9.9. 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.

Lists within lists.

9.9.1. 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.

Example

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

print(len(two_dim_list))

Console Output

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.

Example

Use one set of brackets to access a nested list, and add a second set of brackets to access the values inside that list.

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

Console Output

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

c
101
True

9.9.2. 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()

Example

Examine how including bracket notation affects how the reverse method changes each list.

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)

Console Output

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

9.9.2.1. Try It!

Using the lists below in the text editor, work through the following challenge:

id_number_lists = [ [111, 222, 333], [987, 654], [1010, 1100, 0, 1] ]
new_nums = [1138, 42, 5]

repl.it

1. Append new_nums to id_number_lists, and append the value 444 to the first list in id_number_lists.

2. Print id_number_lists to verify your changes. Also, print the length of id_number_lists (which should be 4).

3. Print a slice of the last two elements in id_number_lists. Also, print a slice of the first two elements from the third list in id_number_lists.

4. Sample output:

   [ [111, 222, 333, 444], [987, 654], [1010, 1100, 0, 1], [1138, 42, 5] ]
   4
   [ [1010, 1100, 0, 1], [1138, 42, 5] ]
   [1010, 1100]
   

5. When done with steps 1 - 4, paste these statements to the bottom of your code. Run the program and examine the output. Write down your explanation of what happens at each step in the loops.

   for id_numbers in id_number_lists:
      print('Outer loop:', id_numbers)
   
      for number in id_numbers:
         print('Nested loop:', number)
   
      print('--------------')
   

9.9.3. 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.

9.9.4. Check Your Understanding

Use the following list to answer the questions:

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

Question

Which of the following will access the name "Jones"?

1. data[0][0]

2. data[0][1]

3. data[1][0]

4. data[1][1]

Question

How would you add "dance" to the first nested list?

1. data.append('dance')

2. data[0].append('dance')

3. data[1].append('dance')