F16JQF (BLAS_DAMAX_VAL) (PDF version)
F16 Chapter Contents
F16 Chapter Introduction
NAG Library Manual

NAG Library Routine Document

F16JQF (BLAS_DAMAX_VAL)

Note:  before using this routine, please read the Users' Note for your implementation to check the interpretation of bold italicised terms and other implementation-dependent details.

+ Contents

    1  Purpose
    7  Accuracy

1  Purpose

F16JQF (BLAS_DAMAX_VAL) computes, with respect to absolute value, the largest component of a real vector, along with the index of that component.

2  Specification

SUBROUTINE F16JQF ( N, X, INCX, K, R)
INTEGER  N, INCX, K
REAL (KIND=nag_wp)  X(1+(N-1)*ABS(INCX)), R
The routine may be called by its BLAST name blas_damax_val.

3  Description

F16JQF (BLAS_DAMAX_VAL) 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=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  Parameters

1:     N – INTEGERInput
On entry: n, the number of elements in x.
2:     X(1+N-1×INCX) – REAL (KIND=nag_wp) arrayInput
On entry: the vector x. Element xi is stored in Xi-1×INCX+1, for i=1,2,,n.
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 1,1+INCX,,1+N-1×INCX , of the largest component of x with respect to absolute value. 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 with respect to absolute value. 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  Further Comments

None.

9  Example

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

9.1  Program Text

Program Text (f16jqfe.f90)

9.2  Program Data

Program Data (f16jqfe.d)

9.3  Program Results

Program Results (f16jqfe.r)


F16JQF (BLAS_DAMAX_VAL) (PDF version)
F16 Chapter Contents
F16 Chapter Introduction
NAG Library Manual

© The Numerical Algorithms Group Ltd, Oxford, UK. 2012