Introduction to Python While Loop

What Is Python While Loop?

A While loop is used to repeat a section of code an unknown number of times until a specific condition is met.

Shown below is the syntax of a Python while loop.

Syntax: WHILE EXPRESSION:

STATEMENT(S)

Flowchart:

Fig: Operation of Python while loop

Example:

Here's a simple example of a while loop in Python that counts from 1 to 5:

count = 1
while count <= 5:
    print(count)
    count += 1

This loop will repeatedly execute the code block inside it as long as the condition count <= 5 is true. It starts with count initialized to 1, and with each iteration, it prints the current value of count and then increments it by 1. The loop stops when count becomes greater than 5.

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Break Statement in While Loop

The Python break statement is used to exit from the loop immediately after a particular condition is met.

Example:

Here's an example of using a while loop with a break statement to find the first even number in a sequence:

# Initialize a variable to store the current number
num = 1

# Iterate through numbers until we find the first even number
while True:
    if num % 2 == 0:
        print("First even number found:", num)
        break  # Exit the loop if an even number is found
    num += 1

In this example:

• We start with num initialized to 1.
• The while True: statement creates an infinite loop.
• Inside the loop, we check if num is divisible by 2 without a remainder (i.e., if it's even). If it is, we print a message indicating that the first even number is found, and then we break out of the loop.
• If num is not even, we increment it by 1 and continue the loop until an even number is found.

Continue Statement in While Loop

The function of the continue statement is to skip the current iteration of a loop and continue with the next one.

Example:

Here's an example of using a while loop with a continue statement to skip printing odd numbers in a sequence:

# Initialize a variable to store the current number
num = 1

# Iterate through numbers and print only even numbers
while num <= 10:
    if num % 2 != 0:
        num += 1
        continue  # Skip the rest of the loop and move to the next iteration if num is odd
    print(num)
    num += 1

In this example:

• We start with num initialized to 1.
• The while num <= 10: statement sets the condition for the loop to execute until num reaches 10.
• Inside the loop, we check if num is odd by using the condition num % 2 != 0. If it is odd, we continue, skipping the rest of the loop body and moving to the next iteration.
• If num is even, we print its value and then increment num.
• This loop prints only the even numbers between 1 and 10, skipping the odd numbers.

While Loop With Else

Python allows an else clause at the end of a while loop. The else part is executed if the loop terminates naturally.

Example:

Here's an example of a while loop with an else block that prints a message if the loop completes without encountering a break statement:

# Initialize a variable
count = 0

# Iterate through numbers
while count < 5:
    print("Inside the loop:", count)
    count += 1
else:
    print("Loop completed without encountering a break statement.")

In this example:

• We start with count initialized to 0.
• The while count < 5: statement sets the condition for the loop to execute until count reaches 5.
• Inside the loop, it prints the current value of count and then increments it by 1.
• After the loop completes all iterations, the else block is executed, printing a message indicating that the loop completed without encountering a break statement.
• Since there are no break statements in this loop, the else block will always execute after the loop completes its iterations.

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Python While Loop Exercise

We would suggest attempting the following question on your own before you refer to the solutions.

Question: Write a program to add all the numbers less than 10 using while loop.

Solution:

# Initialize variables
total = 0
num = 1

# Iterate through numbers less than 10 and add them to the total
while num < 10:
    total += num
    num += 1

# Print the total sum
print("The sum of all numbers less than 10:", total)

Ending a While Loop in Python

In Python, you can end a while loop using the break statement. The break statement immediately terminates the innermost enclosing loop. Here's an example:

count = 0
while True:
    print(count)
    count += 1
    if count >= 5:
        break  # End the loop if count is greater than or equal to 5

In this example, the loop will continue indefinitely (while True: creates an infinite loop) until the count variable becomes greater than or equal to 5. Once count reaches 5 or more, the break statement is executed, terminating the loop.

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FAQs

1. What is a do-while loop in Python with an example?

Python doesn't have a built-in do-while loop like some other programming languages do. However, you can achieve similar behavior using a while loop with an initial condition and then checking the condition again at the end of the loop. Here's an example:

# Example of a do-while loop in Python
count = 0
while True:
 print(count)
 count += 1
 if count >= 5:
 break

In this example, the loop will execute at least once, and then the condition if count >= 5 acts as the exit condition, similar to the condition typically checked at the start of a do-while loop in other languages.

2. What is called a while loop?

A while loop is a control flow statement that allows code to be executed repeatedly based on a given condition. The loop continues to execute the block of code as long as the condition remains true. Once the condition becomes false, the loop terminates. Here's a general syntax:

while condition:
# Code block to execute

3. What is the difference between while and for loop?

• • while loop: A while loop repeats a block of code as long as a specified condition is true. It is useful when you don't know in advance how many times the loop will iterate.
  • for loop: A for loop iterates over a sequence (such as a list, tuple, string, etc.) or other iterable objects. It executes the block of code for each item in the sequence. It's typically used when you know the number of iterations or when you need to iterate over a collection of elements.

4. How to restart a while loop in Python?

You can restart a while loop by using the continue statement. When the continue statement is encountered within a loop, the current iteration of the loop is terminated, and the next iteration begins. Here's a simple example:

count = 0
while count < 5:
 count += 1
 if count == 3:
 continue # Restart the loop when count is 3
 print(count)

In this example, when count becomes 3, the continue statement is executed, skipping the rest of the loop's code for that iteration and restarting the loop with the next iteration.