Question

I was trying to write recursion function,to find factorial of a number.

    int factorial(int input,int *answer)
    {
       if ( input ==0 )        
       {                       
        return 0;
       }

       *answer  = *answer * input;
       factorial(input -1, answer);
    }

What will you say about this function? Is it tail recursive?

Answer 1

When doing tail recursive functions (especially tail recursive functions) it is often helpful to have a helper function in addition to another function which has a more friendly interface. The friendly interface function really just sets up the less friendly function’s arguments.

static unsigned factorial_helper(unsigned input, unsigned acc) {
       if (intput == 0) {
           return acc;
       }
       return factorial_helper(input-1, acc * input);
}

unsigned factorial(int input) {
    if (input < 0) {
        do_something_bad();
    }
    return factorial_helper(input, 1);
}

By passing an accumulator value you avoid having to use pointers or do any computations upon returning from called functions, which makes the functions truely tail recursive.