Question

I have been trying out some basic exercises involving loops.

Can someone tell me why the following snippets have different outputs?

---

While Loop

while (i<3)
{
    while(j<3)
    {
        printf("(%d %d) ",i,j);
        j++;
    }
    i++;
}

Output

(0 0) (0 1) (0 2)

For Loop

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

Output

(0 0) (0 1) (0 2) (1 0) (1 1) (1 2) (2 0) (2 1) (2 2) 

Aren’t they supposed to have the same output?

Answer 1

Can someone tell me why the following snippets have different outputs?

Yes, they are not equivalent. To make the two snippets of code equivalent, you need to initialize i = 0 and especially j = 0 inside the while loop like so:

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

Remember that

for(init-statement condition; expression) {
    statement
}

is translated to

init-statement
while(condition) {
    statement
    expression
}

In particular,

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

is translated to

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

As such, you are missing the very key j = 0 initialization before entering the inner while loop as well as the i = 0 initialization before entering the outer while loop.

So to wrap it all up, the translation of

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

is (first pass)

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

and finally

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