NAG Library Routine Document

f16jnf  (dmax_val)

 Contents

    1  Purpose
    7  Accuracy

1
Purpose

f16jnf (blas_dmax_val) computes the largest component of a real vector, along with the index of that component.

2
Specification

Fortran Interface
Subroutine f16jnf ( n, x, incx, k, r)
Integer, Intent (In):: n, incx
Integer, Intent (Out):: k
Real (Kind=nag_wp), Intent (In):: x(1+(n-1)*ABS(incx))
Real (Kind=nag_wp), Intent (Out):: r
C Header Interface
#include nagmk26.h
void  f16jnf_ ( const Integer *n, const double x[], const Integer *incx, Integer *k, double *r)
The routine may be called by its BLAST name blas_dmax_val.

3
Description

f16jnf (blas_dmax_val) computes the largest component, r, of an n-element real vector x, and determines the smallest index, k, such that
r=xk=maxjxj.  

4
References

Basic Linear Algebra Subprograms Technical (BLAST) Forum (2001) Basic Linear Algebra Subprograms Technical (BLAST) Forum Standard University of Tennessee, Knoxville, Tennessee http://www.netlib.org/blas/blast-forum/blas-report.pdf

5
Arguments

1:     n – IntegerInput
On entry: n, the number of elements in x.
2:     x1+n-1×incx – Real (Kind=nag_wp) arrayInput
On entry: the n-element vector x.
If incx>0, xi must be stored in xi-1×incx+1, for i=1,2,,n.
If incx<0, xi must be stored in xn-i×incx+1, for i=1,2,,n.
Intermediate elements of x are not referenced. If n=0, x is not referenced.
3:     incx – IntegerInput
On entry: the increment in the subscripts of x between successive elements of x.
Constraint: incx0.
4:     k – IntegerOutput
On exit: k, the index, from the set 1,2,,n , of the largest component of x. If n0 on input then k is returned as 0.
5:     r – Real (Kind=nag_wp)Output
On exit: r, the largest component of x. If n0 on input then r is returned as 0.0.

6
Error Indicators and Warnings

If incx=0, an error message is printed and program execution is terminated.

7
Accuracy

The BLAS standard requires accurate implementations which avoid unnecessary over/underflow (see Section 2.7 of Basic Linear Algebra Subprograms Technical (BLAST) Forum (2001)).

8
Parallelism and Performance

f16jnf (blas_dmax_val) is not threaded in any implementation.

9
Further Comments

None.

10
Example

This example computes the largest component and index of that component for the vector
x= 1,10,11,-2,9T .  

10.1
Program Text

Program Text (f16jnfe.f90)

10.2
Program Data

Program Data (f16jnfe.d)

10.3
Program Results

Program Results (f16jnfe.r)

© The Numerical Algorithms Group Ltd, Oxford, UK. 2017