …, 2012-12-19

Python

Here is a example of going thru a list by element.

# -*- coding: utf-8 -*-
# python

aa = ['one', 'two', 'three', 'infinity']
for xx in aa:
    print xx

Python provides a way to loop thru a list and give both its index and the item value.

# -*- coding: utf-8 -*-
# python

bb = ['one', 'two', 'three', 'infinity']
for ii, vv in enumerate(bb):
    print ii, vv

# 0 one
# 1 two
# 2 three
# 3 infinity

The following construct loops thru a dictionary, each time assigning both keys and values to variables.

# -*- coding: utf-8 -*-
# python

dd = {'john':3, 'mary':4, 'jane':5, 'vicky':7}
for kk, vv in dd.iteritems():
    print kk, ' is ', vv

# jane  is  5
# john  is  3
# mary  is  4
# vicky  is  7

Ruby

Loop thru a list.

# -*- coding: utf-8 -*-
# ruby

aa = ['one', 'two', 'three', 'infinity']

# loop thru a list
aa.each {    # opening curly bracket must be on this line, else syntax error.
  |xx|          # each element is set to a dummy variable xx
  p xx          # prints xx
}

In the above, the “each” is a method for list object. This method takes a “block” argument in the form of { … }.

A Ruby “block” typically has the form { |‹var›| … } OR do |‹var›| … end, where the ‹var› is a dummy variable. Ruby's “block” is similar to a lambda (aka pure function).

When a method takes a “block”, you can think of it as taking a pure function written in the form { |‹var›| ‹body› }.

Loop thru a list, get both index and value.

# -*- coding: utf-8 -*-
# ruby

aa = ['one', 'two', 'three', 'infinity']

# loop thru a list and gets its value and index
aa.each_with_index do
  |vv, ii|                      # first item is value
  p vv, ii                      # print value and index
end

The following construct loops thru a dictionary, each time assigning both keys and values to variables.

# -*- coding: utf-8 -*-
# ruby

hh = { :john => 3, :mary => 4, :jane => 5, :vicky => 7}

# loop thru a hash, and get key and value
hh.each do
  |kk,vv|
  p kk, vv                      # print key and value
end

=begin
prints

:john
3
:mary
4
:jane
5
:vicky
7

=end

Perl

In Perl, looping thru a list is typically done with “for”:

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

@myList = ('one', 'two', 'three', 'infinity');
for $xx (@myList) {
    print $xx, "\n";            # print each element
}

To loop thru a list using indexs, do like this:

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

@myList = ('one', 'two', 'three', 'infinity');
for ($i=0; $i < scalar(@myList); $i++) {
    print "$i $myList[$i], ";
} # prints: 0 one, 1 two, 2 three, 3 infinity,

A typical way to loop thru a hash is:

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

%myHash = ('john',3, 'mary',4, 'jane',5, 'vicky',7);
for $key (keys %myHash) {
    print "Key and Value pair is: $key, $myHash{$key} \n";
}

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