On Sun, Dec 15, 2013 at 05:54:10PM +0100, Rafael Knuth wrote:
> Hej,
>
> I stumbled upon this program here (Python 3.3.0) and I don't quite
> understand how the for loop plays with the return True statement:
>
> def is_prime(number):
> for element in range(2, number):
> if number % element == 0:
> return False
> return True
Let's run through that, with number = 5.
When we reach the for-loop, we get:
for element in range(2, 5)
which loops over [2, 3, 4] (the stop value of range doesn't get
used). So the first time around the loop, element = 2 and we test:
if 5 % 2 == 0
which is false. Since there is no "else" matching the "if", the if
statement is complete. There is nothing in the loop after the "if", so
we return to the beginning of the loop, and test:
if 5 % 3 == 0
which is also false. Again, there is no "else" clause matching the "if",
and nothing following the "if", so we return to the beginning of the
loop again. Here we execute:
if 5 % 4 == 0
which is again false, so we return to the beginning of the for-loop.
This time there are no more values to use, so we continue just past the
for-loop, which gives us:
return True
and we exit the function.
Now this time let's do it again with number = 6. This one will be much
shorter: we loop over the elements [2, 3, 4, 5], and the first time
around the loop we test:
6 % 2 == 0
which is true. Since it is true, we execute the body of the "if" clause,
which is
return False
which exits the loop and the function, and we're done.
> Now, I would expect the following in case I call the function with the
> variable 3:
>
> number = 3
> for element in range(2, 3):
> 3 % 2 != 0:
> Loop ends and program returns True.
Correct. But the loop ends because there are no more values to get:
range(2, 3) starts at 2 and stops *before* 3, which means you only get
one value: [2]. So after handling 2, there are no more values to handle.
> Let's do the same with the variable 9:
>
> number = 9
> for element in range(2,9):
> 3 % 2 != 0:
> My assumption is that the program should end the loop after the first
> iteration again and it then should return True.
No. If it did that, it wouldn't be a *loop* at all, would it? The whole
reason loops (for and while) exist is to run the code repeatedly. If
they only ran once, no matter what, they would be useless.
Unless you exit a loop early (with a return, a break, or by raising an
exception) the loop will jump back to the beginning until such time as
the loop is completed. Then it will jump to the code following the loop.
So, here's a simple example:
for element in range(5):
print(element)
With your assumption, you might think it will print 0, then stop. But
it doesn't. It prints 0, then 1, then 2, then 3, then 4. Each time
through the loop it jumps back to the beginning, gets the next value
from the range, and only when there are no more values to get does the
for-loop finish.
--
Steven
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