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DOUBLE PRECISION FUNCTION DDOT(N,DX,INCX,DY,INCY) |
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* .. Scalar Arguments .. |
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INTEGER INCX,INCY,N |
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* .. |
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* .. Array Arguments .. |
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DOUBLE PRECISION DX(*),DY(*) |
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* .. |
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* |
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* Purpose |
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* ======= |
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* |
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* forms the dot product of two vectors. |
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* uses unrolled loops for increments equal to one. |
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* jack dongarra, linpack, 3/11/78. |
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* modified 12/3/93, array(1) declarations changed to array(*) |
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* |
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* |
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* .. Local Scalars .. |
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DOUBLE PRECISION DTEMP |
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INTEGER I,IX,IY,M,MP1 |
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* .. |
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* .. Intrinsic Functions .. |
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INTRINSIC MOD |
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* .. |
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DDOT = 0.0d0 |
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DTEMP = 0.0d0 |
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IF (N.LE.0) RETURN |
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IF (INCX.EQ.1 .AND. INCY.EQ.1) GO TO 20 |
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* |
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* code for unequal increments or equal increments |
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* not equal to 1 |
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* |
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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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DTEMP = DTEMP + DX(IX)*DY(IY) |
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IX = IX + INCX |
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IY = IY + INCY |
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10 CONTINUE |
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DDOT = DTEMP |
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RETURN |
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* |
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* code for both increments equal to 1 |
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* |
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* |
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* clean-up loop |
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* |
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20 M = MOD(N,5) |
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IF (M.EQ.0) GO TO 40 |
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DO 30 I = 1,M |
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DTEMP = DTEMP + DX(I)*DY(I) |
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30 CONTINUE |
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IF (N.LT.5) GO TO 60 |
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40 MP1 = M + 1 |
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DO 50 I = MP1,N,5 |
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DTEMP = DTEMP + DX(I)*DY(I) + DX(I+1)*DY(I+1) + |
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+ DX(I+2)*DY(I+2) + DX(I+3)*DY(I+3) + DX(I+4)*DY(I+4) |
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50 CONTINUE |
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60 DDOT = DTEMP |
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RETURN |
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END |