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Iterating over a list
Modifying lists
Working with multiple lists
16. Summary


Well done! It's time to wrap things up. Let's summarize what we've learned before we take the quiz.

  1. Use a for loop to iterate over a list. You may need one or more variables defined before the loop.
    numbers = [2, 45, 32, 1]
    sum = 0
    for number in numbers:
      sum += number
  2. You can usethe enumerate() function with a for loop.
    first_sum = 0
    second_sum = 0
    for i, number in enumerate(numbers):
      if i < 2:
        first_sum += number
        second_sum += number
  3. If you want to update list elements, iterate over the indexes.
    for i in range(len(numbers)):
      numbers[i] = ...
  4. To change the element order, iterate over the indexes.
    for i in range(0, len(numbers), 2):
      numbers[i], numbers[i+1] = numbers[i+1], numbers[i]
  5. To safely delete list elements, iterate over a copy of the list using [:].
    for number in numbers[:]:
  6. When creating a new list based on an existing one, you will typically need an empty list before the loop with an append() function inside it.
    neg_numbers = []
    for number in numbers:
      neg_numbers.append(-1 * number)
  7. To compare two same-sized lists element by element, iterate over the indexes of either list.
    numbers1 = [4, 98, 23, 43]
    numbers2 = [2, 45, 32, 1]
    for i in range (len(numbers1)):
      if numbers1[i] > numbers2[i]:
  8. When comparing two lists with different sizes, watch out for IndexErrors: iterate over the shorter list or check if the current index is within the bounds.
    numbers1 = [2, 45]
    numbers2 = [2, 45, 32, 1]
    • iterate over the shorter list
      for i in numbers1:
    • check the index inside
      for i in numbers2:
        if i < len(numbers1):
  9. When merging sorted lists or searching for common elements, you will typically need a while loop and two variables to store the current list indexes.
    i, j = 0, 0
    while i < len(list1) and j < len(list2):
      if ...: i += 1
      if ...: j += 1

Quite a lot! All right, it's time for a short quiz.


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