while Loops

Repeat until a condition changes, and avoid infinite loops.

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In this lesson9 parts

01A question with no known count
02The while loop: condition, body and iteration
03Tracing the loop, one iteration at a time
04How many times does the body run?
05The infinite loop, and the rule that prevents it
06A sentinel: looping until an empty entry
07Input validation: asking again until the answer is usable
08The assignment expression: reading and testing in one line
09Recap and your challenge

Key terms

The words this lesson introduces, each in one line. The module’s glossary collects them all.

assignment expression: name := value: assigns a value and gives that value, inside an expression. while (price := input("Price: ")) != "": reads and tests in one condition. Also called the walrus operator.
infinite loop: A loop whose condition never becomes False, because nothing in the body changes what it reads.
iteration: One run of the loop body.
loop: Code that runs its body again and again: a while statement or a for statement, with its block.
loop body: The indented block that repeats.
sentinel: A value that marks the end of the input, and is not processed, such as an empty entry at a checkout.

Quiz 5 questions

Your first pick on each question is the one that counts, and a right one earns a coin. Getting one wrong here is how the lesson sticks.

Practice

Problems to solve in your own notebook. Each states the problem, not the steps: working out the steps is the exercise. Level A applies the lesson, B combines it with earlier ones, C stretches it.

The self-checking notebook for this lesson is Notebook 2 · Loops.

Common mistakes

What you will see when it goes wrong, why it happens, and the fix.

Where it’s used

Where this lesson’s ideas turn up in real work.