Python Quick Guide - Step 2 Control Flow (4) - Loops (2): while Loops

This is a continuation of “Step 2 Control Flow.”

2.2. Loops

Last time, we learned about loops using the for statement.
This time, we’ll learn about another loop structure, the while statement.

2.2.2. while Loops

Basics of while Loops

The while statement is a loop structure that executes repeated processing as long as the specified condition is True.

• In a while statement, the loop process is executed repeatedly as long as the condition is True
• When the condition is evaluated as False, the loop ends and execution proceeds to the next process

flowchart TD
    START[Start] --> INIT["counter = 5"]
    INIT --> WHILE{"while counter > 0"}
    WHILE -->|False| END["Loop ended"]
    WHILE -->|True| LOOP_BODY["Loop process\n(... counter -= 1)"]
    LOOP_BODY --> WHILE

linkStyle 2 stroke:red,stroke-width:2px;
linkStyle 3 stroke:green,stroke-width:2px;

Loop Control Statements

Just like with for loops, you can use break and continue in while loops.

break: Breaking Out of a Loop

The break statement allows you to exit a loop immediately, regardless of the condition.

In the example above, when number becomes 5, the break statement executes and exits the loop. As a result, numbers 6 and above aren’t processed.

Infinite Loops

A loop whose condition is always True is called an infinite loop. Infinite loops are sometimes used in combination with break statements.

• while True always evaluates to True, so it will continue executing forever unless explicitly exited using a break statement
• Be careful of unintended infinite loops, such as forgetting to update a counter

continue: Skipping Loop Iterations

The continue statement allows you to skip the remaining process in the current iteration and proceed to the next condition check.

In this example, if number is even, the continue statement skips the remaining process (print(f"Processing: {number}")) and proceeds to the next iteration of the loop.

break and continue in Nested Loops

When loops are nested, break and continue statements, just like in for loops, only affect the innermost loop that contains them.

In the example above, the break in the inner loop only exits the inner loop, and the outer loop continues to execute.

while-else Statement

Like the for statement, you can also add an else clause to a while statement.
The else block is executed when the loop terminates normally (i.e., not interrupted by break).

In the example above, once an even number is found, the loop is immediately exited using the break statement, so the else block isn’t executed. The else block is only executed if the loop completes normally without finding an even number.

Choosing Between for and while

For choosing between for and while loops, choose the one that can be written more simply.

When a for loop is appropriate:

• Repetitive processing with a predetermined number of iterations
• Processing each element in an iterable (list, tuple, string, etc.)
• Repetition over a fixed range that can be expressed using the range() function

When a while loop is appropriate:

• When the termination condition changes dynamically
• When an infinite loop is needed, ending only under specific conditions
• When the number of iterations isn’t known beforehand

Summary of while Loops