Python while Loop

Introduction

The while loop in Python is used to repeatedly execute a block of code as long as a given condition is true. This type of loop is useful for situations where the number of iterations is not known beforehand and depends on a condition being met.

Syntax

while condition:
    # block of code to be executed

Example

i = 1
while i < 6:
    print(i)
    i += 1

Output:

1
2
3
4
5

In this example, the loop continues to execute as long as i is less than 6. The variable i is incremented by 1 in each iteration.

How the while Loop Works

1. Initialization: Before the loop starts, you typically initialize a variable (e.g., i = 1).
2. Condition Check: The condition (i < 6) is evaluated.
   • If the condition is True, the block of code inside the loop is executed.
   • If the condition is False, the loop terminates, and the program continues with the next statement after the loop.
3. Update: Inside the loop, you update the variable (e.g., i += 1) so that the condition eventually becomes False.

Using else with while Loop

You can use an else block with a while loop. The else block is executed when the loop condition becomes False.

Example

i = 1
while i < 6:
    print(i)
    i += 1
else:
    print("i is no longer less than 6")

Output:

1
2
3
4
5
i is no longer less than 6

Infinite Loop

If the condition never becomes False, the loop will continue to execute indefinitely. This is known as an infinite loop. You need to ensure that the loop condition eventually becomes False.

Example of Infinite Loop

while True:
    print("This will run forever.")

To stop an infinite loop, you can use a break statement or interrupt the execution (e.g., pressing Ctrl+C).

Control Statements within while Loop

break Statement

The break statement is used to exit the loop prematurely, regardless of the loop condition.

Example

i = 1
while i < 6:
    print(i)
    if i == 3:
        break
    i += 1

Output:

1
2
3

continue Statement

The continue statement skips the current iteration and proceeds to the next iteration of the loop.

Example

i = 0
while i < 6:
    i += 1
    if i == 3:
        continue
    print(i)

Output:

1
2
4
5
6

pass Statement

The pass statement does nothing and can be used as a placeholder for future code.

Example

i = 1
while i < 6:
    if i == 3:
        pass
    print(i)
    i += 1

Output:

1
2
3
4
5

Nested while Loop

A while loop can be nested inside another while loop to handle more complex scenarios.

Example

i = 1
while i < 4:
    j = 1
    while j < 4:
        print(f"i = {i}, j = {j}")
        j += 1
    i += 1

Output:

i = 1, j = 1
i = 1, j = 2
i = 1, j = 3
i = 2, j = 1
i = 2, j = 2
i = 2, j = 3
i = 3, j = 1
i = 3, j = 2
i = 3, j = 3

Simple Programs Using while Loop

Program 1: Sum of First N Natural Numbers

This program calculates the sum of the first n natural numbers.

n = 10
sum = 0
i = 1

while i <= n:
    sum += i
    i += 1

print(f"The sum of the first {n} natural numbers is {sum}")

Output:

The sum of the first 10 natural numbers is 55

Program 2: Factorial of a Number

This program calculates the factorial of a given number.

num = 5
factorial = 1
i = 1

while i <= num:
    factorial *= i
    i += 1

print(f"The factorial of {num} is {factorial}")

Output:

The factorial of 5 is 120

Program 3: Reverse a Number

This program reverses the digits of a given number.

num = 12345
reversed_num = 0

while num > 0:
    digit = num % 10
    reversed_num = reversed_num * 10 + digit
    num //= 10

print(f"The reversed number is {reversed_num}")

Output:

The reversed number is 54321

Conclusion

The while loop in Python is used for executing a block of code repeatedly as long as a condition is true. By understanding how to use the while loop, along with control statements like break and continue, you can write more efficient and flexible code. Nested while loops and the use of an else block with while further enhance the capability of this looping construct. The provided examples demonstrate practical applications of the while loop in solving common programming tasks.