relibc_openlibm/slatec/idamax.f

*DECK IDAMAX
      INTEGER FUNCTION IDAMAX (N, DX, INCX)
C***BEGIN PROLOGUE  IDAMAX
C***PURPOSE  Find the smallest index of that component of a vector
C            having the maximum magnitude.
C***LIBRARY   SLATEC (BLAS)
C***CATEGORY  D1A2
C***TYPE      DOUBLE PRECISION (ISAMAX-S, IDAMAX-D, ICAMAX-C)
C***KEYWORDS  BLAS, LINEAR ALGEBRA, MAXIMUM COMPONENT, VECTOR
C***AUTHOR  Lawson, C. L., (JPL)
C           Hanson, R. J., (SNLA)
C           Kincaid, D. R., (U. of Texas)
C           Krogh, F. T., (JPL)
C***DESCRIPTION
C
C                B L A S  Subprogram
C    Description of Parameters
C
C     --Input--
C        N  number of elements in input vector(s)
C       DX  double precision vector with N elements
C     INCX  storage spacing between elements of DX
C
C     --Output--
C   IDAMAX  smallest index (zero if N .LE. 0)
C
C     Find smallest index of maximum magnitude of double precision DX.
C     IDAMAX = first I, I = 1 to N, to maximize ABS(DX(IX+(I-1)*INCX)),
C     where IX = 1 if INCX .GE. 0, else IX = 1+(1-N)*INCX.
C
C***REFERENCES  C. L. Lawson, R. J. Hanson, D. R. Kincaid and F. T.
C                 Krogh, Basic linear algebra subprograms for Fortran
C                 usage, Algorithm No. 539, Transactions on Mathematical
C                 Software 5, 3 (September 1979), pp. 308-323.
C***ROUTINES CALLED  (NONE)
C***REVISION HISTORY  (YYMMDD)
C   791001  DATE WRITTEN
C   890531  Changed all specific intrinsics to generic.  (WRB)
C   890531  REVISION DATE from Version 3.2
C   891214  Prologue converted to Version 4.0 format.  (BAB)
C   900821  Modified to correct problem with a negative increment.
C           (WRB)
C   920501  Reformatted the REFERENCES section.  (WRB)
C***END PROLOGUE  IDAMAX
      DOUBLE PRECISION DX(*), DMAX, XMAG
      INTEGER I, INCX, IX, N
C***FIRST EXECUTABLE STATEMENT  IDAMAX
      IDAMAX = 0
      IF (N .LE. 0) RETURN
      IDAMAX = 1
      IF (N .EQ. 1) RETURN
C
      IF (INCX .EQ. 1) GOTO 20
C
C     Code for increments not equal to 1.
C
      IX = 1
      IF (INCX .LT. 0) IX = (-N+1)*INCX + 1
      DMAX = ABS(DX(IX))
      IX = IX + INCX
      DO 10 I = 2,N
        XMAG = ABS(DX(IX))
        IF (XMAG .GT. DMAX) THEN
          IDAMAX = I
          DMAX = XMAG
        ENDIF
        IX = IX + INCX
   10 CONTINUE
      RETURN
C
C     Code for increments equal to 1.
C
   20 DMAX = ABS(DX(1))
      DO 30 I = 2,N
        XMAG = ABS(DX(I))
        IF (XMAG .GT. DMAX) THEN
          IDAMAX = I
          DMAX = XMAG
        ENDIF
   30 CONTINUE
      RETURN
      END