I'm confused by recursion in my “convert decimal to binary string” code

Question

void print_binary(int number)
{
    if (number){
        print_binary(number/2);
        putc((number % 2) ? '1' : '0', stdout);
    }
}
int main(void) {
    print_binary(8);
}

The code above returns "1000". But when I reverse the two lines in print_binary() and write it like this:

void print_binary(int number)
{
    if (number){
        putc((number % 2) ? '1' : '0', stdout);
        print_binary(number/2);
    }
}

the string is "0001".

I can't understand why this happens. Any explanation is appreciated. Thanks

Answer 1

In first code sample, execution goes like this:

print_binary(8/2)
    print_binary(4/2)
        print_binary(2/2)
            print_binary(1/2)    // 1/2 = 0 => this is terminating condition of recursion
                                 // stack windup from here
            putc((1 % 2) ? '1' : '0', stdout);   --> output 1
        putc((2 % 2) ? '1' : '0', stdout);  --> output 0
    putc((4 % 2) ? '1' : '0', stdout);   --> output 0
putc((8 % 2) ? '1' : '0', stdout);    --> output 0

Hence, the output is 1000 .

In second code sample, execution goes like this:

putc((8 % 2) ? '1' : '0', stdout);    --> output 0
print_binary(8/2)

    putc((4 % 2) ? '1' : '0', stdout);   --> output 0
    print_binary(4/2)

        putc((2 % 2) ? '1' : '0', stdout);  --> output 0
        print_binary(2/2)

            putc((1 % 2) ? '1' : '0', stdout);   --> output 1
            print_binary(1/2)    // 1/2 = 0 => this is terminating condition of recursion
                                 // stack windup from here and nothing to do after last 
                                 // recursive call

Hence, the output is 0001 .

Answer 2

When you call a function, program puts the current address into stack memory where stack pointer points so it could come back when the execution ends and jumps to function's address. Stack is a LIFO (Last In First Out) structure so when you call a function, program finishes the instructions in the function than it turns back where it been. that's why putting order changes.

void print_binary(int number)
{
    if (number){
        putc((number % 2) ? '1' : '0', stdout);  // 1st instruction
        print_binary(number/2); // 2nd instruction
    }
}

The program works like this:

1. 8%2 is 0 so print 0. // 1st instruction of 1st instance
2. A new function appeared. jump to it and push current address to stack. //2st instruction of 1st instance
3. 4%2 is 0 so print 0. //1st instruction of 2nd instance
4. A new function appeared. jump to it and push current address to stack. //2nd instruction of 2nd instance
5. 2%2 is 0 so print 0 // 1st instruction of 3rd instance
6. A new function appeared. jump to it and push current address to stack. //2nd instruction of 3rd instance
7. 1%2 is 0 so print 1 //1st instruction of 4th instance
8. ...

So the output is 0001