Lists within Lists

Earlier we learned that lists can store values of any data type. Does this mean we can store lists inside of lists? Well, yes we can…

A multi-dimensional list is a list of lists, meaning that the values inside the list are also lists.

A nested list is a list that appears as an element inside another list. Nested lists can store strings, numbers, and even more lists.

multidim_list = ['a', 'b', [1, 2, 3], 'rutabaga']

In multidim_list, the element at index 2 is a nested list. If we print(multidim_list[2]), we see [1, 2, 3] appear in the console.

The figure below shows a courses list that holds lists at each index position. Each nested list contains classes from the same subject area. Notice that each nested list has its own set of index values.

Two Dimensional Lists

The simplest form of a multi-dimensional list is a two dimensional list. Each element is a nested list, which contains multiple data values.

two_dim_list = [['a', 'b', 'c'], [90, 101], [True, False, False, True]]

print(len(two_dim_list))

3

two_dim_list holds three elements, each of which is a list. Note that the len() function only counts these three elements and NOT the total number of items inside of the nested lists.

To access the values from a nested list, use two sets of square brackets and two index values. The indexes evaluate from left to right. The first index selects one of the nested lists, and the second index selects an element from that nested list.

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two_dim_list = [['a', 'b', 'c'], [90, 101], [True, False, False, True]]

print(two_dim_list[0])     # Print the lists at indexes 0, 1, and 2
print(two_dim_list[1])
print(two_dim_list[2])

print(two_dim_list[0][2])  # Print the element from list 0, index 2
print(two_dim_list[1][1])  # Print the element from list 1, index 1
print(two_dim_list[2][3])  # Print the element from list 2, index 3

['a', 'b', 'c']
[90, 101]
[True, False, False, True]

c
101
True

Applying Methods to Nested Lists

We can apply list methods to either the nested or outer lists. However, we must use bracket notation carefully.

To apply a method to the outer list, the syntax is:

list_name.method()

To apply a method to one of the nested lists, the syntax is:

list_name[index_of_nested_list].method()

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list_a = [ ['a', 'b', 'c'], ['A', 'b', 'c'], ['A', 'B', 'C'] ]
list_b = [ ['a', 'b', 'c'], ['A', 'b', 'c'], ['A', 'B', 'C'] ]

list_a.reverse()     # Change the order of the 3 nested lists, but NOT their elements.
list_b[2].reverse()  # Change the order of the list at index 2.

print(list_a, '\n', list_b)

[ ['A', 'B', 'C'], ['A', 'b', 'c'], ['a', 'b', 'c'] ] 
[ ['a', 'b', 'c'], ['A', 'b', 'c'], ['C', 'B', 'A'] ]

Beyond Two Dimensional lists

There is no limit to how many layers we can have inside our lists. However, it is rare to use more than two dimensions.

Check Your Understanding

Use the following list to answer the questions:

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data = [
    ["science", "computer", "art"],
    ["Jones", "Diaz", "Rhodes"]
]