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

David Goldsmith d.l.goldsmith@gmail....
Fri Dec 18 17:48:22 CST 2009

```On Fri, Dec 18, 2009 at 3:40 PM, Wayne Watson
<sierra_mtnview@sbcglobal.net> wrote:
> Well, they aren't quite the same. If a is the length of A, and b is the
> length of B, then a*b = A dot B* cos (theta).  I'm still not familiar
> enough with numpy or math to know if there's some function that will
> produce a from A. It's easy enough to do, a = A(0)**2 + ..., but I would
> like to think it's a common enough need that there would be something
> available like sumsq().

In your usage, dot product and scalar product are synonymous:

a = sqrt(A dot A)

There are some contexts in which "scalar" product and "dot" product
don't mean exactly the same thing (e.g., tensors, where "dot" is
typically synonymous w/ "inner," which, in the general case, does not
result in a scalar, or a multiplication-like functional where a
function is mapped to a scalar, in which context we typically - but
not uniformly - do not describe the product as a dot product) but
unless you're working in one of those advanced contexts, scalar and
dot are typically used interchangeably.  In particular, IIUC, in
NumPy, unless your using it to calculate a tensor product that doesn't
result in a scalar, dot and scalar product are synonymous.

DG
>
>
> Keith Goodman wrote:
>> On Fri, Dec 18, 2009 at 3:22 PM, Wayne Watson
>> <sierra_mtnview@sbcglobal.net> wrote:
>>
>>> Is there a scalar product in numpy?
>>>
>>
>> Isn't that the same thing as a dot product? np.dot doesn't do what you want?
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>>
>
> --
>           Wayne Watson (Watson Adventures, Prop., Nevada City, CA)
>
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>
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>
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```