```      SUBROUTINE SSWAP(N,SX,INCX,SY,INCY)
*     .. Scalar Arguments ..
INTEGER INCX,INCY,N
*     ..
*     .. Array Arguments ..
REAL SX(*),SY(*)
*     ..
*
*  Purpose
*  =======
*
*     interchanges two vectors.
*     uses unrolled loops for increments equal to 1.
*     jack dongarra, linpack, 3/11/78.
*     modified 12/3/93, array(1) declarations changed to array(*)
*
*
*     .. Local Scalars ..
REAL STEMP
INTEGER I,IX,IY,M,MP1
*     ..
*     .. Intrinsic Functions ..
INTRINSIC MOD
*     ..
IF (N.LE.0) RETURN
IF (INCX.EQ.1 .AND. INCY.EQ.1) GO TO 20
*
*       code for unequal increments or equal increments not equal
*         to 1
*
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
STEMP = SX(IX)
SX(IX) = SY(IY)
SY(IY) = STEMP
IX = IX + INCX
IY = IY + INCY
10 CONTINUE
RETURN
*
*       code for both increments equal to 1
*
*
*       clean-up loop
*
20 M = MOD(N,3)
IF (M.EQ.0) GO TO 40
DO 30 I = 1,M
STEMP = SX(I)
SX(I) = SY(I)
SY(I) = STEMP
30 CONTINUE
IF (N.LT.3) RETURN
40 MP1 = M + 1
DO 50 I = MP1,N,3
STEMP = SX(I)
SX(I) = SY(I)
SY(I) = STEMP
STEMP = SX(I+1)
SX(I+1) = SY(I+1)
SY(I+1) = STEMP
STEMP = SX(I+2)
SX(I+2) = SY(I+2)
SY(I+2) = STEMP
50 CONTINUE
RETURN
END

```