3.9. While Loops With Lists

It’s often useful to use a loop to iterate through a list. Suppose you had the following list of colours. You can print out each colour using the following code:

colours = ["red", "yellow", "pink", "green", "purple", "orange", "blue"]

print(colours[0])
print(colours[1])
print(colours[2])
print(colours[3])
print(colours[4])
print(colours[5])
print(colours[6])

What you’ll have noticed is that the code is quite repetitive. If you ever see code that is repetitive, it often means it can be replaced by a loop! Have a look at the example below.

Recall that ``len()`` tells you the number of elements in the given list. In this case is ``len(colours)`` is 7.

colours = ["red", "yellow", "pink", "green", "purple", "orange", "blue"]

i = 0
while i < len(colours):
    print(colours[i])
    i = i + 1

Question 1

What do you think the output of the following will be?

numbers = ['one', 'two', 'three', 'four', 'five']

i = len(numbers) - 1
while i >= 0:
   print(numbers[i])
   i = i - 1

Solution

Let’s trace through this code.

len(numbers) gives us the number of elements in the list numbers, which in this case is 5. This means that initially i is 4 since i = len(numbers - 1).

We can also see that the condition we have set for our while loop is i > 0.

At the first iteration i = 4. This means the condition is True. We execute the code block:

print(numbers[i])
i = i - 1

This means that we output numbers[4]:

five

At the second iteration i = 3. This means the condition is True. Again, we execute the code block and we output numbers[3]:

four

At the third iteration i = 2. This means that the condition is True. Again, we execute the code block and we output numbers[2]:

three

At the fourth iteration i = 1. This means that the condition is True. Again, we execute the code block and we output numbers[1]:

two

At the fifth iteration i = 0. This means that the condition is True. Again, we execute the code block and we output numbers[0]:

one

At the sixth iteration i = -1. This means that the condition is False. We do not execute the code block.

Question 2

What do you think the output of the following will be?

numbers = ['one', 'two', 'three', 'four', 'five']
new_list = []

i = len(numbers) - 1
while i >= 0:
   new_list.append(numbers[i])
   i = i - 1

print(new_list)

1. ['five', 'four', 'three', 'two', 'one']
   

2. ['four', 'three', 'two', 'one']
   

3. ['one', 'two', 'three', 'four', 'five']
   

4. ['two', 'three', 'four', 'five']
   

Solution

Solution is locked

Question 3

What do you think the output of the following will be?

fruits = ['apple', 'banana', 'cherry', 'date', 'eggplant', 'fig', 'grape']

i = 0
while i <= len(fruits):
   print(fruits[i])
   i = i + 2

Solution

Solution is locked

Question 4

What do you think the output of the following will be?

numbers = [5, 3, 8, -3, 0, 2]

total = 0

i = 0
while i < len(numbers):
   total = total + numbers[i]
   i = i + 1

print(total)

Solution

Solution is locked

Question 5

Rewrite the following code using a while loop.

symbols = ['@', '%', '#', '+', '^']

print(symbols[0])
print(symbols[0] + symbols[0] + symbols[0])
print(symbols[1])
print(symbols[1] + symbols[1] + symbols[1])
print(symbols[2])
print(symbols[2] + symbols[2] + symbols[2])
print(symbols[3])
print(symbols[3] + symbols[3] + symbols[3])
print(symbols[4])
print(symbols[4] + symbols[4] + symbols[4])

Solution

Solution is locked

Code challenge: All the numbers!

You have been provided with a list of numbers.

numbers = [58, 67, 48, 12, 67, 88, 50, 54, 13, 46, 89, 98, 27, 13, 83]

Write a program that prints out each of these numbers on a new line and at the end says That’s all the numbers!

The output of your program should look like this:

58
67
48
...
13
83
That's all the numbers!

Note

The … indicates that there are more numbers that are just not shown in the example. Here we only show the first 3 lines and last 3 lines of output.

Solution

Solution is locked