Why doesn't my program that computes perfect numbers print anything?

Question

I got an assignment to create a program which displays the perfect integers between one and 100. Here is the actual assignment:

Create a PerfectIntegers application that displays all perfect integers up to 100. A perfect integer is a number which is equal to the sum of all its factors except itself. For example, 6 is a perfect number because 1 + 2 + 3 = 6. The application should include a boolean method isPerfect().

I tried and came up with this:

import java.util.ArrayList;
public class PerfectIntegers {
public static boolean isPerfect(int a){
    ArrayList<Integer> factors = new ArrayList<Integer>();
    int sum=0;
    boolean is;
    for (int i=1; i<=100; i++){
        double r=a/i;
        if (r%1==0){
            factors.add(i);
        }
    }for (int i=0;i<factors.size();i++){
        sum+=factors.get(i);
    }if (sum==a){
        is=true;
    }else{
        is=false;
    }return is;
}
public static void getInts(){
    for (int i=2; i<=100; i++){
        boolean is=isPerfect(i);
        if (is!=false){
            System.out.print(i+" ");
        }
    }
}
public static void main(String[] args) {
    getInts();
}

}

Eclipse did not show any errors but when I try to run it, the program is terminated and I get nothing.

The problem is likely with double r, as it is not dividing properly 100% of the time.

Answer 1

Your factorization code is wrong. You can fix it like this:

for (int i = 1 ; i < a; i++) {
    if (a % i == 0) {
        factors.add(i);
    }
}

One reason your old code did not work is that you misunderstood the workings of the % operator. It computes the remainder of the division of the left-hand side by the right-hand side, so r % 1 == 0 will be true for all numbers, because 1 divides everything; r % 2 == 0 is a way to detect even numbers, and so on.

The other reason is that you went all the way to 100 in search of divisors. This necessarily includes a, which automatically puts the total above the number itself, because 1 is already on the list.

Once you get this working, you could simplify the code by dropping the list of factors. Since the sum of all factors is all that you need, you might as well compute it in the factorization loop, and drop the loop that follows it:

sum = 0;
for (int i = 1 ; i < a; i++) {
    if (a % i == 0) {
        sum += i;
    }
}