While else in Python

In Python, the while...else construct is a control structure that combines the while loop with an optional else block. The else block is executed when the loop’s condition becomes false and the loop completes all its iterations without being prematurely terminated by a break statement. This can be useful for performing actions after the loop has finished its normal execution.

Here’s the general syntax of a while loop with an else clause:

while condition:
    # Code to be executed while the condition is true
    # This code will be repeatedly executed until the condition becomes false
else:
    # Code to be executed when the condition becomes false
    # This code is executed after the loop completes if it finishes normally

Here’s a breakdown of how the while...else construct works:

1. The while loop checks the specified condition. If the condition is true, the loop’s body is executed, and the process continues.
2. If the condition becomes false, the loop exits, and the control flows to the else block (if present). This block is executed only if the loop completes its iterations normally (i.e., without encountering a break statement).
3. If the loop is terminated by a break statement, the else block is skipped.

Here’s an example to illustrate the usage of while with else:

count = 0
while count < 5:
    print(f"Count is {count}")
    count += 1
else:
    print("Loop is finished")

In this example, the while loop runs as long as count is less than 5. Each iteration prints the value of count and increments it. Once count reaches 5, the loop’s condition becomes false, and the else block is executed, printing “Loop is finished.”

Note: if you use a break statement to exit the loop prematurely, the else block will not be executed.

count = 0
while count < 5:
    if count == 3:
        break
    print(f"Count is {count}")
    count += 1
else:
    print("Loop is finished")

In this example, the loop will break when count becomes 3. As a result, the else block will not be executed, and the output will only show the counts up to 2.

While else in the Python example

Here are a few more examples of using the while loop with the else clause in Python:

Printing numbers from 1 to 10 using a while loop:

num = 1
while num <= 10:
    print(num)
    num += 1
else:
    print("Loop completed")

Finding the factorial of a number using a while loop:

num = int(input("Enter a number: "))
factorial = 1
while num > 0:
    factorial *= num
    num -= 1
else:
    print(f"The factorial is {factorial}")

Searching for an element in a list using a while loop:

numbers = [10, 20, 30, 40, 50]
search_value = int(input("Enter a number to search: "))
index = 0
while index < len(numbers):
    if numbers[index] == search_value:
        print(f"Found {search_value} at index {index}")
        break
    index += 1
else:
    print(f"{search_value} not found in the list")

Summing numbers entered by the user until a negative number is encountered:

total = 0
while True:
    num = int(input("Enter a number (negative to quit): "))
    if num < 0:
        break
    total += num
else:
    print(f"Sum of numbers: {total}")

Find whether a number is prime:

num = int(input("Enter a number: "))
is_prime = True

if num <= 1:
    is_prime = False
else:
    divisor = 2
    while divisor * divisor <= num:
        if num % divisor == 0:
            is_prime = False
            break
        divisor += 1
    else:
        print(f"{num} is prime")

if not is_prime:
    print(f"{num} is not prime")

Output:

Note: IDE: PyCharm 2021.3.3 (Community Edition)

Windows 10

Python 3.10.1

All Python Examples are in Python 3, so Maybe its different from python 2 or upgraded versions.