user3798602 - 1 year ago 102

Java Question

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

answer+=a;

if(b<10000){

//b is an amicable number under 10000

answer+=b;

}

}

}

System.out.println(answer);

}

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;

}

}

My(incorrect) answer:

63252

What is wrong with my code?

The correct answer is 31626

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Answer Source

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
answer += a;
System.out.println(a); //NOTE I AM PRINTING 'a'
if (b < 10000) {
// b is an amicable number under 10000
answer += b;
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
answer += a;
l.add(a); //add a to list
if (b < 10000) {
// b is an amicable number under 10000
answer += b;
l.add(b); //add b to list
}
}
}
System.out.println(answer);
```

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