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
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")
OutputHello 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
OutputCurrent 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
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)
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")
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.")
OutputCount 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())
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")
OutputHello 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!
Python While Loop – FAQs
How do we create a while loop in Python?
A while loop in Python is created using the following syntax:
while condition:
# code block to be executed
The loop runs as long as the condition is true.
What are typical use cases for using while loops in Python?
Typical use cases for while loops include:
- Repeating tasks until a condition is met, such as reading lines from a file.
- Continuously prompting the user for input until valid data is provided.
- Running the main loop of a game until the player quits.
- Polling or continuously checking the status of a resource or condition.
How to control a while loop with break and continue in Python?
break: Exits the loop immediately, regardless of the condition.
while True:
user_input = input("Enter something (or 'quit' to stop): ")
if user_input == 'quit':
break
print(f"You entered: {user_input}")
continue: Skips the rest of the code inside the loop for the current iteration and jumps back to the beginning to re-evaluate the condition.
counter = 0
while counter < 10:
counter += 1
if counter % 2 == 0:
continue
print(counter) # prints only odd numbers
What are the dangers of using a while loop in Python?
The dangers include:
- Infinite Loops: If the condition never becomes false, the loop will run indefinitely, potentially causing the program to hang.
- Resource Exhaustion: Continuously running loops can consume system resources, such as memory and CPU, leading to performance issues.
- Complexity and Readability: While loops can make code harder to read and maintain if not used properly, especially when nested deeply or when the loop conditions are complex.
while True:
print("This will run forever!")
How can we use a while loop to read user input in Python?
You can use a while loop to repeatedly prompt the user for input until a valid response is received:
user_input = ""
while user_input.lower() != "exit":
user_input = input("Enter something (type 'exit' to stop): ")
if user_input.lower() != "exit":
print(f"You entered: {user_input}")
print("Goodbye!")
