while Loops and Stopping Conditions

Unit ID: M04-U04 Estimated active time: 20-30 minutes

Repeat while a condition remains true

attempt = 1

while attempt <= 3:
    print(f"Attempt {attempt}")
    attempt = attempt + 1

The loop has three visible parts:

1. initial state: attempt = 1;
2. condition: attempt <= 3;
3. progress: attempt increases each time.

Without progress, the condition may stay true forever.

Prefer for when iterating over a known collection

Use for to process every item in a list. Use while when repetition depends on a changing condition and the number of repetitions is not naturally the collection length.

A safe countdown

remaining = 3

while remaining > 0:
    print(remaining)
    remaining = remaining - 1

print("Complete")

Trace remaining: 3, 2, 1, then 0. At 0, the condition is false and the loop ends.

Infinite-loop warning

This loop never changes remaining:

remaining = 3

while remaining > 0:
    print(remaining)

Interrupt the kernel if code continues unexpectedly. Then repair the stopping logic before rerunning.

Use a maximum as a safeguard

When retrying a simulated operation, keep an explicit limit:

attempt = 1
maximum_attempts = 3
success = False

while attempt <= maximum_attempts and not success:
    print(f"Checking attempt {attempt}")
    success = attempt == 2
    attempt = attempt + 1

This deterministic example succeeds on attempt 2. Real retries need carefully defined failure handling and should not repeat unsafe actions automatically.

Practice

Start with current_hours = 0. Add 2 during each loop until the value reaches 8. Print each new value. Then explain the initial state, condition, progress step, and final state.

Takeaway

Every while loop needs visible state, a condition, and reliable progress toward stopping. Prefer for when the collection already defines the repetitions. Next, we will control searches with break, continue, and loop else.