4. break and continue Statements¶

Python includes ways to make your loops even more modifiable. There are times when you will want to change the typical flow of control of the loop body itself. For these instances, Python has break and continue statements that allow you to precisely tweak the flow of control in both your for and while loops.

For example, there are times when you may want to terminate your loop if a given condition — besides the controlling condition — is met. This is where the break statement is useful. Looking at our previous while loop code, let’s say that we want to terminate the loop not only if the value of num exceeds the value of n, but also if the value of num exceeds 100. We could amend our code as follows to allow for that condition. (Note that we’ll also refactor to include a main function.)

def sum_to(n): """ Return the sum of 1+2+3 ... n, up to 100""" sum = 0 num = 0 while num <= n: sum = sum + num num = num + 1 if num > 100: break return sum def main(): print(sum_to(4)) print(sum_to(100)) print(sum_to(500)) if __name__ == "__main__": main()

You can see from the print statements that the last two calls to sum_to returned the same result. The flow of control for the last function call was to execute the while loop until, inside the loop body, the if num > 100 condition became True. When that happened, the break statement executed and the flow of execution moved outside of the while loop and down to the return statement. Unlike the continue statement, which we will examine next, the break statement did not cause the flow of control to return again to the loop header to check the controlling condition of num <= n.

The flow of control for the continue statement is, like the break statement, to halt execution of the loop body when a given condition is met. But unlike the break statement, the flow of execution does not move outside the loop entirely and onto the next statement after it. Instead, the flow of execution “continues” by returning to the loop header and checking the controlling condition again.

Now let’s look at a modification we can make to our example that will use the continue statement. Let’s say that we only want to add to the sum numbers which are odd. We still want the range of numbers to be from 1 up to n, but we only want to add the odd numbers to our running total. Obviously, the break statement wouldn’t be a good choice for this task, since if we make the condition for the break be if num % 2 == 0, then the loop would be exited the first time we encountered an even number. Not at all what we want! Instead, we want to skip adding even numbers to our running total, but we still want to iterate through all the numbers in our range. We can use the continue statement to accomplish this:

def sum_to(n): """ Return the sum of odd numbers from 1 to n """ sum = 0 num = 0 while num <= n: if num % 2 == 0: num = num + 1 continue sum = sum + num num = num + 1 return sum def main(): print(sum_to(4)) print(sum_to(100)) if __name__ == "__main__": main()

Note that we have to increment num inside the if statement in order to avoid an infinite loop. If we only increment it outside of that statement, then the while loop would go on “continuing” forever!

Check your understanding

Which 2 lines of code could you add to the following for loop, before sum = sum + num, in order to add only even numbers in the range to the running total?

  def sum_to(n):
      sum = 0
      for num in range(1, n+1):
          sum = sum + num

      return sum

  print(sum_to(500))

::

  a. if num % 2 == 0:
         continue

  b. if num % 2 != 0:
         break

  c. if num % 2 != 0:
         continue

• a
• Since even numbers would result in a True value for this condition, the continue statement would execute and therefore you would not add the even number to the total; you'd only be adding odd numbers to the total.
• b
• This is the correct testing condition, but you don't want to use a break statement since that would result in not adding any number to the total since the first number that is tested is 1 and then we would break out of the loop entirely.
• c
• If a number is odd (not evenly divisible by 2), then you want to move back up to the loop controlling condition without adding that number to the running total.