[Jprogramming] Simple scalar multiplication of matrices

['Jon Hough' via Programming]

Thanks,

I was actually just trying that. Problem is, it has a dangling dimension (shape 1 4 2), which can be removed, but seems a little messy.


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On Mon, 4/18/16, EelVex <eelvex at gmail.com> wrote:

 Subject: Re: [Jprogramming] Simple scalar multiplication of matrices
 To: "Programming forum" <programming at jsoftware.com>
 Date: Monday, April 18, 2016, 5:54 PM
 
 Of course, you can still
 go by the dot product. Just don't sum:
 
    a (] . *) |:b
 10 14
 22 30
 36
 48
 52 68
 
 
 On Mon, Apr 18, 2016 at 11:49 AM, EelVex <eelvex at gmail.com>
 wrote:
 
 > If you remove
 the first axis of a you can multiply-1:
 >
 >  (,a) *"1 b
 > 10 22 36 52
 > 14 30 48
 68
 >
 > Do you really
 need a to be 1xn ?
 >
 >
 > On Mon, Apr 18, 2016
 at 11:41 AM, 'Jon Hough' via Programming <
 > programming at jsoftware.com>
 wrote:
 >
 >> I am
 sure there is a simple answer to this, but I would like to
 know it.
 >>
 >>
 Say I have two matrices:
 >>
 >> a =: 1 4 $ 1 2 3 4
 >> b =: 2 4 $ 10 11 12 13 14 15 16 17
 >>
 >> and I want to
 multiply a's single row element-wise with each of
 b's rows.
 >> Not matrix
 multiplication, so the resulting matrix should have
 dimensions 2
 >> 4, same as b.
 >>
 >> Obviously a *
 b gives a length error,
 >>
 >> So does a *"1 1 b
 >>
 >> My best
 solution is pretty ugly,
 >>
 >> (($b)$,a ) * b
 >>
 >> i.e. reshape
 a so it has the same number of rows as b, which
 duplicates
 >> the items of its top row
 into the second row and then multiply row-wise.
 >>
 >> Is there a
 better way?
 >> Incidentally, isn't
 a*1 1 b a row-to-row multiplication. I thought this
 >> would have worked, and am slightly
 confused why it doesn't.
 >>
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 >
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