Problem with Ackermann-Funktion

[whyunomad]

Hey guys, my program is stuck in the endless loop, and i can't seem to find a problem. It works for A(1,2) but for (4,3) it gives endless loop.

Here's the code:

#include <iostream>

using namespace std;

int A(int n, int m)
{
    if(n == 0)
    {
        return (m + 1);
    }
    else if(m ==0 && n>0)
    {
        return (A(n-1,1));
    }
    else if(n>0 && m>0)
    {
        return(A(n-1, A(n, m-1)));
    }
}

int main()
{
    int n, m = 0;
    cout << "Enter n and m : " << endl;
    cin >> n;
    cin >> m;

    cout << A(n,m);

    return 0;
}

Here is the task i need to do :

A(0,m)=m+1
A(n,0)=A(n-1,1) falls n>0
A(n,m)=A(n-1,A(n,m-1)) falls n>0 und m>0

[TarikNeaj]

One problem I see is this:

In your function. You have if statements. But what if non of the 3 statements ever come true? What if it skips the first, second and third one. What will it then return? It's an int function, it has to return an integer.

[whyunomad]

So is this better ?? I just added return 0 on the end of the function.

#include <iostream>

using namespace std;

int A(int n, int m)
{
    if(n == 0)
    {
        return m + 1;
    }
    if(m == 0 && n > 0)
    {
        return A(n - 1, 1);
    }
    if(n > 0 && m > 0)
    {
        return A(n - 1, A(n, m - 1));
    }
    return 0;
}

int main()
{
    int n, m = 0;
    cout << "Enter n and m: " << endl;
    cin >> n >> m;
    cout << A(n,m);
}

[TheIdeasMan]

Hi,

The compiler confirms what TarikNeaj has said:

In function 'int A(int, int)': 
19:1: warning: control reaches end of non-void function [-Wreturn-type]

So it's a good idea to have the compiler warning options at their highest. :+)

Good Luck !!

[whyunomad]

Hey,

I think i solved the problem. I just changed line number 27 to:

cout << A(m,n);

Now it is not giving an endless loop. I guess the second number must be higher than the first number, that's all. :)
Thanks TarikNeaj and TheIdeasMan for help. :)

[MikeyBoy]

Don't you think it would make more sense to fix your function, so that it can correctly handle a situation where the second number is lower that the first?

[whyunomad]

What do you mean MikeyBoy ? You mean that it should ask a user to repeat the input if input is wrong ?