The else Statement with while in Python

The else Statement with while

Overview

In Python, the else statement can be used with loops (for and while). When used with a while loop, the else block is executed after the loop terminates, but only if the loop terminates normally. If the loop is terminated by a break statement, the else block is skipped.

Syntax

Here is the basic syntax for using else with a while loop: 

while condition:
    # Loop body
    if break_condition:
        break
else:
    # Code to execute after the loop

 How It Works

• Normal Termination: If the while loop terminates due to the condition becoming False (i.e., it finishes all iterations without hitting a break), the else block is executed.
• Break Termination: If the while loop is terminated by a break statement, the else block is not executed.

Example 1: Normal Termination 

i = 0
while i < 3:
    print(f"i is {i}")
    i += 1
else:
    print("Loop terminated normally.")
# Output:
# i is 0
# i is 1
# i is 2
# Loop terminated normally.

 Explanation:

• The while loop increments i and prints its value until i is no longer less than 3.
• Once i reaches 3, the loop condition becomes False, and the else block is executed.

Example 2: Terminated by break 

i = 0
while i < 5:
    if i == 3:
        break
    print(f"i is {i}")
    i += 1
else:
    print("Loop terminated normally.")
# Output:
# i is 0
# i is 1
# i is 2

 Explanation:

• The while loop continues as long as i is less than 5.
• When i equals 3, the break statement is executed, which immediately exits the loop.
• Since the loop was terminated by break, the else block is not executed.

Common Use Cases

• Search or Validation Loops:
  • When searching for an item or validating a condition, you may use else to handle cases where the item or condition is not found.

Example: 

numbers = [1, 2, 4, 6, 8]
target = 3
i = 0
while i < len(numbers):
    if numbers[i] == target:
        print("Target found!")
        break
    i += 1
else:
    print("Target not found.")

Explanation:

• • The loop searches for the target in the numbers list.
  • If the loop completes without finding the target (i.e., without a break), the else block prints “Target not found.”
• Processing Until Completion:
  • Use else to perform final actions after a loop completes successfully without interruption.

Example: 

count = 0
while count < 10:
    print(f"Count is {count}")
    count += 1
else:
    print("All numbers processed.")

Explanation:

1. • The loop processes numbers from 0 to 9.
   • After completing all iterations, the else block is executed to indicate that all numbers have been processed.

Key Points

• else and break:
  • The else block will not execute if the loop is terminated by a break statement.
• Readability:
  • Using else with loops can make certain algorithms more readable by clearly separating the normal end of the loop from cases where the loop is interrupted.
• Use Cases:
  • It is particularly useful when dealing with searches or validations where the loop may end either by completion or by an early exit due to a condition.

Summary

The else statement with a while loop in Python provides a way to execute a block of code when the loop terminates normally. It is not executed if the loop is exited via a break statement. This can be particularly useful for situations like searches, validations, or any case where you need to handle normal completion and early termination differently.