isamax, idamax, icamax, izamax 

Index of the element of a vector with maximum absolute value

FORMAT

  I{S,D,C,Z}AMAX (n, x, incx)

Function Value

  imax: integer*4
  The index of the element of the vector x that is the largest in absolute
  value of all elements of the vector.  If n<=0, imax returns the value 0.

Arguments

  n                   integer*4
                      On entry, the number of elements in the vector x.
                      On exit, n is unchanged.

  x                   real*4 | real*8 | complex*8 | complex*16
                      On entry, a one-dimensional array X of length at least
                      (1+(n-1)*|incx|), containing the elements of the vector
                      x.
                      On exit, x is unchanged.

  incx                integer*4
                      On entry, the increment for the array X.
                      If incx >= 0, vector x is stored forward in the array,
                      so that x(i) is stored in location X(1+(i-1)*incx).
                      If incx < 0, vector x is stored backward in the array,
                      so that x(i) is stored in location X(1+(n-i)*|incx|).
                      On exit, incx is unchanged.

Description

  These functions determine the first integer i among the elements of the
  vector x such that: |x(i)| = MAX{|x(j)|, j = 1,2, ...,n}
  You can use these functions to obtain the pivots in Gaussian elimination.

  For complex vectors, each element of the vector is a complex number. In
  these subprograms, the absolute value of a complex number is defined as the
  absolute value of the real part plus the absolute value of the imaginary
  part: |x(j)| = |a(j)| + |b(j)| = |real| + |imaginary|

  If incx < 0, the result depends on how the program is processed.  See the
  coding information in this document for a discussion of the possible
  results.  If incx = 0, the computation is a time-consuming way of setting
  imax = 1.

Example

  INTEGER*4 IMAX, N, INCX
  REAL*4 X(40)
  INCX = 2
  N = 20
  IMAX = ISAMAX(N,X,INCX)

  This FORTRAN code shows how to compute the index of a real vector element
  with maximum absolute value.