3.7. The "while" loop

In previous section, we learned about the for loop that we can use to iterate over an iterable. In this section, we will take a look at the while loop.

Understanding while loop

A while loop, unlike for loop, does not iterate over an iterable. This loop is used repeatedly execute a code block until a condition is True.

The basic syntax of this loop is like so:

while condition:
    # loop body here

Here, until condition could be a logical expression, a boolean or any true-like value, the loop body would be executed until the condition is True.

Let us see an example, the following code prints out the numbers from 0 to 9:

CodeOutput

i = 0

while i < 10:
    print(i)
    i = i + 1

0
1
2
3
4
5
6
7
8
9

Until the variable i is less than 10, the while loop is going to execute.

Caveats

while loop comes with some extra caveats that must be taken into account.

Variable Definition

In a while loop, the variables that are being used to control the loop (e.g. one used in condition) must be initialized before the loop. This isn't the case in a for loop.

For example, in above example, since we use i in the condition, we must define this variable beforehand and give it an initial value. In a for loop, we don't have to do this.

while loopfor loop

i = 0

while i < 10:
    print(i)
    i = i + 1

for i in range(10):
    print(i)

Loop termination

while loop will execute until the condition is True. It is a common pitfall to run into an "infinite loop" when using while loop.

For illustration purpose, if you run the following code, you will see that the program prints Hello World repeatedly and never stops:

CodeOutput

while True:
    print('Hello World')

Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
[...]

In the numbers example shown above, the loop terminates because the line i = i + 1 keeps incrementing i until i becomes equal to 10 and the condition (i < 10) becomes False.

i = 0

while i < 10:
    print(i)
    i = i + 1

If we remove that line, look at what happens:

CodeOutput

i = 0

while i < 10:
    print(i)

0
0
0
0
0
0
0
0
0
[...]

The i variable never updates and ultimately the i < 10 condition is always True causing the loop to run forever.

Note

In a for loop, incrementing the loop controlling variable is handled by range() and you don't have to do that manually.

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Exercise

ProblemSolution

Write a program that repeatedly takes numbers as user input until user does not enter stop. Once stop is given as input, the program should output the sum of numbers that were input.

Example program output:

Enter a number or type "stop": 20
Enter a number or type "stop": 10
Enter a number or type "stop": 5
Enter a number or type "stop": 30
Enter a number or type "stop": 5
Enter a number or type "stop": stop
Sum of given numbers is 70

Highlighted lines are user inputs.

prompt = 'Enter a number or type "stop": '
result = 0
value = input(prompt)

while value != 'stop':
    result = result + int(value)
    value = input(prompt)

print('Sum of given numbers is', result)