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.

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.

Flowchart of Python while Loop

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:

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.

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.

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

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