3.4 List basics

Learning objectives

By the end of this section you should be able to

Use indexes to access individual elements in a list.
Use indexes to modify individual elements in a list.
Use len function to find the length of a list.
Demonstrate that lists can be changed after creation.

Lists

A list object can be used to bundle elements together in Python. A list is defined by using square brackets with comma separated values within the square brackets. Ex: list_1 = [1, 2, 4].

Empty lists can be defined in two ways:

list_1 =
list_1 = list

Lists can be made of elements of any type. Lists can contain integers, strings, floats, or any other type. Lists can also contain a combination of types. Ex: [2, "Hello", 2.5] is a valid list.

Python lists allow programmers to change the contents of the list in various ways.

Using indexes

Individual list elements can be accessed directly using an index. Indexes begin at 0 and end at one less than the length of the sequence. Ex: For a sequence of 50 elements, the first position is 0, and the last position is 49.

The index number is put in square brackets and attached to the end of the name of the list to access the required element. Ex: new_list[3] accesses the 4th element in new_list. An expression that evaluates to an integer number can also be used as an index. Similar to strings, negative indexing can also be used to address individual elements. Ex: Index -1 refers to the last element and -2 the second-to-last element.

The len function, when called on a list, returns the length of the list.

Example 1 List indexes and len function

The following code demonstrates the use of list indexes and the len function. Line 6 shows the use of the len function to get the length of the list. Line 10 shows how to access an element using an index. Line 14 shows how to modify a list element using an index.

# Setup a list of numbers
num_list = [2, 3, 5, 9, 11]
print(num_list)

# Print the length of the list
print("Length: ", len(num_list))

# Print the 4th element in the list
# The number 3 is used to refer to the 4th element
print("4th element:", num_list[3])

# The desired value of the 4th element is actually 7
# Update the value of the 4th element to 7
num_list[3] = 7

# The list of the first 5 prime numbers
print(num_list)

The above code's output is:


    [2, 3, 5, 9, 11]
    Length: 5
    4th element: 9
    [2, 3, 5, 7, 11]

Note

List basics

Write a program to complete the following:

Create a list with the following elements: 2, 23, 39, 6, -5.
Change the third element of the list (index 2) to 35.
Print the resulting list and the list's length.

# Create the list from the given elements

# Change the third element to 35

# Print the resultant list

# Print the length of the list