Question

Hey i got the program to work but i cant get the merge sorter from big to little

import java.util.*;

class MergeSorter
{

   public static void sort(int[] a)
   {
if (a.length <= 1) { return; }
int[] first = new int[a.length / 2];
int[] second = new int[a.length - first.length];
for (int i = 0; i < first.length; i++)
{
   first[i] = a[i];
}
for (int i = 0; i < second.length; i++)
{
   second[i] = a[first.length + i];
}
sort(first);
sort(second);
merge(first, second, a);
   }

   private static void merge(int[] first, int[] second, int[] a)
   {
int iFirst = 0;
int iSecond = 0;
int j = 0;

while (iFirst < first.length && iSecond < second.length)
{
   if (first[iFirst] < second[iSecond])
   {
a[j] = first[iFirst];
iFirst++;
   }
   else
   {
a[j] = second[iSecond];
iSecond++;
   }
   j++;
}

while (iFirst < first.length)
{
   a[j] = first[iFirst];
   iFirst++; j++;
}
while (iSecond < second.length)
{
   a[j] = second[iSecond];
   iSecond++; j++;
}
   }
}

public class MergeSortDemo888888{
   public static void main(String[] args) {
int [] myAry = {3,2,6,7};
System.out.println("myAry is " + Arrays.toString(myAry));
MergeSorter.sort(myAry);
System.out.println("myAry is sorted descendingly using selection sort: "+Arrays.toString(myAry));
   }
}

Answer 1

The code does not sort in descending order because of the logical error in the if condition below:

while (iFirst < first.length && iSecond < second.length)
{
   if (first[iFirst] < second[iSecond]) //This if condition, sorts the elements from small to big. Hence, we have to change the '<' to '>' so that it would sort the elements from big to small.
   {
a[j] = first[iFirst];
iFirst++;
   }
   else
   {
a[j] = second[iSecond];
iSecond++;
   }

Note:  If you have trouble in copying the code, kindly check the code in the screenshots provided below

Correct Code:

import java.util.*;
class MergeSorter
{

public static void sort(int[] a)
{
if (a.length <= 1) { return; }
int[] first = new int[a.length / 2];
int[] second = new int[a.length - first.length];
for (int i = 0; i < first.length; i++)
{
first[i] = a[i];
}
for (int i = 0; i < second.length; i++)
{
second[i] = a[first.length + i];
}
sort(first);
sort(second);
merge(first, second, a);
}

private static void merge(int[] first, int[] second, int[] a)
{
int iFirst = 0;
int iSecond = 0;
int j = 0;

while (iFirst < first.length && iSecond < second.length)
{
if (first[iFirst] > second[iSecond]) //modified if condition with '>'
{
a[j] = first[iFirst];
iFirst++;
}
else
{
a[j] = second[iSecond];
iSecond++;
}
j++;
}

while (iFirst < first.length)
{
a[j] = first[iFirst];
iFirst++; j++;
}
while (iSecond < second.length)
{
a[j] = second[iSecond];
iSecond++; j++;
}
}
}

public class MergeSortDemo888888{
public static void main(String[] args) {
int [] myAry = {3,2,6,7};
System.out.println("myAry is " + Arrays.toString(myAry));
MergeSorter.sort(myAry);
System.out.println("myAry is sorted descendingly using selection sort: "+Arrays.toString(myAry));
}
}

Code Screenshot:

import java.util.*; class MergeSorter public static void sort(int[] a) if (a.length <= 1) { return; } int[] first = new int[a.length / 2]; int[] second = new int[a. length - first.length]; for (int i = 0; i < first.length; i++), first[i] = a[i]; for (int i = 0; i < second.length; i++), second[i] = a[first.length + i]; sort(first); sort(second); merge(first, second, a); } private static void merge(int[] first, int[] second, int[] a), int iFirst = 0; int isecond = 0; int j = 0; while (iFirst < first.length && isecond < second. length), if (first[iFirst] > second[isecond]), a[j] = first[iFirst]; iFirst++;

if (first[iFirst] > second[isecond]), { a[j] = first[iFirst]; iFirst++; else a[j] = second[isecond]; isecond++; } j++; while (iFirst < first.length), a[j] = first[iFirst]; iFirst++; j++; while (isecond < second.length) a[j] = second[isecond]; isecond++; j++; public class MergeSortDemo888888{ public static void main(String[] args) { int [] myAry = {3,2,6,7}; System.out.println("myAry is " + Arrays.toString(myAry)); MergeSorter.sort(myAry); System.out.println("myAry is sorted descendingly using selection sort: "+Arrays.toString(myAry));

Output:

if (first[iFirst] > second[isecond]), 39 - a[j] = first[iFirst]; iFirst++; 43 else 44 a[j] = second[isecond]; isecond++; 48 49 } j++; 50 51 while (iFirst < first.length), 52 -{ 53 a[j] = first[iFirst]; 54 iFirst++; j++; 55 ] 56 while (iSecond < second.length) - 57 - input myAry is [3, 2, 6, 7] myAry is sorted descendingly using selection sort: [7, 6, 3, 2]