While Loops

A while loop lets us execute a series of statements as long as the condition (boolean expression) remains True.

A while loop has the basic structure:

while <condition>:
    # Do something until the condition is False

1. While Loop Using a Counter

A good portion of the while loops that you write will involve a counter variable.

The following example demonstrates a basic while loop that prints out the numbers 1 - 5:

i = 1 # Define a variable i and bind the value 1 to it
while i < 6: 
    print(i)  # print out the value of i
    i += 1    # This increments i by 1. i.e. i = i + 1

print("Program has ended!")

Note the indentation here! The indented block is the loop body (the code that is executed inside the loop).

The above code first defines a counter variable i = 1. It then defines a while loop that tests whether i < 6. If this is True it prints out the value of i and then adds 1 to i.

Note it is traditional to use i for a counter variable.

The image below demonstrates how the code runs:

2. While Loop Using a Boolean Variable

Consider the following program which just keeps asking the user for a number until they enter a 0. Then the program ends.

# loop until the user enters 0

input_string = input("Please enter a number:\n")
num = int(input_string)

while num != 0:
    input_string = input("Please enter a number:\n")
    num = int(input_string)

print("Program has ended!")

We can rewrite this program by using a bool variable (program_over) which keeps track of whether the while loop should continue or stop.

After a number is entered, we use an if statement to test if the number entered was 0, if it was, then we set program_over to True. This will then stop the while loop and the program will end.

# loop until the user enters 0

program_over = False

while not program_over:
    input_string = input("Please enter a number:\n")
    num = int(input_string)
    if num == 0:
         program_over = True

print("Program has ended!")

3. Non-terminating While Loop

Now consider the code if you forget to increment (add to) i.

i = 1 # Define a variable i and bind the value 1 to it
while i < 6: 
    print(i)  # print out the value of i

This loop will repeat forever! i will never change its value from 1 and therefore the condition i < 6 will forever remain True.

• What will be the output?
• Try it in Python. To stop (exit) the program in the terminal press Ctrl + c.

4. Nested While Loops

Just like if statements, we can nest loops.

The following program shows an outer and an inner loop. This prints out the multiplication table.

i = 1
while i < 13:
    j = 1
    while j < 13:
        print(f"{i} x {j} = {i*j}")
        j += 1
    i += 1

The outer loop has a counter variable i and the inner loop has a counter variable j. Below is a flow diagram of the program.

The following is of an animation of a similar program (below), but the conditions are changed to allow us to visualise it quicker.

i = 1
while i < 3:
    j = 1
    while j < 3:
        print(f"{i} x {j} = {i*j}")
        j += 1
    i += 1

=== TASK ===

Use a while loop to print out the even numbers starting at 2 and up to and including 100.

2
4
6
.
.
.
100

Note that the dots represent numbers between 6 and 100. It saves us writing it all out!

HINT: You only need a single while loop.