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Python While Loop

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Python While Loop is used to execute a block of statements repeatedly until a given condition is satisfied. When the condition becomes false, the line immediately after the loop in the program is executed.

Syntax of while loop in Python

while expression:
statement(s)

Flowchart of Python While Loop

Python While Loop

 

While loop falls under the category of indefinite iteration. Indefinite iteration means that the number of times the loop is executed isn’t specified explicitly in advance. 

Statements represent all the statements indented by the same number of character spaces after a programming construct are considered to be part of a single block of code. Python uses indentation as its method of grouping statements. When a while loop is executed, expr is first evaluated in a Boolean context and if it is true, the loop body is executed. Then the expr is checked again, if it is still true then the body is executed again and this continues until the expression becomes false.

Difference between Python For Loop and Python While Loop

The main difference between Python For Loop Versus Python While Loop is that Python for loop is usually used when the number of iterations is known, whereas Python while loop is used when the number of iterations is unknown

Python While Loop

In this example, the condition for while will be True as long as the counter variable (count) is less than 3. 

Python

# Python program to illustrate
# while loop
count = 0
while (count < 3):
    count = count + 1
    print("Hello Geek")

                    

Output
Hello Geek
Hello Geek
Hello Geek

Infinite while Loop in Python

Here, the value of the condition is always True. Therefore, the body of the loop is run infinite times until the memory is full.

Python

age = 28
  
# the test condition is always True
while age > 19:
    print('Infinite Loop')

                    

Control Statements in Python with Examples

Loop control statements change execution from their normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed. Python supports the following control statements.

Python while loop with continue statement

Python Continue Statement returns the control to the beginning of the loop.

Python

# Prints all letters except 'e' and 's'
i = 0
a = 'geeksforgeeks'
  
while i < len(a):
    if a[i] == 'e' or a[i] == 's':
        i += 1
        continue
          
    print('Current Letter :', a[i])
    i += 1

                    

Output
Current Letter : g
Current Letter : k
Current Letter : f
Current Letter : o
Current Letter : r
Current Letter : g
Current Letter : k

Python while loop with a break statement

Python Break Statement brings control out of the loop.

Python

# break the loop as soon it sees 'e'
# or 's'
i = 0
a = 'geeksforgeeks'
  
while i < len(a):
    if a[i] == 'e' or a[i] == 's':
        i += 1
        break
          
    print('Current Letter :', a[i])
    i += 1

                    

Output
Current Letter : g

Python while loop with a pass statement

The Python pass statement to write empty loops. Pass is also used for empty control statements, functions, and classes.

Python

# An empty loop
a = 'geeksforgeeks'
i = 0
  
while i < len(a):
    i += 1
    pass
    
print('Value of i :', i)

                    

Output
Value of i : 13

While loop with else

As discussed above, while loop executes the block until a condition is satisfied. When the condition becomes false, the statement immediately after the loop is executed. The else clause is only executed when your while condition becomes false. If you break out of the loop, or if an exception is raised, it won’t be executed.

Note: The else block just after for/while is executed only when the loop is NOT terminated by a break statement. 

Python

# Python program to demonstrate
# while-else loop
  
i = 0
while i < 4:
    i += 1
    print(i)
else# Executed because no break in for
    print("No Break\n")
  
i = 0
while i < 4:
    i += 1
    print(i)
    break
else# Not executed as there is a break
    print("No Break")

                    

Output
1
2
3
4
No Break

1

Sentinel Controlled Statement

In this, we don’t use any counter variable because we don’t know how many times the loop will execute. Here user decides how many times he wants to execute the loop. For this, we use a sentinel value. A sentinel value is a value that is used to terminate a loop whenever a user enters it, generally, the sentinel value is -1.

Python while loop with user input

Here, It first asks the user to input a number. if the user enters -1 then the loop will not execute, i.e.

  • The user enters 6 and the body of the loop executes and again asks for input
  • Here user can input many times until he enters -1 to stop the loop
  • User can decide how many times he wants to enter input

Python

a = int(input('Enter a number (-1 to quit): '))
  
while a != -1:
    a = int(input('Enter a number (-1 to quit): '))

                    

Output:

Output Screen Image

While loop with Boolean values

One common use of boolean values in while loops are to create an infinite loop that can only be exited based on some condition within the loop. 

Example:

In this example, we initialize a counter and then use an infinite while loop (True is always true) to increment the counter and print its value. We check if the counter has reached a certain value and if so, we exit the loop using the break statement.

Python

# Initialize a counter
count = 0
  
# Loop infinitely
while True:
    # Increment the counter
    count += 1
    print(f"Count is {count}")
  
    # Check if the counter has reached a certain value
    if count == 10:
        # If so, exit the loop
        break
  
# This will be executed after the loop exits
print("The loop has ended.")

                    

Output
Count is 1
Count is 2
Count is 3
Count is 4
Count is 5
Count is 6
Count is 7
Count is 8
Count is 9
Count is 10
The loop has ended.

Python while loop with Python list

In this example, we have run a while loop over a list that will run until there is an element present in the list.

Python

# checks if list still
# contains any element
a = [1, 2, 3, 4]
  
while a:
    print(a.pop())

                    

Output
4
3
2
1

Single statement while block

Just like the if block, if the while block consists of a single statement we can declare the entire loop in a single line. If there are multiple statements in the block that makes up the loop body, they can be separated by semicolons (;). 

Python

# Python program to illustrate
# Single statement while block
count = 0
while (count < 5):
    count += 1
    print("Hello Geek")

                    

Output
Hello Geek
Hello Geek
Hello Geek
Hello Geek
Hello Geek

Python While Loop Exercise Questions

Below are two exercise questions on Python while loop. We have covered 2 important exercise questions based on the bouncing ball program and the countdown program.

Q1. While loop exercise question based on bouncing ball problem

Python

initial_height = 10 
bounce_factor = 0.5 
height = initial_height
while height > 0.1:  
    print("The ball is at a height of", height, "meters.")
    height *= bounce_factor  
print("The ball has stopped bouncing.")

                    

Output

The ball is at a height of 10 meters.
The ball is at a height of 5.0 meters.
The ball is at a height of 2.5 meters.
The ball is at a height of 1.25 meters.
The ball is at a height of 0.625 meters.
The ball is at a height of 0.3125 meters.
The ball is at a height of 0.15625 meters.
The ball has stopped bouncing.

Q2. Simple while-loop exercise code to build countdown clock

Python

countdown = 10
while countdown > 0:
    print(countdown)
    countdown -= 1
print("Blast off!")

                    

Output

10
9
8
7
6
5
4
3
2
1
Blast off!


Last Updated : 27 Dec, 2023
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