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Python Data Structures in Practice

Introduction to nested lists
Iterating over nested lists
Modifying nested lists
Working with multiple lists
15. Transposing nested lists
Summary

Instruction

Very well done! Now, let's tackle a classic programming problem. Suppose we need to transpose a rectangular nested list, i.e. change its rows into columns:

transpose

How can we do that in Python? Take a look:

numbers_list = [
  [1, 4, 7, 10],
  [2, 5, 8, 11],
  [3, 6, 9, 12]
]

list_to_return = []
for i in range(len(numbers_list[0])):
  new_row = []
  for j in range(len(numbers_list)):
    new_row.append(numbers_list[j][i])
  list_to_return.append(new_row)

Note what happens:

  1. In the outer loop (i), we iterate over the length of the first nested list, i.e. over the "columns".
  2. Inside the loop, we create a variable for a new row (an empty list).
  3. In the inner loop (j), we iterate over the outer list's length, i.e. over the "rows".
  4. Inside the inner loop, we add the i-th element from each nested list:
  5. We add the new row to the new list.

Exercise

Write a function named vertical_flip(input_list) that accepts a 2D list and flips it vertically:

flip

Do not return a copy of the list. Instead, modify the original one.

Stuck? Here's a hint!

Use the following nested for loops:

for i in range(len(input_list)):
  for j in range(len(input_list[i]) // 2):

Inside the nested loop, swap two elements:

input_list[i][j]
and 
input_list[i][len(input_list[i]) - j - 1]