Services & Resources / Wolfram Forums / MathGroup Archive
-----

MathGroup Archive 2012

[Date Index] [Thread Index] [Author Index]

Search the Archive

multiplying, element wise, a row by each column of a matrix.

  • To: mathgroup at smc.vnet.net
  • Subject: [mg126975] multiplying, element wise, a row by each column of a matrix.
  • From: "Nasser M. Abbasi" <nma at 12000.org>
  • Date: Thu, 21 Jun 2012 05:19:52 -0400 (EDT)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • Reply-to: nma at 12000.org

Any one can think of a better way to do this?

The problem: Given a row vector

    v={v1,v2}

and matrix with same number of columns as v, such as

mat={{a11, a12},
      {a21, a22},
      {a31, a32}};

I need to multiply v1 by the first column, then
multiply v2 by the second column. (i.e. scale each column
of the matrix by the corresponding "weight" from the
row).

This results in

  {{a11 v1, a12 v2},
    {a21 v1, a22 v2},
   {a31 v1, a32 v2}}

I wanted to find a 'smart' way or good command to do
it, but so far, had to do it the hard way. I'll show 2
methods. May be you can find a better functional way
to solve this:

1)
Transpose[MapThread[#1 #2 &, {v, Transpose[mat]}]]

This is not natural solution. need to transposes
2 times.

2)
KroneckerProduct[{v[[#]]},mat[[All,#]]]&/@ Range[1,Length[v]]
Transpose[Flatten[%, 1]]

also not too natural. Faltten, transpose, etc... not good.

Fyi, using another system, which starts with the letter
'O' and ends with the letters 'VE', I can do the above
using

       v .* mat

Where the ".*" above means element by element product.

I hope the experts here can find a short/better way to do
this. I am sure there is, I just can't find it yet, and
I am not happy with what I came up with so far.

thanks,
--Nasser



  • Prev by Date: Re: Bug in Sort !?!?
  • Next by Date: Re: Moment function
  • Previous by thread: Re: Mathematica freezes?
  • Next by thread: Re: multiplying, element wise, a row by each column of a matrix.