NAG Library Routine Document

f16ecf (blas_daxpby) (daxpby)

1
Purpose

f16ecf (blas_daxpby) computes the sum of two scaled vectors, for real vectors and scalars.

2
Specification

Fortran Interface
Subroutine f16ecf ( n, alpha, x, incx, beta, y, incy)
Integer, Intent (In):: n, incx, incy
Real (Kind=nag_wp), Intent (In):: alpha, x(1+(n-1)*ABS(incx)), beta
Real (Kind=nag_wp), Intent (Inout):: y(1+(n-1)*ABS(incy))
C Header Interface
#include <nagmk26.h>
void  f16ecf_ (const Integer *n, const double *alpha, const double x[], const Integer *incx, const double *beta, double y[], const Integer *incy)
The routine may be called by its BLAST name blas_daxpby.

3
Description

f16ecf (blas_daxpby) performs the operation
yα x+β y  
where x and y are n-element real vectors, and α and β real scalars. If n is equal to zero, or if α is equal to zero and β is equal to 1, this routine returns immediately.

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 and y.
2:     alpha – Real (Kind=nag_wp)Input
On entry: the scalar α.
3:     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.
4:     incx – IntegerInput
On entry: the increment in the subscripts of x between successive elements of x.
Constraint: incx0.
5:     beta – Real (Kind=nag_wp)Input
On entry: the scalar β.
6:     y1+n-1×incy – Real (Kind=nag_wp) arrayInput/Output
On entry: the n-element vector y.
If incy>0, yi must be stored in yi-1×incy+1, for i=1,2,,n.
If incy<0, yi must be stored in yn-i×incy+1, for i=1,2,,n.
Intermediate elements of y are not referenced.
On exit: the updated vector y stored in the array elements used to supply the original vector y.
Intermediate elements of y are unchanged.
7:     incy – IntegerInput
On entry: the increment in the subscripts of y between successive elements of y.
Constraint: incy0.

6
Error Indicators and Warnings

If incx=0 or incy=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

f16ecf (blas_daxpby) makes calls to BLAS and/or LAPACK routines, which may be threaded within the vendor library used by this implementation. Consult the documentation for the vendor library for further information.
Please consult the X06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this routine. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

9
Further Comments

None.

10
Example

This example computes the result of a scaled vector accumulation for
α=3,   x = -6,4.5,3.7,2.1,-4T , β=-1,   y = -5.1,-5,6.4,-2.4,-3T .  
x and y are stored in reverse order.

10.1
Program Text

Program Text (f16ecfe.f90)

10.2
Program Data

Program Data (f16ecfe.d)

10.3
Program Results

Program Results (f16ecfe.r)