Thread: ackermann function

Is my implementation correct? I'm assuming it's not because I keep getting zero returned... Where am I going wrong?

Code:

//implementation of the recursive ackermann function
int ack(int m, int n)
{
    if (m == 0){
        return n+1;
    }
    else if(m > 0){
        ack(m-1, 1);
    }
    else if((m > 0) && (n > 0)){
        ack(m-1, ack(m, n-1));
    }
}//end ack

The third condition is never used. If m>0 the second will be evaluated whether n>0 or not. You also need to return ack(whatever) instead of just invoking it. As is the value is never used or returned, just computed and thrown out

Originally Posted by Kernel Sanders

The third condition is never used. If m>0 the second will be evaluated whether n>0 or not. You also need to return ack(whatever) instead of just invoking it. As is the value is never used or returned, just computed and thrown out

My mistake on the missing return bit. I'm not sure why I overlooked that... Anyway, I've updated the conditions of the second else if - they should now properly match the wikipedia entry.

Code:

//implementation of the recursive ackerman function
int ack(int m, int n)
{
    if (m == 0){
        return n+1;
    }
    else if((m > 0) && (n == 0)){
        return ack(m-1, 1);
    }
    else if((m > 0) && (n > 0)){
        return ack(m-1, ack(m, n-1));
    }
}//end ack

A quick question about the above, however. The book I'm working through suggested I implement the function to learn about recursion. The book also stated that this is a popular demonstration of recursion that is commonly used in CS courses all over the place.

anyway, my question is: What exactly is the point of the function? What purpose does it serve? The wikipedia page turned out to be a little too 'mathematically verbose'.