Question

I get index out of bounds exception when i try to run this code .

we are using two different array s left and right for merging ..

and using recursive merge sort to divide the array into individaul elements and merging them…

Here’s the algorithm from CLRS i am using :

    Merge(A, p, q, r)
    n1 ← q - p + 1
    n2 ← r - q
    create arrays L[1..n1 + 1] and R[1..n2 + 1]
    for i ← 1 to n1
    do L[i] ← A[p + i - 1]
    for j ← 1 to n2
      do R[j] ← A[q + j]
    L[n1 + 1] ← ∞
    L[n2 + 1] ← ∞
    i ← 1
    j ← 1
    for k ≤ p to r
        do if L[i] ≤ R[j]
          then A[k] ← L[i]
             i ← i + 1
          else A[k] ← R[j]
              j ← j + 1

     MergeSort(A, p, r)
     if p < r
      then q ← ⌊(p + r)/2⌋
       MergeSort(A, p, q)
       MergeSort(A, q + 1, r)
       Merge(A, p, q, r)

here’s the code:

import java.util.Arrays;
import java.util.Scanner;

public class MergeSort {
   public static void main(String[] args) {
      Scanner input = new Scanner(System.in);
      System.out.println("Enter the size of array to be sorted");
      int size = input.nextInt();
      int[] A = new int[size];
      System.out.println("Enter the elements of array");
      for (int i = 0; i < A.length; i++) {
         A[i] = input.nextInt();
      }
      System.out.println("The UNSORTED array elements are" + Arrays.toString(A));
      int p = 0, r = size;
      mergeSort(A, p, r);
      System.out.println("The SORTED array elements are" + Arrays.toString(A));
   }

   public static void mergeSort(int[] A, int p, int r) {
      if (p < r) {
         int q = (p + r) / 2;
         mergeSort(A, p, q);
         mergeSort(A, q + 1, r);
         merge(A, p, q, r);
      }
   }

   public static void merge(int[] A, int p, int q, int r) {
      int n1 = q - p + 1;
      int n2 = r - q;
      int[] L = new int[n1 + 1];
      int[] R = new int[n2 + 1];
      L[n1] = Integer.MAX_VALUE;
      R[n2] = Integer.MAX_VALUE;
      for (int i = 0; i < n1; i++) {
         L[i] = A[p + i];
      }
      for (int j = 0; j < n2; j++) {
         R[j] = A[q + j + 1];
      }
      int x = 0, y = 0;
      for (int k = p; k < r; k++) {
         if (L[x] <= R[y]) {
            A[k] = L[x];
            x++;
         } else {
            A[k] = R[y];
            y++;
         }
      }
   }
}

Answer 1

After a second but longer look, your implementation of the algorithm seems to be right, i got confused regarding your input parameters because they did not fit your zero based implementation. So maybe you do not want to call it like this:

int p = 0, r = size;
mergeSort(A, p, r);

but rather like this:

int p = 0, r = size - 1;
mergeSort(A, p, r);