# [Numpy-discussion] dot function or dot notation, matrices, arrays?

Wayne Watson sierra_mtnview@sbcglobal....
Sat Dec 19 06:51:25 CST 2009

```I'm trying to compute the angle between two vectors in three dimensional
space. For that, I need to use the "scalar (dot) product" , according to
a calculus book (quoting the book) I'm holding in my hands right now.
I've used dot() successfully to produce the necessary angle. My program
works just fine.

In the case of the dot(function), one must use np.dev(x.T,x), where x is
1x3.

I'm not quite sure what your point is about dot()* unless you are
thinking in some non-Euclidean fashion. One can form np.dot(a,b) with a
and b arrays of 3x4 and 4x2 shape to arrive at a 3x2 array. That's
definitely not a scalar. Is there a need for this sort of calculation in
non-Euclidean geometry, which I have never dealt with?

*Maybe it's about something else related to it.

David Goldsmith wrote:
> np.dot(x.flat, x.flat) _is exactly_ "sum of squares"(x.flat).  Your
> math education appears to have drawn a distinction between "dot
> product" and "scalar product," that, when one is talking about
> Euclidean vectors, just isn't there: in that context, they are one and
> the same thing.
>
> DG
>
> On Fri, Dec 18, 2009 at 9:29 PM, Wayne Watson
> <sierra_mtnview@sbcglobal.net> wrote:
>
>> I'll amend that. I should have said, "Dot's all folks." -- Bugs Bunny
>>
>> --
>>
>>             (121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
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>>
>>             "... humans'innate skills with numbers isn't much
>>              better than that of rats and dolphins."
>>                       -- Stanislas Dehaene, neurosurgeon
>>
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>>
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--

(121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
Obz Site:  39° 15' 7" N, 121° 2' 32" W, 2700 feet

"... humans'innate skills with numbers isn't much
better than that of rats and dolphins."
-- Stanislas Dehaene, neurosurgeon

Web Page: <www.speckledwithstars.net/>

```