idamax.f

Go to the documentation of this file.

*> \brief \b IDAMAX
*
*  =========== DOCUMENTATION ===========
*
* Online html documentation available at 
*            http://www.netlib.org/lapack/explore-html/ 
*
*  Definition:
*  ===========
*
*       INTEGER FUNCTION IDAMAX(N,DX,INCX)
* 
*       .. Scalar Arguments ..
*       INTEGER INCX,N
*       ..
*       .. Array Arguments ..
*       DOUBLE PRECISION DX(*)
*       ..
*  
*
*> \par Purpose:
*  =============
*>
*> \verbatim
*>
*>    IDAMAX finds the index of the first element having maximum absolute value.
*> \endverbatim
*
*  Authors:
*  ========
*
*> \author Univ. of Tennessee 
*> \author Univ. of California Berkeley 
*> \author Univ. of Colorado Denver 
*> \author NAG Ltd. 
*
*> \date November 2015
*
*> \ingroup aux_blas
*
*> \par Further Details:
*  =====================
*>
*> \verbatim
*>
*>     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(*)
*> \endverbatim
*>
*  =====================================================================
      INTEGER FUNCTION idamax(N,DX,INCX)
*
*  -- Reference BLAS level1 routine (version 3.6.0) --
*  -- Reference BLAS is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*     November 2015
*
*     .. Scalar Arguments ..
      INTEGER INCX,N
*     ..
*     .. Array Arguments ..
      DOUBLE PRECISION DX(*)
*     ..
*
*  =====================================================================
*
*     .. Local Scalars ..
      DOUBLE PRECISION DMAX
      INTEGER I,IX
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC dabs
*     ..
      idamax = 0
      IF (n.LT.1 .OR. incx.LE.0) RETURN
      idamax = 1
      IF (n.EQ.1) RETURN
      IF (incx.EQ.1) THEN
*
*        code for increment equal to 1
*
         dmax = dabs(dx(1))
         DO i = 2,n
            IF (dabs(dx(i)).GT.dmax) THEN
               idamax = i
               dmax = dabs(dx(i))
            END IF
         END DO
      ELSE
*
*        code for increment not equal to 1
*
         ix = 1
         dmax = dabs(dx(1))
         ix = ix + incx
         DO i = 2,n
            IF (dabs(dx(ix)).GT.dmax) THEN
               idamax = i
               dmax = dabs(dx(ix))
            END IF
            ix = ix + incx
         END DO
      END IF
      RETURN
      END