Understanding lists

Python supports a data structure called list, which is a mutable and ordered sequence of elements. Each element in that list is called as item. Lists are defined by inserting values between square brackets [ ]. Each element of list is given a number, which we call as a position or index. The index starts from zero; that is, the first index is zero, the second index is 1, and so on. We can perform the following operations on lists: indexing, slicing, adding, multiplying, and checking for membership.

Python's built-in length function returns the length of that list. Python also has function for finding the largest and smallest item of list. Lists can be numbered lists, string lists, or mixed list.

The following is the code for creating a list:

l = list()
numbers = [10, 20, 30, 40]
animals = ['Dog', 'Tiger', 'Lion']
list1 = ['John', 5.5, 500, [110, 450]]

Here, we've created three lists: the first is numbers, the second is animals, and the third is list1. A list within another list is called as nested list. Our list1 is a nested list. A list containing no elements is called an empty list; you can create one with empty brackets, [].

As you might expect, you can assign list values to variables:

>>> cities = ['Mumbai', 'Pune', 'Chennai']
>>> numbers_list = [75, 857]
>>> empty_list = []
>>> print (cities, numbers_list, empty_list)
['Mumbai', 'Pune', 'Chennai'] [75, 857] []

Accessing values in lists

We can access the values from a list by using index values. We will specify the index number in [ and ]. Index starts from 0. Refer to the given example:

#!/usr/bin/python3
cities = ['Mumbai', 'Bangalore', 'Chennai', 'Pune']
numbers = [1, 2, 3, 4, 5, 6, 7 ]
print (cities[0])
print (numbers[1:5])


Output:
Mumbai
[2, 3, 4, 5]

Updating lists

You can update elements of lists, as shown in the following code:

#!/usr/bin/python3
cities = ['Mumbai', 'Bangalore', 'Chennai', 'Pune']
print ("Original Value: ", cities[3])
cities[3] = 'Delhi'
print ("New value: ", cities[3])


Output:
Original Value: Pune
New value: Delhi

Deleting list elements

To remove a list element, you can use either the del statement if you know exactly which element(s) you are deleting. You can use the remove() method if you do not know exactly which items to delete. Refer to the following example:

#!/usr/bin/python3
cities = ['Mumbai', 'Bangalore', 'Chennai', 'Pune']
print ("Before deleting: ", cities)
del cities[2]
print ("After deleting: ", cities)

Output:
Before deleting: ['Mumbai', 'Bangalore', 'Chennai', 'Pune']
After deleting: ['Mumbai', 'Bangalore', 'Pune']

Basic list operations

There are five basic list operations:

• Concatenation
• Repetition
• Length
• Membership
• Iteration

Description	Expression	Result
Concatenation	[30, 50, 60] + ['Hello', 75, 66]	[30,50,60,'Hello',75,66]
Membership	45 in [45,58,99,65]	True
Iteration	for x in [45,58,99] : print (x,end = ' ')	45 58 99
Repetition	['Python'] * 3	['python', 'python', 'python']
Length	len([45, 58, 99, 65])	4

List operations

In this section, we are going to learn about basic list operations: concatenation and repetition.

The + operator concatenates lists:

>>> a = [30, 50, 60]
>>> b = ['Hello', 75, 66 ]
>>> c = a + b
>>> print c
[30,50,60,'Hello',75,66]

Similarly, the * operator repeats a list a given number of times:

>>> [0] * 4
[0, 0, 0, 0]
>>> ['Python'] * 3
['python', 'python', 'python']

Indexing, slicing, and matrices

List indices work the same way as string indices. Values can be accessed using index. If you try to read or write an element that does not exist, you get IndexError. If an index has a negative value, it counts backward from the end of the list.

Now, we will create a list named cities and we will see the index operations:

cities = ['Mumbai', 'Bangalore', 'Chennai', 'Pune']

Description	Expression	Results
Index start at zero	cities[2]	'Chennai'
Slicing: getting sections	cities[1:]	['Bangalore', 'Chennai', 'Pune']
Negative: count from the right	cities[-3]	'Bangalore'