NAG CL Interface
f16ehc (dwaxpby)

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1 Purpose

f16ehc computes the sum of two scaled vectors, preserving input, for real scalars and vectors.

2 Specification

#include <nag.h>
void  f16ehc (Integer n, double alpha, const double x[], Integer incx, double beta, const double y[], Integer incy, double w[], Integer incw, NagError *fail)
The function may be called by the names: f16ehc, nag_blast_dwaxpby or nag_dwaxpby.

3 Description

f16ehc performs the operation
w αx+βy,  
where x and y are n-element real vectors, and α and β are real scalars.

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, y and w.
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] const double Input
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. If β=0.0 or n=0, y is not referenced and may be NULL.
7: incy Integer Input
On entry: the increment in the subscripts of y between successive elements of y.
Constraint: incy0.
8: w[dim] double Input/Output
Note: the dimension, dim, of the array w must be at least max(1,1+(n-1)×|incw|).
On entry: if |incw|1, intermediate elements of w may contain values and will not be referenced; the other elements will be overwritten and need not be set.
On exit: the elements wi of the vector w will be stored in w as follows.
If incw>0, wi is in w[(i-1)×incw], for i=1,2,,n.
If incw<0, wi is in w[(n-i)×|incw|], for i=1,2,,n.
Intermediate elements of w are not referenced.
9: incw Integer Input
On entry: the increment in the subscripts of w between successive elements of w.
Constraint: incw0.
10: 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, incw=value.
Constraint: incw0.
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.
f16ehc is not threaded in any implementation.

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, and also the sum vector w, are stored in reverse order.

10.1 Program Text

Program Text (f16ehce.c)

10.2 Program Data

Program Data (f16ehce.d)

10.3 Program Results

Program Results (f16ehce.r)