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

Hector hector1618@gmail....
Fri Mar 4 03:13:00 CST 2011

```On Fri, Mar 4, 2011 at 12:59 PM, eat <e.antero.tammi@gmail.com> wrote:

> 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.
>>
>
Hello Daniel Hyams and group,

I guess the following code serves the purpose but I think something needs to
be highlighted. I think Eat forgot to paste it here.

No function needed for this case, just:
>

In [ ] : import numpy.random as rand

> 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
>
>>
>>
>>
I guess you can use 'rand' only this after  import.

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>
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>

--
-Regards
Hector

Whenever you think you can or you can't, in either way you are right.
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