9.4 Nested lists

Learning objectives

By the end of this section you should be able to

• Demonstrate the use of a list-of-lists to structure data.
• Demonstrate individual element addressing using multi-dimensional indexing.
• Use nested loops to iterate a list-of-lists.

List-of-lists

Lists can be made of any type of element. A list element can also be a list. Ex: [2, [3, 5], 17] is a valid list with the list [3, 5] being the element at index 1.

When a list is an element inside a larger list, it is called a nested list. Nested lists are useful for expressing multidimensional data. When each of the elements of a larger list is a smaller list, the larger list is called a list-of-lists.

Ex: A table can be stored as a two-dimensional list-of-lists, where each row of data is a list in the list-of-lists.

  print(matA[0])

Using nested loops to iterate nested lists

A nested loop structure can be used to iterate a list-of-lists. For a two-dimensional list-of-lists, an outer for loop can be used for rows, and an inner for loop can be used for columns.

1
"""Iterating a list-of-lists."""
2
 
3
    # Create a list of numbers
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    list1 = [[1, 2, 3],
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            [1, 4, 9],
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            [1, 8, 27]]
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    # Iterating the list-of-lists
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    for row in list1:
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      for num in row:
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        print(num, end=" ")
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      print()
    
The above code's output is:
    
    1 2 3
    1 4 9
    1 8 27
    
    

my_list = [[7, 4, 5, 12],
          [24, 3, 9, 16],
          [12, 8, 91, -5]]