# [R] Strange Matrix Multiplication Behaviour

Prof Brian Ripley ripley at stats.ox.ac.uk
Mon Oct 4 17:04:42 CEST 2004

```This is *not* matrix multiplication, which uses %*% in R.
Might that help explain your confusion?

The difference is explained in `An Introduction to R'.

On Mon, 4 Oct 2004, Wayne Jones wrote:

>
> Hi there fellow R-users,
>
> Im seeing some strange behaviour when I multiply a vector by a matrix
>
> Here is my script:
>
> > tr
>         1         2         3         4         5         6
> 0.2217903 0.1560525 0.1487908 0.1671354 0.1590643 0.1471667
> >
> > ex1
>            a         b          c          d          e          f
> 1  0.2309579 -3.279045 -0.6694697 -1.1024404  0.2303928 -1.5527404
> 2 -0.2865357 -2.519789 -0.1138712 -0.3571832 -2.6727913 -0.3296945
>
> > is.vector(tr)
> [1] TRUE
>
> > is.data.frame(ex1)
> [1] TRUE
>
> > tr* ex1[1,]
>            a          b           c          d          e          f
> 1 0.05122423 -0.5117031 -0.09961092 -0.1842569 0.03664727 -0.2285117
>
> > (tr* ex1)[1,]
>            a          b          c          d          e          f
> 1 0.05122423 -0.4878917 -0.1064887 -0.2445106 0.03428033 -0.2469855
>
>
> Notice that the output from tr * ex[1,] is different from (tr* ex1)[1,]
> Especially element number 4.
>
> I would naturally expect both vectors to be equivalent.

Why?  The second multiplies the elements of ex by tr, in column-major
order, and what is `natural' about that?

--
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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