Java - Merge sort with Strings

Question

In this program, I am sorting Olympic medals using mergeSort.

Something seems a little off with my code since sometimes it will give me an java.lang.ArrayIndexOutOfBoundsException and sometimes, it won't.

---

For a little background explanation:

I have a method that randomly generates Olympic countries and their medals won in a scoreboard. Returns a String[] array of results in the form of:

CAN 1 1 1

USA 1 1 2

GBR 0 0 1

CHN 0 0 2

---

However the scoreboard needs to be organized y descending medals of gold, silver and bronze. So it would have to be like:

USA 1 1 2

CAN 1 1 1

CHN 0 0 2

GBR 0 0 1

Using bubblesort and quicksort to sort the board works fine, but mergesort does not. At times it will be fine, but more often of the time it gives me ArrayIndexOutOfBoundsException.

public static void main(String[] args) {    

  Olympic_Results score = new Olympic_Results();

  //print a return value of an array    

  String[] countries = score.OlympicResult(7); //input how many game results
  mergeSort(countries, 0, countries.length - 1);
  for (String value:countries)
  System.out.println(value);
}

public static void mergeSort(String array[], int lo, int n) {
  int low = lo;
  int high = n;
  if (low >= high) {
    return;
  }

  int middle = (low + high) / 2;
  mergeSort(array, low, middle);
  mergeSort(array, middle + 1, high);
  int end_low = middle;
  int start_high = middle + 1;
  while ((lo <= end_low) && (start_high <= high)) {
    if ((array[low].substring(4,8)).compareTo(array[high].substring(4,8)) > 0) {
      low++;
    } 

    else {
      String Temp = array[start_high];
      for (int k = start_high - 1; k >= low; k--) {
        array[k + 1] = array[k];
      }
      array[low] = Temp;
      low++;
      end_low++;
      start_high++;
    }
  }
}  

Any idea why this code isn't working properly? Thank you!

Answer 1

I think you go down too deep in dividing the array into parts:

if (low >= high) {
    return;
    }

Try to start fixing it with stopping here when length is 1.

if (high - low <=1) {
    return;
    }

BTW, if the length is 2 then you can compare the values in place and return them already sorted immediately.

UPD

You seem to add too many variables with similar names and lost yourself in them :)..

This does not look correct :

while (( lo <= end_low) && (start_high <= high)) {

Answer 2

Two major errors:

1. You keep two variables with similar names, but different purpose. As a result the while loop test the lo value, despite never changing it (the loop works with low variable).

2. The if inside while should compare the initial items of two sorted series to choose one of them. Your series start at low and start_high , so you should compare array[low] to array[start_high] , but you compare it to array[high] instead. If array[high] data happens to be the smallest in the subinterval being sorted, then the low value will get incremented past the array size (and while condition doesn't catch it, as it tests lo instead).