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Python

Posted by james malloy
james malloy
7,959 Points

else statement on python for loops

Hi,

looking at some example code given in the python docs to demonstrate use of for loops with an else statement.

for n in range(2, 10):
  for x in range(2, n):
    if n % x == 0:
      print(n, "equals", x, "*", n/x)
      break
  else:
    print(n, "is a prime number)"

my question is why this returns "2 is a prime number" (which it is, of course) ? I can't see how n = 2 would be processed by the inner loop when n = 2 as the range would be (2, 1), so the inner loop would never run for n = 2. If the inner loop doesn't run, then the else statement which is nested to the inner loop couldn't run either.

I'd be grateful if someone could explain what i'm not getting.

TIA.

Jason Anello
Jason Anello
Courses Plus Student 94,610 Points

fixed code formatting

2 Answers

Jason Anello
Jason Anello
Courses Plus Student 94,610 Points

Hi James,

When n = 2 the range would be (2, 2) for the inner loop but you're correct that the loop wouldn't run. The else: clause would still execute though. It will execute as long as a break statement wasn't executed.

It looks like you got your example from the tutorials page here https://docs.python.org/3/tutorial/controlflow.html#break-and-continue-statements-and-else-clauses-on-loops

That section doesn't really completely clarify what happens if the loop doesn't run but it does say

...and a loop’s else clause runs when no break occurs.

If you follow the for loop link to the reference page it does a better job of describing when the else clause will execute.

From https://docs.python.org/3/reference/compound_stmts.html#for

When the items are exhausted (which is immediately when the sequence is empty or an iterator raises a StopIteration exception), the suite in the else clause, if present, is executed, and the loop terminates.

So what happens when n=2 is that you have an empty sequence and its immediately exhausted.

james malloy
james malloy
7,959 Points
james malloy
james malloy
7,959 Points

Hi Jason,

thanks for the reponse and the link was good.

Cheers.