5.5 Loop else

Python's loop else statement is a unique feature that enhances control flow in for and while loops. It executes when a loop completes without encountering a break, providing a clear way to handle loop completion scenarios.

Loop else simplifies code by eliminating the need for flag variables in search and validation tasks. It streamlines control flow, making code more readable and efficient, especially when combined with exception handling for robust error management.

Loop Else Statement

Loop else execution behavior

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• Loop else is an optional clause added after a for or while loop
• If the loop completes all iterations without encountering a break statement, the code block under the else clause executes
  • Indicates the loop ran to completion without interruption (loop completion)
• If a break statement is encountered within the loop, it immediately exits the loop and skips the else block
  • Signifies the loop was interrupted and did not complete all iterations (loop interruption)

Loop else with for and while

• Loop else can be used with for loops
  • Searching for a specific item in a list using a for loop
    • If the item is found, a break statement exits the loop early
    • If the loop completes without finding the item, the else block executes, indicating the item was not found
• Loop else can also be used with while loops
  • Validating user input using a while loop
    • If the user enters valid input, a break statement exits the loop
    • If the loop completes without a break, the else block executes, indicating the user entered valid input

Simplifying code with loop else

• Loop else can simplify code by eliminating the need for additional flag variables in certain scenarios
• Searching for a specific item in a list
  • Without loop else, you might use a flag variable to track whether the item was found
    1. Initialize a flag variable (

       found = False

       ) before the loop
    2. If the item is found, set the flag variable to

       True

       and break out of the loop
    3. After the loop, check the flag variable to determine if the item was found
  • With loop else, you can eliminate the flag variable
    1. If the item is found, simply break out of the loop
    2. If the loop completes without a break, the else block executes, indicating the item was not found
• Validating user input
  • Without loop else, you might use a flag variable to track whether the input is valid
    1. Initialize a flag variable (

       valid_input = False

       ) before the loop
    2. If the input is valid, set the flag variable to

       True

       and break out of the loop
    3. After the loop, check the flag variable to determine if the input was valid
  • With loop else, you can eliminate the flag variable
    1. If the input is valid, simply break out of the loop
    2. If the loop completes without a break, the else block executes, indicating the input was valid

Control Flow and Exception Handling

• Loop else affects the control flow of a program by providing an alternative execution path
• Loop termination can occur through normal completion or by using break statements
• Exception handling can be combined with loop else to manage errors that may occur during loop execution