c++ multiple recursive functions

Question

How do I use a function before it gets declared?

I have 3 functions, each one of them has to interact with each other, but I can't find a way to make the earlier declared functions to interact with the later declared functions. Here's the code:

int f(int a){
    if (a==0){return 1;}
    if (a==1){return 2;}
    if (a==2){return 7;}
    return 2*f(a-1)+3*f(a-2)+3*f(a-3)+6*g(a-2)+2*g(a-3)+2*h(a-2);
}

int g(int b){
    if (b==0){return 0;}
    if (b==1){return 1;}
    return f(b-1)+g(b-1)+h(b-1);
}

int h(int c){
    if (c==0){return 0;}
    if (c==1){return 1;}
    return f(c-2)+g(c-1)+g(c-2);
}

What you are looking for is called a "forward declaration". You will find more information in Google. — Sam Varshavchik, Feb 28 '17 at 14:46
Use a forward declaration, where you declare the function without filling out its body. — David Choweller, Feb 28 '17 at 14:46

score 6 · Accepted Answer · edited Feb 28 '17 at 15:11

A function only needs to know another function's name, its argument types and its return type to be able to call it. It doesn't need to be fully implemented at the point where you call it. Simply declare your functions first and implement them later.

// Declare before use
int g(int b);
int h(int c);

// Implement here
int f(int a){
    if (a==0){return 1;}
    if (a==1){return 2;}
    if (a==2){return 7;}
    return 2*f(a-1)+3*f(a-2)+3*f(a-3)+6*g(a-2)+2*g(a-3)+2*h(a-2);
}

int g(int b){
    if (b==0){return 0;}
    if (b==1){return 1;}
    return f(b-1)+g(b-1)+h(b-1);
}

int h(int c){
    if (c==0){return 0;}
    if (c==1){return 1;}
    return f(c-2)+g(c-1)+g(c-2);
}

c++ multiple recursive functions

1 Answers1