Python - While Loop

Python uses the while and for keywords to constitute a conditional loop, by which repeated execution of a block of statements is done until the specified boolean expression is true.

The following is the while loop syntax.

Syntax:

while [boolean expression]:
    statement1
    statement2
    ...
    statementN

Python keyword while has a conditional expression followed by the : symbol to start a block with an increased indent. This block has statements to be executed repeatedly. Such a block is usually referred to as the body of the loop. The body will keep executing till the condition evaluates to True. If and when it turns out to be False, the program will exit the loop. The following example demonstrates a while loop.

Example: while loop

num =0

while num < 5:
    num = num + 1
    print('num = ', num)

Try it

Output

num = 1
num = 2
num = 3
num = 4
num = 5

Here the repetitive block after the while statement involves incrementing the value of an integer variable and printing it. Before the block begins, the variable num is initialized to 0. Till it is less than 5, num is incremented by 1 and printed to display the sequence of numbers, as above.

All the statements in the body of the loop must start with the same indentation, otherwise it will raise a IndentationError.

Example: Invalid Indentation

num =0
while num < 5:
    num = num + 1
      print('num = ', num)

Try it

Output

  print('num = ', num)
  ^
IndentationError: unexpected indent

Exit from the While Loop

Use the break keyword to exit the while loop at some condition. Use the if condition to determine when to exit from the while loop, as shown below.

Example: Breaking while loop

num = 0

while num < 5:
    num += 1   # num += 1 is same as num = num + 1
    print('num = ', num)
    if num == 3: # condition before exiting a loop
        break

Try it

Output

num = 1
num = 2
num = 3

Continue Next Iteration

Use the continue keyword to start the next iteration and skip the statements after the continue statement on some conditions, as shown below.

Example: Continue in while loop

num = 0

while num < 5:
	num += 1   # num += 1 is same as num = num + 1
	if num > 3: # condition before exiting a loop
		continue
	print('num = ', num)

Try it

Output

num = 1
num = 2
num = 3

While Loop with else Block

The else block can follow the while loop. The else block will be executed when the boolean expression of the while loop evaluates to False.

Use the continue keyword to start the next iteration and skip the statements after the continue statement on some conditions, as shown below.

Example: while loop with else block

num = 0

while num < 3:
	num += 1   # num += 1 is same as num = num + 1
	print('num = ', num)
else:
    print('else block executed')

Output

num = 1
num = 2
num = 3
else block executed

The following Python program successively takes a number as input from the user and calculates the average, as long as the user enters a positive number. Here, the repetitive block (the body of the loop) asks the user to input a number, adds it cumulatively and keeps the count if it is non-negative.

Example: while loop

num=0
count=0
sum=0

while num>=0:
    num = int(input('enter any number .. -1 to exit: '))
    if num >= 0:
        count = count + 1 # this counts number of inputs
        sum = sum + num # this adds input number cumulatively.
avg = sum/count
print('Total numbers: ', count, ', Average: ', avg)

Try it

When a negative number is provided by the user, the loop terminates and displays the average of the numbers provided so far. A sample run of the above code is below:

Output

enter any number .. -1 to exit: 10
enter any number .. -1 to exit: 20
enter any number .. -1 to exit: 30
enter any number .. -1 to exit: -1
Total numbers: 3, Average: 20.0