Ema Ema - 18 days ago 5
C Question

How to convert recursion to tail recursion in this example?

I have this recursive function to add the cubes of n even numbers and I want't to turn it to a tail recursion.

int sum_even_cubes_rec(int n) {
if (n < 2)
return 0;
if ((n % 2) == 0) {
return (n*n*n + sum_even_cubes_rec(n - 1));
} else {
return (0 + sum_even_cubes_rec(n - 1));
}
}


This is what I wrote but it is wrong and I don't know how to fix it.
Can you please help me.

int sum_even_cubes_rec2(int n, int acc) {
if ((n % 2) == 0) {
return sum_even_cubes_rec2 (n-1, acc + n*n*n);
} return acc;
}

int sum_even_cubes_helperFunktion(int n) {
return sum_even_cubes_rec2(n, 0);
}

Answer

Your approach is correct. You have already added acc argument, so that's what you need to return for the base case.

The rest of your code is almost right - you need to adjust what you add to acc for the next invocation:

int sum_even_cubes_rec2(int n, int acc) {
    if (n < 2) {
        return acc;
    }
    int nextAcc = (n % 2) == 0 ? acc + n*n*n : acc;
    return sum_even_cubes_rec2 (n-1, nextAcc); 
}