NAG CL Interface
f16jqc (damax_​val)

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1 Purpose

f16jqc computes, with respect to absolute value, the largest component of a real vector, along with the index of that component.

2 Specification

#include <nag.h>
void  f16jqc (Integer n, const double x[], Integer incx, Integer *k, double *r, NagError *fail)
The function may be called by the names: f16jqc, nag_blast_damax_val or nag_damax_val.

3 Description

f16jqc computes, with respect to absolute value, the largest component, r, of an n-element real vector x, and determines the smallest index, k, such that
r=|xk|=maxj|xj|.  

4 References

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

5 Arguments

1: n Integer Input
On entry: n, the number of elements in x.
Constraint: n0.
2: x[dim] const double Input
Note: the dimension, dim, of the array x must be at least max(1,1+(n-1)×|incx|).
On entry: the n-element vector x.
If incx>0, xi must be stored in x[(i-1)×incx], for i=1,2,,n.
If incx<0, xi must be stored in x[(n-i)×|incx|], for i=1,2,,n.
Intermediate elements of x are not referenced. If n=0, x is not referenced and may be NULL.
3: incx Integer Input
On entry: the increment in the subscripts of x between successive elements of x.
Constraint: incx0.
4: k Integer * Output
On exit: k, the index, from the set {0,1,,n-1} , of the largest component of x with respect to absolute value. If n=0 on input then k is returned as −1.
5: r double * Output
On exit: r, the largest component of x with respect to absolute value. If n=0 on input then r is returned as 0.0.
6: fail NagError * Input/Output
The NAG error argument (see Section 7 in the Introduction to the NAG Library CL Interface).

6 Error Indicators and Warnings

NE_ALLOC_FAIL
Dynamic memory allocation failed.
See Section 3.1.2 in the Introduction to the NAG Library CL Interface for further information.
NE_BAD_PARAM
On entry, argument value had an illegal value.
NE_INT
On entry, incx=value.
Constraint: incx0.
On entry, n=value.
Constraint: n0.
NE_NO_LICENCE
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library CL Interface for further information.

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

Background information to multithreading can be found in the Multithreading documentation.
f16jqc is not threaded in any implementation.

9 Further Comments

None.

10 Example

This example computes the largest component with respect to absolute value and index of that component for the vector
x= (1,10,11,−2,9) T .  

10.1 Program Text

Program Text (f16jqce.c)

10.2 Program Data

Program Data (f16jqce.d)

10.3 Program Results

Program Results (f16jqce.r)