dasum.f

贝尔实验室多年开发的矩阵计算程序库的说明文件

      DOUBLE PRECISION FUNCTION DASUM(N,DX,INCX)
*     .. Scalar Arguments ..
      INTEGER INCX,N
*     ..
*     .. Array Arguments ..
      DOUBLE PRECISION DX(*)
*     ..
*
*  Purpose
*  =======
*
*     takes the sum of the absolute values.
*     jack dongarra, linpack, 3/11/78.
*     modified 3/93 to return if incx .le. 0.
*     modified 12/3/93, array(1) declarations changed to array(*)
*
*
*     .. Local Scalars ..
      DOUBLE PRECISION DTEMP
      INTEGER I,M,MP1,NINCX
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC DABS,MOD
*     ..
      DASUM = 0.0d0
      DTEMP = 0.0d0
      IF (N.LE.0 .OR. INCX.LE.0) RETURN
      IF (INCX.EQ.1) GO TO 20
*
*        code for increment not equal to 1
*
      NINCX = N*INCX
      DO 10 I = 1,NINCX,INCX
          DTEMP = DTEMP + DABS(DX(I))
   10 CONTINUE
      DASUM = DTEMP
      RETURN
*
*        code for increment equal to 1
*
*
*        clean-up loop
*
   20 M = MOD(N,6)
      IF (M.EQ.0) GO TO 40
      DO 30 I = 1,M
          DTEMP = DTEMP + DABS(DX(I))
   30 CONTINUE
      IF (N.LT.6) GO TO 60
   40 MP1 = M + 1
      DO 50 I = MP1,N,6
          DTEMP = DTEMP + DABS(DX(I)) + DABS(DX(I+1)) + DABS(DX(I+2)) +
     +            DABS(DX(I+3)) + DABS(DX(I+4)) + DABS(DX(I+5))
   50 CONTINUE
   60 DASUM = DTEMP
      RETURN
      END