# [Numpy-discussion] Taking a large number of dot products at once

eat e.antero.tammi@gmail....
Fri Mar 4 01:29:37 CST 2011

```Hi,

On Fri, Mar 4, 2011 at 8:19 AM, Daniel Hyams <dhyams@gmail.com> wrote:

> This is probably so easy, I'm embarrassed to ask it...but I've been casting
> around trying things to no avail for the last hour and a half, so here
> goes...
>
> I have a lot of dot products to take.  The length-3 vectors that I want to
> dot are stacked in a 2D array like this:
>
> U = [u1 u2 u3....]
>
> and
>
> V = [v1 v2 v3....]
>
> So both of these arrays, are, say, 3x100 each.  I just want to take the dot
> product of each of the corresponding vectors, so that the result is
>
> [u1.v1 u2.v2  u3.v3 ....]
>
> which would be a 1x100 array in this case.
>
> Which function do I need to use?  I thought tensordot() was the one, but I
> couldn't make it work....pure user error I'm sure.
>
No function needed for this case, just:
In []: x= rand(3, 7)
In []: y= rand(3, 7)
In []: d= (x* y).sum(0)
In [490]: d
Out[490]:
array([ 1.25404683,  0.19113117,  1.37267133,  0.74219888,  1.55296562,
0.15264303,  0.72039922])
In [493]: dot(x[:, 0].T, y[:, 0])
Out[493]: 1.2540468282421895

Regards,
eat

>
>
>
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