NAG CL Interface
f16ecc (daxpby)

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1 Purpose

f16ecc computes the sum of two scaled vectors, for real vectors and scalars.

2 Specification

#include <nag.h>
void  f16ecc (Integer n, double alpha, const double x[], Integer incx, double beta, double y[], Integer incy, NagError *fail)
The function may be called by the names: f16ecc, nag_blast_daxpby or nag_daxpby.

3 Description

f16ecc 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 function 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 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 and y.
Constraint: n0.
2: alpha double Input
On entry: the scalar α.
3: 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.
4: incx Integer Input
On entry: the increment in the subscripts of x between successive elements of x.
Constraint: incx0.
5: beta double Input
On entry: the scalar β.
6: y[dim] double Input/Output
Note: the dimension, dim, of the array y must be at least max(1,1+(n-1)×|incy|).
On entry: the n-element vector y.
If incy>0, yi must be stored in y[(i-1)×incy], for i=1,2,,n.
If incy<0, yi must be stored in y[(n-i)×|incy|], 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 Integer Input
On entry: the increment in the subscripts of y between successive elements of y.
Constraint: incy0.
8: 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, incy=value.
Constraint: incy0.
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.
f16ecc 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 function. 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,−4) T , β=−1,   y = (-5.1,−5,6.4,-2.4,−3) T .  
x and y are stored in reverse order.

10.1 Program Text

Program Text (f16ecce.c)

10.2 Program Data

Program Data (f16ecce.d)

10.3 Program Results

Program Results (f16ecce.r)