Stopping early: break and continue

What you will be able to do

After this lesson you can

• leave a loop early with break,
• skip a round with continue,
• write a while True loop that stops from the middle, and
• stop a loop with a special value, a sentinel.

Leaving a loop early

Sometimes you find what you are looking for in the middle of a loop. Then there is no need to go on. Imagine that you look for a lost key in ten drawers. When you find it in the third, you do not open the other seven.

• break leaves the loop at once. The program goes on with the line after the loop.
• continue skips the rest of this round and goes on with the next round.

Both are used with an if inside the loop, so that they happen only in a special case.

break

break works in both kinds of loops, for and while.

continue

A caution for while loops: if the line that changes the condition comes after the continue, the continue skips it, and the loop may never end. In a while loop, change the variable before the continue.

Put the continue number after the break number. It never gets its turn, because the loop has already ended.

while True

The condition True is always true. So while True: is an endless loop, unless a break stops it. It looks strange, but it is useful when the stopping test comes in the middle of the body, and not at the top.

Every while True loop must have a break inside it, in a way that will really happen. When you write one, ask yourself: "How does this loop end?"

A sentinel

A sentinel is a special value that means "stop". You ask for numbers until the user types 0, for example. The sentinel is not part of the data. It is only the sign of the end. You will meet this in the next lesson, with input().

break or condition?

Both can end a loop. When the reason to stop is simple, put it in the condition: while savings < 500. When the reason comes in the middle of the body, or there are several different reasons, use while True and break. Choose the one that is easier to read.

Common mistakes

• A break outside a loop. It is a SyntaxError.
• A continue that skips the line that moves the condition, in a while loop. The loop never ends.
• Using break when you meant continue. One leaves the loop, the other only skips one round.
• A while True with no break that can really happen.
• Forgetting that break leaves only the loop that it is in, not an outer one.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the lab Find the first and use break to stop at the right number.
2. Build. Open the problem The first multiple and the problem Skip the even numbers.
3. Stretch. Open the problem Add until it passes 50 and write it with while True and break.

Remember these

• break leaves the loop now. continue skips to the next round.
• Use them with an if.
• while True needs a break that can happen.
• In a while loop, change the variable before a continue.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What does break do?

2. What does the loop for n in range(1, 10): if n == 5: continue; print(n) show?

3. How can a while True loop end?