Nested Loops and Loop Control

A loop does not always need to decide whether to continue before it begins its next iteration. With break, the loop can begin, perform work, check a condition at a chosen point in its body, and exit immediately when that condition is met. The key behavior is simple: break exits the loop at once and skips any code that remains in the current loop body.

The important trace is the difference between a non-stop input and the stop input. For hello, the equality check is false, so break does not execute. The program prints hello and returns to the top of the loop. The same happens for world. For done, the equality check is true, so break executes. The loop skips the print(line) statement and continues with the statement after the loop.

while True creates an intentional infinite loop. The loop is designed to keep iterating until a break statement is reached. This pattern is useful when the program discovers its exit condition inside the loop rather than knowing that condition before the loop starts.

A traditional loop condition controls whether the next iteration begins. By contrast, break allows the loop to begin first and then check for an exit at a chosen point in the body. That check can occur after input is received or after other loop work has taken place. When the condition becomes true, the remaining statements in that iteration are skipped and control moves outside the loop.

This placement matters because break does not merely prevent a future iteration. It interrupts the current one. Any statements below break in the same loop body are skipped. This is why the done input is not printed by print(line) in the source example.

Nested loops place one loop inside another. The loop-control idea remains the same: break exits a loop immediately and skips the rest of that loop body. When reading nested loops, trace the execution one loop body at a time. Identify which loop body is currently running, locate the condition that can trigger break, and then follow control to the code after the loop that break exits.

The while True and break pattern lets you state the stop condition affirmatively: stop when this happens. The alternative is to state a negative condition: keep going while the stop event has not happened. Both approaches can perform the same task, but the affirmative form can be clearer because the reader sees the stopping event directly. This advantage is especially useful when the stop condition is complex or when there are multiple possible exit points.