[Tutor] Two subsequent for loops in one function

[Peter Otten]

Rafael Knuth wrote:

> Hej there,
> 
> newbie question: I struggle to understand what exactly those two
> subsequent for loops in the program below do (Python 3.3.0):
> 
> for x in range(2, 10):
>     for y in range(2, x):
>         if x % y == 0:
>             print(x, "equals", y, "*", x//y)
>             break
>     else:
>         print(x, "is a prime number")
> 
> The result is:
> 
>>>>
> 2 is a prime number
> 3 is a prime number
> 4 equals 2 * 2
> 5 is a prime number
> 6 equals 2 * 3
> 7 is a prime number
> 8 equals 2 * 4
> 9 equals 3 * 3
> 
> I have a very basic understanding of for loops, so for example:
> 
> for everything in range(10):
>     print(everything)
> 
> ... the for loop grabs everything in that given range and prints it.
> But I feel confused by the double use of for loops as show above.
> 
> Can anyone explain?

Try to understand the inner for loop first. Once you understand what it does 
treat it as a black box like so:

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

for x in range(2, 10):
    unknown(x)