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# C++/qsort()

Question
Hi,
How are you?
I have a qsort() question. Can you do qsort only on a part of an array (not the entire array).
For example if I have:
int myArray[50];
and I want to sort myArray between entries 10 and 20 only (and not touch the array between 0...9 and 21...49), would that be possible.
What would be the syntax of such a sort.
Thank you very much.
Tom

yes you can where you will send the lower index as 10 and higher index as 20. Here is a smaple code for q sort.

class QSortAlgorithm extends SortAlgorithm
{
/** This is a generic version of C.A.R Hoare's Quick Sort
* algorithm.  This will handle arrays that are already
* sorted, and arrays with duplicate keys.
*
* If you think of a one dimensional array as going from
* the lowest index on the left to the highest index on the right
* then the parameters to this function are lowest index or
* left and highest index or right.  The first time you call
* this function it will be with the parameters 0, a.length - 1.
*
* @param a       an integer array
* @param lo0     left boundary of array partition
* @param hi0     right boundary of array partition
*/
void QuickSort(int a[], int lo0, int hi0) throws Exception
{
int lo = lo0;
int hi = hi0;
int mid;

// pause for redraw
pause(lo, hi);
if ( hi0 > lo0)
{

/* Arbitrarily establishing partition element as the midpoint of
* the array.
*/
mid = a[ ( lo0 + hi0 ) / 2 ];

// loop through the array until indices cross
while( lo <= hi )
{
/* find the first element that is greater than or equal to
* the partition element starting from the left Index.
*/
while( ( lo < hi0 ) && ( a[lo] < mid ) )
++lo;

/* find an element that is smaller than or equal to
* the partition element starting from the right Index.
*/
while( ( hi > lo0 ) && ( a[hi] > mid ) )
--hi;

// if the indexes have not crossed, swap
if( lo <= hi )
{
swap(a, lo, hi);
// pause
pause();

++lo;
--hi;
}
}

/* If the right index has not reached the left side of array
* must now sort the left partition.
*/
if( lo0 < hi )
QuickSort( a, lo0, hi );

/* If the left index has not reached the right side of array
* must now sort the right partition.
*/
if( lo < hi0 )
QuickSort( a, lo, hi0 );

}
}

private void swap(int a[], int i, int j)
{
int T;
T = a[i];
a[i] = a[j];
a[j] = T;

}

public void sort(int a[]) throws Exception
{
QuickSort(a, 0, a.length - 1);
}
}

C++

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