sort.c

用c++实现的决策树算法

/*************************************************************************/
/*									 */
/*	Sorting utilities						 */
/*	-----------------						 */
/*									 */
/*************************************************************************/


#include "defns.i"
#include "types.i"
#include "extern.i"



/*************************************************************************/
/*									 */
/*	Sort items from Fp to Lp on attribute a				 */
/*									 */
/*************************************************************************/


void    Quicksort(ItemNo Fp,ItemNo Lp,Attribute Att, void (*Exchange)(ItemNo,ItemNo))
{
    register ItemNo Lower, Middle;
    register float Thresh;
    register ItemNo i;

    if ( Fp < Lp )
    {
		Thresh = CVal(Item[Lp], Att);
		
		/*  Isolate all items with values <= threshold  */
		
		Middle = Fp;
		
		for ( i = Fp ; i < Lp ; i++ )
		{ 
			if ( CVal(Item[i], Att) <= Thresh )
			{ 
				if ( i != Middle ) (*Exchange)(Middle, i);
				Middle++; 
			} 
		} 
		
		/*  Extract all values equal to the threshold  */
		
		Lower = Middle - 1;
		
		for ( i = Lower ; i >= Fp ; i-- )
		{
			if ( CVal(Item[i], Att) == Thresh )
			{ 
				if ( i != Lower ) (*Exchange)(Lower, i);
				Lower--;
			} 
		} 
		
		/*  Sort the lower values  */
		
		Quicksort(Fp, Lower, Att, Exchange);
		
		/*  Position the middle element  */
		
		(*Exchange)(Middle, Lp);
		
		/*  Sort the higher values  */
		
		Quicksort(Middle+1, Lp, Att, Exchange);
    }
}