Thread: My code works... but I don't understand why. Can someone explain?

Hi all. I'm new to C programming and I am having A LOT of trouble understanding nested for loops. Here is a program I wrote.

Code:

#include <stdio.h>

int main(void){
    int userNum = 0;
    int i, j;

    printf("Enter an integer: ");   scanf("%d", &userNum);
    printf("\n");

    for(i = 0; i <= userNum; ++i){
        for(j = 1; j <= i; ++j){
            printf(" ");
        }
        printf("%d", i);
        printf("\n");
    }
    return(0);
}

The desired output is:
0
_1
__2
___3...... and so on, up to the number that was entered. (the underscores should be spaces instead)

I understand why it works for the first couple of iterations. But starting on the third line, why does the inside loop iterate twice, producing 2 spaces? Shouldn't 'j' increment and become equal to 'i', therefore only iterating once and printing one space?

I'm just trying to get a better understanding of all this. Any kind of explanation would be greatly appreciated.

Please post your code in the respective code tags.

That happens because "++j" happens after everything else inside that loop. I don't know how to say it, so I'll show it:

The following pieces of code are equivalent.

Code:

int i;

for (i = 0; i < 10; i++) {
    // Code goes here
}

Code:

int i;

i = 0;
while (i < 10) {
    // Code goes here

    i++;
}

That the behavor of for-loops.

Code:

for ( initialization ; evaluation ; action )
{
    loop-code;
}

initialization will be executed at the start of the loop.
evaluation will be checked every time befor the loop-code runs.
action will be executed every time the loop-code is finish.

You can rebuild your for-loop with while():

Code:

…
    j = 1;           // initialization
    while (j <= i)   // evaluation
    {
        printf(" "); // loop-code (one or more lines)
        ++j;         // action (last line)
    }
…

Originally Posted by agamemn0n

After this, since 'i' and 'j' are now equal, why would the inner loop run twice, printing 2 spaces?

In every iteration of the outer loop, the inner loop will start new! This means, the initialization will be executed.
So, the inner loop starts again and initialize j with 1!