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102 lines
3 KiB
Fortran
102 lines
3 KiB
Fortran
*DECK DSWAP
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SUBROUTINE DSWAP (N, DX, INCX, DY, INCY)
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C***BEGIN PROLOGUE DSWAP
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C***PURPOSE Interchange two vectors.
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C***LIBRARY SLATEC (BLAS)
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C***CATEGORY D1A5
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C***TYPE DOUBLE PRECISION (SSWAP-S, DSWAP-D, CSWAP-C, ISWAP-I)
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C***KEYWORDS BLAS, INTERCHANGE, LINEAR ALGEBRA, VECTOR
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C***AUTHOR Lawson, C. L., (JPL)
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C Hanson, R. J., (SNLA)
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C Kincaid, D. R., (U. of Texas)
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C Krogh, F. T., (JPL)
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C***DESCRIPTION
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C
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C B L A S Subprogram
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C Description of Parameters
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C
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C --Input--
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C N number of elements in input vector(s)
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C DX double precision vector with N elements
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C INCX storage spacing between elements of DX
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C DY double precision vector with N elements
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C INCY storage spacing between elements of DY
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C
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C --Output--
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C DX input vector DY (unchanged if N .LE. 0)
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C DY input vector DX (unchanged if N .LE. 0)
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C
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C Interchange double precision DX and double precision DY.
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C For I = 0 to N-1, interchange DX(LX+I*INCX) and DY(LY+I*INCY),
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C where LX = 1 if INCX .GE. 0, else LX = 1+(1-N)*INCX, and LY is
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C defined in a similar way using INCY.
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C
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C***REFERENCES C. L. Lawson, R. J. Hanson, D. R. Kincaid and F. T.
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C Krogh, Basic linear algebra subprograms for Fortran
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C usage, Algorithm No. 539, Transactions on Mathematical
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C Software 5, 3 (September 1979), pp. 308-323.
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C***ROUTINES CALLED (NONE)
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C***REVISION HISTORY (YYMMDD)
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C 791001 DATE WRITTEN
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C 890831 Modified array declarations. (WRB)
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C 890831 REVISION DATE from Version 3.2
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C 891214 Prologue converted to Version 4.0 format. (BAB)
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C 920310 Corrected definition of LX in DESCRIPTION. (WRB)
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C 920501 Reformatted the REFERENCES section. (WRB)
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C***END PROLOGUE DSWAP
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DOUBLE PRECISION DX(*), DY(*), DTEMP1, DTEMP2, DTEMP3
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C***FIRST EXECUTABLE STATEMENT DSWAP
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IF (N .LE. 0) RETURN
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IF (INCX .EQ. INCY) IF (INCX-1) 5,20,60
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C
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C Code for unequal or nonpositive increments.
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C
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5 IX = 1
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IY = 1
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IF (INCX .LT. 0) IX = (-N+1)*INCX + 1
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IF (INCY .LT. 0) IY = (-N+1)*INCY + 1
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DO 10 I = 1,N
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DTEMP1 = DX(IX)
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DX(IX) = DY(IY)
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DY(IY) = DTEMP1
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IX = IX + INCX
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IY = IY + INCY
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10 CONTINUE
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RETURN
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C
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C Code for both increments equal to 1.
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C
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C Clean-up loop so remaining vector length is a multiple of 3.
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C
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20 M = MOD(N,3)
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IF (M .EQ. 0) GO TO 40
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DO 30 I = 1,M
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DTEMP1 = DX(I)
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DX(I) = DY(I)
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DY(I) = DTEMP1
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30 CONTINUE
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IF (N .LT. 3) RETURN
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40 MP1 = M + 1
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DO 50 I = MP1,N,3
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DTEMP1 = DX(I)
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DTEMP2 = DX(I+1)
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DTEMP3 = DX(I+2)
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DX(I) = DY(I)
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DX(I+1) = DY(I+1)
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DX(I+2) = DY(I+2)
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DY(I) = DTEMP1
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DY(I+1) = DTEMP2
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DY(I+2) = DTEMP3
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50 CONTINUE
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RETURN
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C
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C Code for equal, positive, non-unit increments.
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C
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60 NS = N*INCX
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DO 70 I = 1,NS,INCX
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DTEMP1 = DX(I)
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DX(I) = DY(I)
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DY(I) = DTEMP1
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70 CONTINUE
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RETURN
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END
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