SUBROUTINE DSWAP(N,DX,INCX,DY,INCY)
C***BEGIN PROLOGUE  DSWAP
C***DATE WRITTEN   791001   (YYMMDD)
C***REVISION DATE  820801   (YYMMDD)
C***CATEGORY NO.  D1A5
C***KEYWORDS  BLAS,DOUBLE PRECISION,INTERCHANGE,LINEAR ALGEBRA,VECTOR
C***AUTHOR  LAWSON, C. L., (JPL)
C           HANSON, R. J., (SNLA)
C           KINCAID, D. R., (U. OF TEXAS)
C           KROGH, F. T., (JPL)
C***PURPOSE  Interchange d.p. vectors
C***DESCRIPTION
C
C                B L A S  Subprogram
C    Description of Parameters
C
C     --Input--
C        N  number of elements in input vector(s)
C       DX  double precision vector with N elements
C     INCX  storage spacing between elements of DX
C       DY  double precision vector with N elements
C     INCY  storage spacing between elements of DY
C
C     --Output--
C       DX  input vector DY (unchanged if N .LE. 0)
C       DY  input vector DX (unchanged if N .LE. 0)
C
C     Interchange double precision DX and double precision DY.
C     For I = 0 to N-1, interchange  DX(LX+I*INCX) and DY(LY+I*INCY),
C     where LX = 1 if INCX .GE. 0, else LX = (-INCX)*N, and LY is
C     defined in a similar way using INCY.
C***REFERENCES  LAWSON C.L., HANSON R.J., KINCAID D.R., KROGH F.T.,
C                 *BASIC LINEAR ALGEBRA SUBPROGRAMS FOR FORTRAN USAGE*,
C                 ALGORITHM NO. 539, TRANSACTIONS ON MATHEMATICAL
C                 SOFTWARE, VOLUME 5, NUMBER 3, SEPTEMBER 1979, 308-323
C***ROUTINES CALLED  (NONE)
C***END PROLOGUE  DSWAP
C
      DOUBLE PRECISION DX(1),DY(1),DTEMP1,DTEMP2,DTEMP3
C***FIRST EXECUTABLE STATEMENT  DSWAP
      IF(N.LE.0)RETURN
      IF(INCX.EQ.INCY) IF(INCX-1) 5,20,60
    5 CONTINUE
C
C       CODE FOR UNEQUAL OR NONPOSITIVE INCREMENTS.
C
      IX = 1
      IY = 1
      IF(INCX.LT.0)IX = (-N+1)*INCX + 1
      IF(INCY.LT.0)IY = (-N+1)*INCY + 1
      DO 10 I = 1,N
        DTEMP1 = DX(IX)
        DX(IX) = DY(IY)
        DY(IY) = DTEMP1
        IX = IX + INCX
        IY = IY + INCY
   10 CONTINUE
      RETURN
C
C       CODE FOR BOTH INCREMENTS EQUAL TO 1
C
C
C       CLEAN-UP LOOP SO REMAINING VECTOR LENGTH IS A MULTIPLE OF 3.
C
   20 M = MOD(N,3)
      IF( M .EQ. 0 ) GO TO 40
      DO 30 I = 1,M
        DTEMP1 = DX(I)
        DX(I) = DY(I)
        DY(I) = DTEMP1
   30 CONTINUE
      IF( N .LT. 3 ) RETURN
   40 MP1 = M + 1
      DO 50 I = MP1,N,3
        DTEMP1 = DX(I)
        DTEMP2 = DX(I+1)
        DTEMP3 = DX(I+2)
        DX(I) = DY(I)
        DX(I+1) = DY(I+1)
        DX(I+2) = DY(I+2)
        DY(I) = DTEMP1
        DY(I+1) = DTEMP2
        DY(I+2) = DTEMP3
   50 CONTINUE
      RETURN
   60 CONTINUE
C
C     CODE FOR EQUAL, POSITIVE, NONUNIT INCREMENTS.
C
      NS = N*INCX
        DO 70 I=1,NS,INCX
        DTEMP1 = DX(I)
        DX(I) = DY(I)
        DY(I) = DTEMP1
   70   CONTINUE
      RETURN
      END