[R] Strange Matrix Multiplication Behaviour

[Prof Brian Ripley]

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)
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