Lesson 08 · First steps in code

Loops: doing things again

Programs are good at doing the same thing many times without getting bored. A loop runs a block of lines again and again. Step through a short Python program: a for loop adds up 1 + 2 + 3 + 4, the highlight jumps back to the loop's first line after every pass, and the numbers in memory change each time. Then see a while loop stop when its condition turns False, and how break gets you out of a loop that would never end.

10 min10 steps5 questionsBeginner

Fig 8 · Interactive

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5 quick questions

Pick an answer to see why. Your score saves with your progress.

01Which numbers does for i in range(1, 4): give to i?

range(start, stop) counts from start and stops just before stop, so 4 itself is left out: 1, 2, 3.

02What does this print? total = 0 then for n in range(1, 4): total = total + n then print(total)

The loop adds 1, then 2, then 3 to total: 0 + 1 + 2 + 3 = 6. range(1, 4) never gives 4.

03A while loop checks its condition...

Python tests the condition each time it reaches the while line. True runs the body again; False ends the loop.

04Why does x = 5 then while x > 0: print(x) never stop?

It is an infinite loop. The body has to change something the condition checks, such as x = x - 1, or use break.

05What does break do inside a loop?

break leaves the loop it is in straight away, whatever the condition says. The lines after the loop then run.

Key terms

Glossary

Loop: A part of a program that runs the same lines again and again.
for loop: A loop that takes the items of a sequence, such as a list or range(1, 5), one at a time.
range(start, stop): Gives the whole numbers from start up to, but not including, stop: range(1, 4) gives 1, 2, 3.
Loop variable: The name after for (n in for n in ...). It takes the next item on every pass.
Loop body: The indented lines under the loop's header line. They run once per pass.
Pass (iteration): One run through the loop body.
while loop: A loop that runs its body again as long as its condition is True.
break: A statement that ends the loop it is in at once.

Fine print

Notes and sources

range(start, stop) counts up from start and stops just before stop, so range(1, 4) gives 1, 2, 3 and range(1, 5) gives 1, 2, 3, 4. range(5) on its own starts at 0: 0, 1, 2, 3, 4.
Python knows which lines are the loop body from the indentation: the lines under the header that are indented further. The header line ends with a colon.
After a for loop ends, the loop variable keeps its last value: here n is still 4.
Ctrl+C makes Python raise KeyboardInterrupt, which stops a program stuck in an infinite loop.
Python also has a built-in sum(), so sum(range(1, 5)) gives 10 without writing the loop yourself.

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