Python while Loop

In programming, loops are used to repeat a block of code. For example, if we want to show a message 100 times, then we can use a loop. It's just a simple example, we can achieve much more with loops.

In the previous tutorial, we learned about Python for loop . Now we will learn about the while loop.

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Python while Loop

Python while loop is used to run a block code until a certain condition is met.

The syntax of while loop is:

while condition:
    # body of while loop

Here,

1. A while loop evaluates the condition
2. If the condition evaluates to True , the code inside the while loop is executed.
3. condition is evaluated again.
4. This process continues until the condition is False .
5. When condition evaluates to False , the loop stops.

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Flowchart of Python while Loop

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Example: Python while Loop

# program to display numbers from 1 to 5

# initialize the variable
i = 1
n = 5

# while loop from i = 1 to 5
while i <= n:
    print(i)
    i = i + 1

Output

1
2
3
4
5

Here's how the program works:

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Example 2: Python while Loop

# program to calculate the sum of numbers
# until the user enters zero

total = 0

number = int(input('Enter a number: '))

# add numbers until number is zero
while number != 0:
    total += number    # total = total + number
    
    # take integer input again
    number = int(input('Enter a number: '))
    

print('total =', total)

Output

Enter a number: 12
Enter a number: 4
Enter a number: -5
Enter a number: 0
total = 11

In the above example, the while iterates until the user enters zero. When the user enters zero, the test condition evaluates to False and the loop ends.

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Infinite while Loop in Python

If the condition of a loop is always True , the loop runs for infinite times (until the memory is full). For example,

age = 32

# the test condition is always True
while age > 18:
    print('You can vote')

In the above example, the condition always evaluates to True . Hence, the loop body will run for infinite times.

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Python While loop with else

In Python, a while loop may have an optional else block.

Here, the else part is executed after the condition of the loop evaluates to False .

counter = 0

while counter < 3:
    print('Inside loop')
    counter = counter + 1
else:
    print('Inside else')

Output

Inside loop
Inside loop
Inside loop
Inside else

Note : The else block will not execute if the while loop is terminated by a break statement.

counter = 0

while counter < 3:
    # loop ends because of break
    # the else part is not executed 
    if counter == 1:
        break

    print('Inside loop')
    counter = counter + 1
else:
    print('Inside else')

Output

Inside loop

Inside else

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Python for Vs while loops

The for loop is usually used when the number of iterations is known . For example,

# this loop is iterated 4 times (0 to 3)
for i in range(4):
    print(i)

The while loop is usually used when the number of iterations is unknown . For example,

while condition:
    # run code until the condition evaluates to False