user3798602 - 1 year ago 102
Java Question

# Getting 63252 for Project Euler 21. Why is this incorrect?

Project Euler question:

Let d(n) be defined as the sum of proper divisors of n (numbers less than n which divide evenly into n).
If d(a) = b and d(b) = a, where a ≠ b, then a and b are an amicable pair and each of a and b are called amicable numbers.

For example, the proper divisors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55 and 110; therefore d(220) = 284. The proper divisors of 284 are 1, 2, 4, 71 and 142; so d(284) = 220.

Evaluate the sum of all the amicable numbers under 10000.

My solution in Java:

``````public class problem21 {

private static int answer = 0;
public static void main(String[] args){

for(int a = 1; a<10000; a++){
int b = calculateSumOfDivisorsOf(a);
if(calculateSumOfDivisorsOf(b) == a && b!=a){
//amicable numbers
//a is always amicable
if(b<10000){
//b is an amicable number under 10000
}
}
}
}
public static int calculateSumOfDivisorsOf(double num){
String divisors = "1";
int sum = 0;
for(int i = 2; i< Math.sqrt(num); i++){
if(num%i == 0){
divisors+= " " + i;
if(num/i != i){
divisors += " " + num/i;
}
}
}

double[] divisorsArr = new double[divisors.split("\\s+").length];
for(int i = 0; i< divisors.split("\\s+").length; i++)
divisorsArr[i] = Double.parseDouble(divisors.split("\\s+")[i]);

for(int i = 0; i < divisorsArr.length; i++)
sum+= divisorsArr[i];

return sum;
}
``````

}

63252

What is wrong with my code?
The correct answer is 31626

When I try to debug your code I found that you adding a,b (amicable numbers) twice. Check your code and output.

``````    for (int a = 1; a < 10000; a++) {
int b = calculateSumOfDivisorsOf(a);
if (calculateSumOfDivisorsOf(b) == a && b != a) {
// amicable numbers
// a is always amicable
System.out.println(a); //NOTE I AM PRINTING 'a'
if (b < 10000) {
// b is an amicable number under 10000
System.out.println(b); // //NOTE I AM PRINTING 'b'
}
}
}
``````

Output is:

``````220
284
284
220
1184
1210
1210
1184
2620
2924
2924
2620
5020
5564
5564
5020
6232
6368
6368
6232
``````

Now you can see that your are adding a,b twice. You didn't check whether a,b already add or not. Check below code.

``````List<Integer> l = new ArrayList<Integer>(); //List to add a,b
for (int a = 1; a < 10000; a++) {
int b = calculateSumOfDivisorsOf(a);
if (calculateSumOfDivisorsOf(b) == a && b != a && !l.contains(a) && !l.contains(b)) { //Check whether list contain a,b
// amicable numbers
// a is always amicable