Weblog of an Aspiring Computer Scientist
Random header image... Refresh for more!

A Few List Methods

by Shriphani

I hate reading reference manuals. They bug me to no end and I generally do not get beyond page number 10. These excellent habits of mine were the main reason for my taking up this task. I decided to brush up on my knowledge of Python’s built-in sequence-types and of course, since I love using lists in most of my code, I will attempt to write down some of the List methods in Python. I am not too sure if the same algorithms are used in Python but these work. Well, here goes:

First comes the very familiar append() method.

Usage: some_list.append(element)

#!/usr/bin/env python
#Author: Shriphani Palakodety

def append(list_name, element):
        list_name[len(list_name):] = element

>>> list_name = [1,2,3]
>>> list_name.append(4)
>>> print list_name
[1, 2, 3, 4]
>>>

Next comes the extend() method. This method takes a list as an argument and appends its elements to the current list.

def extend(list_name, list_to_add):
        list_name[len(list_name):] = list_to_add

A bunch of no-brainer methods follow. They are:

#insert(position, element) - insert a number at a given position in a list
def insert(list_name, position, element):
        list_name = list_name[0:position] + [element] + list_name[position+1:]

#index(element) - return the index of the first item whose value
#is equal to the supplied argument
def index(list_name, element):
        for i in range(len(list_name)):
                if list_name[i] == element:
                        break
                        return i
                else:
                        continue
        return "Element doesn’t exist"

#remove(element) - remove first item from list whose value is "element"
def remove(list_name, element):
        position = list_name.index(element)
        list_name = list_name[0:position] + list_name[position+1:]

The next method is the count() method. The count method returns the number of times an element has been repeated in a list. This is what I wrote in a few seconds:

def count(list_name, element):
        freq_dict = {}
        index = 0
        freq_dict[element] = 0
        for i in range(len(list_name)):
                if list_name[i] == element:
                        freq_dict[element] += 1
                else:
                        continue
        return freq_dict[element]

That is the count() method. Finally, we have the reverse() method. When used, it reverses all the elements in a list.

def reverse(list_name):
        new_list = copy.copy(list_name)
        for i in range(len(new_list)):
                list_name[i] = new_list[len(new_list)-1 - i]

That session served me quite well. I got to write some code and also got myself to read the reference manual.

Well, Happy Coding ! :)

2 comments

1 Prashanth Mohan { 04.11.08 at 4:46 am }

For reversing lists, look at the inbuilt rev() function. Another cool way to do it is to use list_name[::-1].

P.S: Your blog’s comment template is broken

2 Shriphani { 04.11.08 at 4:56 am }

Thanks for the comments. I seem to notice nothing odd about my blog’s comment template though.

Leave a Comment