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I'm a novice at java language, and I had a problem that I have to solve, I'm fairly sure that I've done it right yet the tester still crashes.

a brief summary of what if has to do is " Inside an array a, an inversion is a pair of positions i and j inside the array that satisfy simultaneously both i < j and a[i] > a[j]. In combinatorics, the inversion count inside an array is a rough measure how "out of order" that array is. If an array is sorted in ascending order, it has zero inversions, whereas an n-element array sorted in reverse order has n(n-1)/2 inversions, the largest number possible. This method should count the inversions inside the given array arr, and return that count "

here's what I've done/tried 


import java.util.Arrays;

public class P2J1
{
public static int countInversions(int[] arr)
{ 
    int inversions = 0; 
    for (int i = 0; i <= arr.length; i++){
        for (int j = i+1; j < i; j++){
           if (arr[i] > arr[j]){
               inversions++; 
           } 
       } 
   } 
  return inversions; 
} 
}

/// here's the tester 

    @Test public void testCountInversions() {
        Random rng = new Random(SEED);
        CRC32 check = new CRC32();
        for(int i = 0; i < 1000; i++) {
            int[] a = new int[i];
            for(int j = 0; j < i; j++) {
                a[j] = rng.nextInt(100000);
            }
            check.update(P2J1.countInversions(a));
        }
        assertEquals(1579619806L, check.getValue());
    }
Heitor Purcino
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    You need `i < arr.length`. Also `(int j = i+1; j < i; j++)` will perform zero iterations, since `j < i` starts out false. – khelwood Aug 10 '19 at 18:53

1 Answers1

2

In Java, the array indexing is from 0 to arr.length - 1, you need to change i <= arr.length in your code to i < arr.length. Otherwise you would get ArrayIndexOutofBoundsException

Also @khelwood's suggestion is true. Change (int j = i+1; j < i; j++) to (int j = i+1; j < arr.length; j++)

SomeDude
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