SUBROUTINE ISORT(X,Y,N,KFLAG)
C***BEGIN PROLOGUE  ISORT
C***DATE WRITTEN   761118   (YYMMDD)
C***REVISION DATE  820801   (YYMMDD)
C***CATEGORY NO.  N6A2A
C***KEYWORDS  QUICKSORT,SINGLETON QUICKSORT,SORT,SORTING
C***AUTHOR  JONES, R. E., (SNLA)
C           KAHANER, D. K., (NBS)
C           WISNIEWSKI, J. A., (SNLA)
C***PURPOSE  ISORT sorts integer array X and optionally makes the same
C            interchanges in integer array Y.  The array X may be
C            sorted in increasing order or decreasing order.  A
C            slightly modified QUICKSORT algorithm is used.
C***DESCRIPTION
C
C     Written by Rondall E Jones
C     Modified by John A. Wisniewski to use the Singleton QUICKSORT
C     algorithm. Date 18 November 1976.
C
C     Further modified by David K. Kahaner
C     NATIONAL BUREAU OF STANDARDS
C     August, 1981
C
C     Abstract
C         ISORT sorts integer array X and optionally makes the same
C         interchanges in integer array Y.  The array X may be sorted in
C         INCREASING order or DECREASING order.  A slightly modified
C         QUICKSORT algorithm is used.
C
C     Reference
C         Singleton,R.C., Algorithm 347, An Efficient Algorithm For
C         Sorting With Minimal Storage, CACM,12(3),1969,185-7.
C
C     Description of Parameters
C         X - integer array of values to be sorted
C         Y - integer array to be (optionally) carried along
C         N - number of values in integer array X to be sorted
C     KFLAG - control parameter
C           = 2 means sort X in INCREASING order and carry Y along.
C           = 1 means sort X in INCREASING order (ignoring Y)
C           =-1 means sort X in DECREASING order (ignoring Y)
C           =-2 means sort X in DECREASING order and carry Y along.
C***REFERENCES  SINGLETON, R. C., ALGORITHM 347, AN EFFICIENT
C                 ALGORITHM FOR SORTING WITH MINIMAL STORAGE, CACM,
C                 VOL. 12, NO. 3, 1969, PP. 185-187.
C***ROUTINES CALLED  XERROR
C***END PROLOGUE  ISORT