SLATEC Routines --- PCHNGS ---

*DECK PCHNGS
      SUBROUTINE PCHNGS (II, XVAL, IPLACE, SX, IX, IRCX)
C***BEGIN PROLOGUE  PCHNGS
C***SUBSIDIARY
C***PURPOSE  Subsidiary to SPLP
C***LIBRARY   SLATEC
C***TYPE      SINGLE PRECISION (PCHNGS-S, DPCHNG-D)
C***AUTHOR  Hanson, R. J., (SNLA)
C           Wisniewski, J. A., (SNLA)
C***DESCRIPTION
C
C     PCHNGS LIMITS THE TYPE OF STORAGE TO A SEQUENTIAL SCHEME.
C     SPARSE MATRIX ELEMENT ALTERATION SUBROUTINE.
C
C     SUBROUTINE PCHNGS() CHANGES ELEMENT II IN VECTOR +/- IRCX TO THE
C     VALUE XVAL.
C
C            II THE ABSOLUTE VALUE OF THIS INTEGER IS THE SUBSCRIPT FOR
C               THE ELEMENT TO BE CHANGED.
C          XVAL NEW VALUE OF THE MATRIX ELEMENT BEING CHANGED.
C     IPLACE POINTER INFORMATION WHICH IS MAINTAINED BY THE PACKAGE.
C   SX(*),IX(*) THE WORK ARRAYS WHICH ARE USED TO STORE THE SPARSE
C               MATRIX. THESE ARRAYS ARE AUTOMATICALLY MAINTAINED BY THE
C               PACKAGE FOR THE USER.
C          IRCX POINTS TO THE VECTOR OF THE MATRIX BEING UPDATED.
C               A NEGATIVE VALUE OF IRCX INDICATES THAT ROW -IRCX IS
C               BEING UPDATED.  A POSITIVE VALUE OF IRCX INDICATES THAT
C               COLUMN IRCX IS BEING UPDATED.  A ZERO VALUE OF IRCX IS
C               AN ERROR.
C
C     SINCE DATA ITEMS ARE KEPT SORTED IN THE SEQUENTIAL DATA STRUCTURE,
C     CHANGING A MATRIX ELEMENT CAN REQUIRE THE MOVEMENT OF ALL THE DATA
C     ITEMS IN THE MATRIX. FOR THIS REASON, IT IS SUGGESTED THAT DATA
C     ITEMS BE ADDED A COL. AT A TIME, IN ASCENDING COL. SEQUENCE.
C     FURTHERMORE, SINCE DELETING ITEMS FROM THE DATA STRUCTURE MAY ALSO
C     REQUIRE MOVING LARGE AMOUNTS OF DATA, ZERO ELEMENTS ARE EXPLICITLY
C     STORED IN THE MATRIX.
C
C     THIS SUBROUTINE IS A MODIFICATION OF THE SUBROUTINE LCHNGS,
C     SANDIA LABS. REPT. SAND78-0785.
C     MODIFICATIONS BY K.L. HIEBERT AND R.J. HANSON
C     REVISED 811130-1000
C     REVISED YYMMDD-HHMM
C
C***SEE ALSO  SPLP
C***ROUTINES CALLED  IPLOC, PRWPGE, XERMSG
C***REVISION HISTORY  (YYMMDD)
C   811215  DATE WRITTEN
C   890531  Changed all specific intrinsics to generic.  (WRB)
C   891214  Prologue converted to Version 4.0 format.  (BAB)
C   900315  CALLs to XERROR changed to CALLs to XERMSG.  (THJ)
C   900328  Added TYPE section.  (WRB)
C   910403  Updated AUTHOR and DESCRIPTION sections.  (WRB)
C***END PROLOGUE  PCHNGS