Question

I am new to C and I’m reading about recursion, but I am totally confused.

The main part where I’m getting confused is how things get unwind when the exit condition is reached. I would like to know how during recursion values got pushed and popped from stack.

Also can anyone please give me a diagramatic view of recursion?

Thanks…

Answer 1

Lets assume a function:

int MyFunc(int counter) {
    // check this functions counter value from the stack (most recent push)

    // if counter is 0, we've reached the terminating condition, return it
    if(counter == 0) {
        return counter;
    }
    else {
        // terminating condition not reached, push (counter-1) onto stack and recurse
        int valueToPrint = MyFunc(counter - 1);

        // print out the value returned by the recursive call 
        printf("%d", valueToPrint);

        // return the value that was supplied to use 
        // (usually done via a register I think)
        return counter;
    }
}

int main() {
    // Push 9 onto the stack, we don't care about the return value...
    MyFunc(9);
}

The output is: 012345678

The first time through MyFunc, count is 9. It fails the terminating check (it is not 0), so the recursive call is invoked, with (counter -1), 8.

This repeats, decrementing the value pushed onto the stack each time until counter == 0. At this point, the terminating clause fires and the function simply returns the value of counter (0), usually in a register.

The next call up the stack, uses the returned value to print (0), then returns the value that was supplied into it when it was called (1). This repeats:

The next call up the stack, uses the returned value to print (1), then returns the value that was supplied into it when it was called (2). etc, till you get to the top of the stack.

So, if MyFunc was invoked with 3, you’d get the equivalent of (ignoring return addresses etc from the stack):

Call MyFunc(3) Stack: [3]
Call MyFunc(2) Stack: [2,3]
Call MyFunc(1) Stack: [1,2,3]
Call MyFunc(0) Stack: [0,1,2,3]
Termination fires (top of stack == 0), return top of stack(0).
// Flow returns to:
MyFunc(1) Stack: [1,2,3]
Print returned value (0)
return current top of stack (1)

// Flow returns to:
MyFunc(2) Stack: [2,3]
Print returned value (1)
return current top of stack (2)

// Flow returns to:
MyFunc(3) Stack: [3]
Print returned value (2)
return current top of stack (3)

// and you're done...