Name

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

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

xreal∗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.

incxinteger∗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.