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Exercise 11

Challenging Problem: Don't feel bad if you struggle with this problem or can't solve it. The problem is not easy. It is designed to demonstrate why we prefer to use for loops when we can, and a big part of that problem is messy code that is difficult to write and understand. See how far you can get, but don't spend much time struggling.

Print all of the even numbers in the following list of nested lists. This time, don't use any for loops.

my_list = [
  [1, 3, 6, 11],
  [4, 2, 4],
  [9, 17, 16, 0],
]

Solution

my_list = [
  [1, 3, 6, 11],
  [4, 2, 4],
  [9, 17, 16, 0],
]

outer_index = 0
while outer_index < len(my_list):
    inner_index = 0
    while inner_index < len(my_list[outer_index]):
        number = my_list[outer_index][inner_index]
        if number % 2 == 0:
            print(number)

        inner_index += 1

    outer_index += 1

Phew! That's messy and hard to follow, isn't it? While the individual loops aren't tough to understand, keeping track of two different indexes and getting them to work together is no mean feat. We also have to use both indexes on lines 10 and 11.

We can simplify this solution a bit by assigning each nested list to a local variable:

my_list = [
  [1, 3, 6, 11],
  [4, 2, 4],
  [9, 17, 16, 0],
]

outer_index = 0
while outer_index < len(my_list):
    inner_list = my_list[outer_index]
    inner_index = 0
    while inner_index < len(inner_list):
        number = inner_list[inner_index]
        if number % 2 == 0:
            print(number)

        inner_index += 1

    outer_index += 1

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