Dead code warning in the i--

Question

I'm going to try to define function which return greatest common divisor. Now I'm just meeting some obstacle. Why do dead code warning appear in the i--? I can feel nothing so please let me know what is wrong.

public class Main {

    public static int function(int a, int b, int c) {
        int min;
        if (a > b) {
            if (b > c) {
                min = c;
            } else {
                min = b;
            }
        } else {
            if (a > c) {
                min = c;
            } else {
                min = a;
            }
        }
        for (int i = min; i > 0; i--) {
            if (a % i == 0 && b % i == 0 && c % i == 0) {
                return i;
            }
            return -1;
        }
        return 0;
    }

    public static void main(String[] args) {

        System.out.println("(400, 300, 750)의 최대 공약수 : " + function(400, 300, 750));

    }

}

Answer 1

The for loop has an if block where you are returning the greatest common divisor if found. If not you are returning -1. So the loop will never continue and "i--" will never be executed. That is why it's a dead code. Remove "return -1", it should work properly.

Answer 2

Stepping through the code in your debugger is often the quickest way to find these bugs. However, you could make the code much faster by only checking values which are a factor of the min value. This reduces the number of iterations dramatically.

public class Main {

    public static int function(int a, int b, int c) {
        int min;
        if (a > b) {
            if (b > c) {
                min = c;
            } else {
                min = b;
            }
        } else {
            if (a > c) {
                min = c;
            } else {
                min = a;
            }
        }
        for (int i = min; i > 0; i--) {
            if (a % i == 0 && b % i == 0 && c % i == 0) {
                System.err.println("Iterations " + (min + 1 - i));
                return i;
            }
        }
        return 0;
    }

    public static long gcd(long a, long b, long c) {
        long min = Math.min(a, Math.min(b, c));
        for (int j = 1, max = (int) Math.sqrt(min); j <= max; j++) {
            long i = min / j;
            if (a % i == 0 && b % i == 0 && c % i == 0) {
                System.err.println("Iterations: " + j);
                return i;
            }
        }
        return 1;
    }

    public static void main(String[] args) {
        System.out.println("(400, 300, 750)의 최대 공약수 : " + function(400, 300, 750));
        System.out.println("(400, 300, 750)의 최대 공약수 : " + gcd(400, 300, 750));
    }
}

prints

(400, 300, 750)의 최대 공약수 : 50
(400, 300, 750)의 최대 공약수 : 50
Iterations 251
Iterations: 6

In your approach, it has to consider all the factors from 300 to 50 (251 values). But considering only the factors of 300 ie 300/1, 300/2, 300/3, 300/4, 300/5, 300/6 (6 values) it is much faster.