CODEWITHSUNDEEP
javaarrays
Laken
Laken

Reputation: 1

Sum of the difference of pairs of numbers in an array - Java

I'm working on an assignment for a beginning Java course and I am completely stuck on this problem.

Write a program that computes the sum of the differences of pairs of numbers in an array. For example if the array is [2, 3, 7, 8, 9, 12] the sum of the differences of pairs is (2-3) + (7-8) + (9-12) ** we are not allowed to use built in Java functions.

Here is what I have so far.. (I know it is terrible)

public static void main(String[] args) 
    {
        int[] A = {3, 4, 5, 6, 1, 2};
        int total = 0; 
        int sum = 0;
        for(int i = 0; i < A.length; i++)
        {
            for(int j = i+1; j < A.length; j++)
            sum = (A[i] - A[j]);

        }
        System.out.println(sum);
    }
}

Upvotes: 0

Views: 1180

Answers (2)

Andre Roda
Andre Roda

Reputation: 11

When you use that nested cycle you're doing this:

i = 0,
j = 1,
sum = 3 - 4;
// next cycle,
i = 0,
j = 2,
sum = 3 - 5;
// etc...,
i = 1,
j = 2,
sum = 4 - 5,
// etc..;

Which means for each value of A[i] you're making the difference of A[i] and all the values in the array for A[j + 1]. Also you're not updating the sum variable. When you do sum = A[i] - A[i + 1] because this operation only gives the variable sum a new value. What you want is sum+= value, which means sum = sum + newValue (newValue = A[i] - A[i +1]). This operation adds the new value to the old value stored in sum. So what you need to do is add the two values and jump 2 indexes (i+=2) so you don't do (for example) 3-4, 4-5, 5-6 etc. what you want is 3-4, 5-6, etc.
Try this:

public static void main(String[] args) 
{
    int[] A = {3, 4, 5, 6, 1, 2};
    int total = 0; 
    int sum = 0;
    for(int i = 0; i < A.length; i+=2)
    {
        sum +=(A[i] - A[i + 1]);
    }
    System.out.println(sum);
   }
}

Upvotes: 1

thetraveller
thetraveller

Reputation: 445

I am not sure what are you stuck on exactly but looks like you are not adding the sum.

public static void main(String[] args) 
{
    int[] A = {3, 4, 5, 6, 1, 2};
    int total = 0; 
    int sum = 0,i=0;
   while(i<A.length){
        sum+= (A[i] - A[i++]);
 }
    System.out.println(sum);
}

Upvotes: 0

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