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Why does the for loop repeat in this recursion? (lemm.ee)

submitted 10 months ago by [email protected] to c/[email protected]

30 comments fedilink hide all child comments

I used the debugger to examine this code but not understanding a couple areas.

Why does the for loop repeat after it exits to print a new line? If it exits the loop, shouldn't it be done with it?
Why is n incremented and not i as stated with i++?


int main(void)
{
    int height = get_int("Height: ");

    draw(height);
}

void draw(int n)
{
    if (n &lt;= 0)
    {
        return;
    }

    draw(n - 1);

    for (int i = 0; i &lt; n; i++)
    {
        printf("#");
    }
    printf("\n");
}

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[–] xmunk 6 points 10 months ago* (last edited 10 months ago) (7 children)

So you're drawing a triangle of # that starts skinny in the top left and grows to the right with a height and width of n?

As an example, Height 3 would be:

#
##
###

I'm not following your questions, but the code seems pretty reasonable... though the use of recursion feels pretty unnecessary when you could easily just use a nested for loop.

[–] [email protected] 2 points 10 months ago (5 children)

It's supposed to be a pyramid but not my code. It's an example of a recursive function from a CS50 lecture and I'm just trying to understand how the code works line by line.

[–] xmunk 2 points 10 months ago (4 children)

So is my example of Height 3 in line with your expectations?

[–] [email protected] 1 points 10 months ago (1 children)

Yep

[–] xmunk 2 points 10 months ago (1 children)

I wrote an equivalent version just using nested loops - reading it might help you understand why the recursion works the way it does.

[–] [email protected] 1 points 10 months ago (1 children)

Thanks. I did see that. I have a general understanding of how recursion works I think where the function calls itself again and again but I don't get why the code (for loop) below the draw(n - 1) is recursive.

[–] xmunk 5 points 10 months ago

The code below the draw(n - 1) isn't recursive... the call to draw(n - 1) is the recursion.

Sometimes, it can be helpful to invert recursion. Think about what draw(0) would be and write it down... then compute draw(1) using the value you previously computed for draw(0).

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