NAG Library Routine Document

f16eaf (blas_ddot) (ddot)

1

Purpose

2

Specification

3

Description

4

References

5

Arguments

1:     $\mathbf{conj}$ – IntegerInput

On entry: conj is not referenced and need not be set. The presence of this argument in the BLAST standard is for consistency with the interface of the complex variant of this routine.

2:     $\mathbf{n}$ – IntegerInput

On entry: $n$, the number of elements in $x$ and $y$.

3:     $\mathbf{alpha}$ – Real (Kind=nag_wp)Input

On entry: the scalar $\alpha$.

4:     $\mathbf{x}\left(1+\left(\mathbf{n}-1\right)\times \left|\mathbf{incx}\right|\right)$ – Real (Kind=nag_wp) arrayInput

On entry: the $n$-element vector $x$.

If $\mathbf{incx}>0$, $x_{\mathit{i}}$ must be stored in $\mathbf{x}\left(\left(\mathit{i}-1\right)\times \mathbf{incx}+1\right)$, for $\mathit{i}=1,2,\dots ,\mathbf{n}$.

If $\mathbf{incx}<0$, $x_{\mathit{i}}$ must be stored in $\mathbf{x}\left(\left(\mathbf{n}-\mathit{i}\right)\times \left|\mathbf{incx}\right|+1\right)$, for $\mathit{i}=1,2,\dots ,\mathbf{n}$.

Intermediate elements of x are not referenced. If $\alpha =0.0$ or $\mathbf{n}=0$, x is not referenced.

5:     $\mathbf{incx}$ – IntegerInput

On entry: the increment in the subscripts of x between successive elements of $x$.

Constraint: $\mathbf{incx}\ne 0$.

6:     $\mathbf{beta}$ – Real (Kind=nag_wp)Input

On entry: the scalar $\beta$.

7:     $\mathbf{y}\left(1+\left(\mathbf{n}-1\right)\times \left|\mathbf{incy}\right|\right)$ – Real (Kind=nag_wp) arrayInput

On entry: the $n$-element vector $y$.

If $\mathbf{incy}>0$, $y_{\mathit{i}}$ must be stored in $\mathbf{y}\left(\left(\mathit{i}-1\right)\times \mathbf{incy}+1\right)$, for $\mathit{i}=1,2,\dots ,\mathbf{n}$.

If $\mathbf{incy}<0$, $y_{\mathit{i}}$ must be stored in $\mathbf{y}\left(\left(\mathbf{n}-\mathit{i}\right)\times \left|\mathbf{incy}\right|+1\right)$, for $\mathit{i}=1,2,\dots ,\mathbf{n}$.

Intermediate elements of y are not referenced. If $\alpha =0.0$ or $\mathbf{n}=0$, y is not referenced.

8:     $\mathbf{incy}$ – IntegerInput

On entry: the increment in the subscripts of y between successive elements of $y$.

Constraint: $\mathbf{incy}\ne 0$.

9:     $\mathbf{r}$ – Real (Kind=nag_wp)Input/Output

On entry: the initial value, $r$, to be updated. If $\beta =0.0$, r need not be set on entry.

On exit: the value $r$, scaled by $\beta$ and updated by the scaled dot product of $x$ and $y$.

6

Error Indicators and Warnings

7

Accuracy

8

Parallelism and Performance

9

Further Comments

10

Example

10.1

Program Text

Program Text (f16eafe.f90)

10.2

Program Data

Program Data (f16eafe.d)

10.3

Program Results

Program Results (f16eafe.r)