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Message:

RE:Sort order

Posted by Kishori Sharan on August 08, 2000 at 2:14 PM

Hi
In merge( int, int, int ) method the following line of code does the actual sorting.
this.compareElementsAt(secondHalf, firstHalf) < 0

So it is the compareElementsAt () method's return value that dictates how to sort the array. If you change < 0 to > 0 that should work. But, But the actual comparision for array elements will go in the compareElementAt () method and the return value will decide how to sort. I have given an example with Integer array just to show how it will work at run time.
Thanx
Kishori

//////////// Example with your original code

abstract class MergeSort extends Object {
protected Object toSort[];
protected Object swapSpace[];

public void sort(Object array[]) {
if( array != null && array.length > 1 )
{
int maxLength;

maxLength = array.length;
swapSpace = new Object[maxLength];
toSort = array;
this.mergeSort(0, maxLength - 1);
swapSpace = null;
toSort = null;
}
}

public abstract int compareElementsAt(int beginLoc, int endLoc);

protected void mergeSort(int begin, int end) {
if(begin != end)
{
int mid;

mid = (begin + end) / 2;
this.mergeSort(begin, mid);
this.mergeSort(mid + 1, end);
this.merge(begin, mid, end);
}
}

protected void merge(int begin, int middle, int end) {
int firstHalf, secondHalf, count;

firstHalf = count = begin;
secondHalf = middle + 1;
while((firstHalf <= middle) && (secondHalf <= end))
{
if(this.compareElementsAt(secondHalf, firstHalf) < 0)
swapSpace[count++] = toSort[secondHalf++];
else
swapSpace[count++] = toSort[firstHalf++];
}
if(firstHalf <= middle)
{
while(firstHalf <= middle)
swapSpace[count++] = toSort[firstHalf++];
}
else
{
while(secondHalf <= end)
swapSpace[count++] = toSort[secondHalf++];
}
for(count = begin;count <= end;count++)
toSort[count] = swapSpace[count];
}
}

public class KSort extends MergeSort {
public int compareElementsAt(int beginLoc, int endLoc) {
Integer a1, a2;
a1 = ( Integer ) toSort[beginLoc] ;
a2 = ( Integer ) toSort[endLoc] ;
int v1, v2 ;
v1 = a1.intValue ( ) ;
v2 = a2.intValue ( ) ;

// When you change the return value here
// it will changet the sort criteria. Try
// commenting following lines and uncommenting the commented ones
if ( v1 > v2)
return 1 ;
else if ( v1 < v2 )
return -1 ;
else return 0;

/* if ( v1 > v2)
return -1 ;
else if ( v1 < v2 )
return 1 ;
else return 0; */

}


public static void main ( String[] args ) {
Integer[] nums = new Integer[] { new Integer(10), new Integer(2), new Integer(30) } ;
new KSort().sort ( nums ) ;
for ( int i = 0; i < nums.length ; i++ )
System.out.println ( nums[i].intValue() ) ;
}

}