Nested Lists

Section 12.2 Nested Lists

A nested list is a list that appears as an element in another list. Nested lists in Python are lists that contain other lists as their elements. They are useful for representing structured data like tables, grids, and trees.

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When you have a nested list, each list within the main list can be accessed using an index. To access elements within those nested lists, you chain multiple indices together.

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In this list, the element with index 3 is a nested list. If we print(nested[3]), we get [10, 20]. To extract an element from the nested list, we can proceed in two steps. First, extract the nested list, then extract the item of interest.

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It is also possible to combine those steps using bracket operators that evaluate from left to right.

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To understand the code above does, let’s to analyze the indices used:

nested[3]: This accesses the fourth element of the nested list, which is [10, 20].

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[1]: This accesses the second element of the list [10, 20], which is 20.

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Putting it all together, nested[3][1] first accesses [10, 20] and then accesses 20

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Check your understanding

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Checkpoint 12.2.1.

What is printed by the following statements?

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alist = [ [4, [True, False], 6, 8], [888, 999] ]
if alist[0][1][0]:
print(alist[1][0])
else:
print(alist[1][1])

6
6 is in the wrong list. alist[1] refers to the second item in alist, namely [888,999].
8
8 is in the wrong list. alist[1] refers to the second item in alist, namely [888,999].
888
Yes, alist[0][1][0] is True and alist[1] is the second list, the first item is 888.
999
alist[0][1][0] is True. Take another look at the if statement.

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