Loops

Learn about loops in Python so that you can do things many times.

Look at each question and see if you can answer it, then check with the answer we’ve given. Do try these out in the The Python REPL.

Remember: understanding is the most imporant thing, don’t move on until you really understand the question and answer.

What does this do?

while True:
  'I run forever!'
  • It sets up a loop, that runs forever.
  • The body of the loop is the string ‘I run forever!’
  • The body is indented by some spaces so that Python knows what is inside the loop.
  • Indentation is really imporant in Python to mark out different blocks of code.
  • The code produces the string ‘I run forever!’ forever!

That’s a lot of new things to take in! Is this code properly indented?

while True:
'banana'
  • No, ‘banana’ is not indented, so there is nothing inside the while loop.

Is this code properly indented?

while True:
                         'lemon'
  • Yes, but it not very tidy. We usually use two spaces to indent.

Well what about these two lines inside the loop, are they properly indented?

while True:
  'apple'
    'orange'
  • No, ‘apple’ and ‘orange’ must be indented by the same amount, they should line up.

Do we ever see the string ‘all done’?

while True:
  'I run forever!'
'all done'
  • No. That string is outside the loop, it will run once the loop has finished, but the loop never finishes.

Does this loop finish?

v = 5
while v > 0:
  v*v
  v = v - 1
  • I’m not sure, what does it do?

It creates a variable v and it starts at 5. While v is greater than 0 we produce its square and then reduce it by 1.

  • So it does finish. And it produces: 25 16 9 4 1.

Does this produce the same thing as the code above?

for v in range(5):
  v*v
  • No, but it is similar. It sets v to each number from 0 to 4 (one less than the number we give it) then produces the square.

So how do we change it to produce exactly the same sequence of squares from 25 to 1, counting down?

for v in range(5, 0, -1):
  v*v
  • OK, so range now produces numbers from 5 down to 0, stepping -1 at a time, so going down by 1 each time.

Let’s try and produce the even squares of numbers 1 to 20. What does the loop look like?

  • This loop produces all of the squares for the numbers 1 to 20, the evens and the odds:
for v in range(1, 21):
  v*v

What is missing?

  • We need to test whether v*v is even.

Read on to the next section to learn how to do this: Conditionals.