For loop inside a for loop

Question

This is a question on a test I have, I'm having trouble understanding the solution which is:

01
12
23

(Which is the output of the program below)

int main()
{
    int i, j;

    for(i=0;i<3;i++)
    {
        for(j=0;j<2;j++)
            printf("%d", i+j);

        printf("\n");
    }

    return (0);
}

Can anyone explain why this happens? I'm completely stumped.

Answer 1

@Barmar is right but maybe you don't know how to substitute yourself as a computer.

Let me show you how I would do it myself.

I get to the inner loop and see that j successively is 0, 1. In the other hand, i is a constant for that loop.

Within the inner loop, we simply display the sum i+j as a digit, two times, that is i+0 followed by i+1, then print new line.

Now, the outer loop increments i from 0, 1, 2 successively.

As for the inner loop, I apply what I've found, and the program will end up, successively with:

01 12 23

Hope it helps.