*DECK SSWAP
      SUBROUTINE SSWAP (N, SX, INCX, SY, INCY)
C***BEGIN PROLOGUE  SSWAP
C***PURPOSE  Interchange two vectors.
C***LIBRARY   SLATEC (BLAS)
C***CATEGORY  D1A5
C***TYPE      SINGLE PRECISION (SSWAP-S, DSWAP-D, CSWAP-C, ISWAP-I)
C***KEYWORDS  BLAS, 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***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       SX  single precision vector with N elements
C     INCX  storage spacing between elements of SX
C       SY  single precision vector with N elements
C     INCY  storage spacing between elements of SY
C
C     --Output--
C       SX  input vector SY (unchanged if N .LE. 0)
C       SY  input vector SX (unchanged if N .LE. 0)
C
C     Interchange single precision SX and single precision SY.
C     For I = 0 to N-1, interchange  SX(LX+I*INCX) and SY(LY+I*INCY),
C     where LX = 1 if INCX .GE. 0, else LX = 1+(1-N)*INCX, and LY is
C     defined in a similar way using INCY.
C
C***REFERENCES  C. L. Lawson, R. J. Hanson, D. R. Kincaid and F. T.
C                 Krogh, Basic linear algebra subprograms for Fortran
C                 usage, Algorithm No. 539, Transactions on Mathematical
C                 Software 5, 3 (September 1979), pp. 308-323.
C***ROUTINES CALLED  (NONE)
C***REVISION HISTORY  (YYMMDD)
C   791001  DATE WRITTEN
C   890831  Modified array declarations.  (WRB)
C   890831  REVISION DATE from Version 3.2
C   891214  Prologue converted to Version 4.0 format.  (BAB)
C   920310  Corrected definition of LX in DESCRIPTION.  (WRB)
C   920501  Reformatted the REFERENCES section.  (WRB)
C***END PROLOGUE  SSWAP
      REAL SX(*), SY(*), STEMP1, STEMP2, STEMP3
C***FIRST EXECUTABLE STATEMENT  SSWAP
      IF (N .LE. 0) RETURN
      IF (INCX .EQ. INCY) IF (INCX-1) 5,20,60
C
C     Code for unequal or nonpositive increments.
C
    5 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
        STEMP1 = SX(IX)
        SX(IX) = SY(IY)
        SY(IY) = STEMP1
        IX = IX + INCX
        IY = IY + INCY
   10 CONTINUE
      RETURN
C
C     Code for both increments equal to 1.
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
        STEMP1 = SX(I)
        SX(I) = SY(I)
        SY(I) = STEMP1
   30 CONTINUE
      IF (N .LT. 3) RETURN
   40 MP1 = M + 1
      DO 50 I = MP1,N,3
        STEMP1 = SX(I)
        STEMP2 = SX(I+1)
        STEMP3 = SX(I+2)
        SX(I) = SY(I)
        SX(I+1) = SY(I+1)
        SX(I+2) = SY(I+2)
        SY(I) = STEMP1
        SY(I+1) = STEMP2
        SY(I+2) = STEMP3
   50 CONTINUE
      RETURN
C
C     Code for equal, positive, non-unit increments.
C
   60 NS = N*INCX
      DO 70 I = 1,NS,INCX
        STEMP1 = SX(I)
        SX(I) = SY(I)
        SY(I) = STEMP1
   70 CONTINUE
      RETURN
      END
