[R] Strange Matrix Multiplication Behaviour

Liaw, Andy andy_liaw at merck.com
Mon Oct 4 16:48:55 CEST 2004

```What you need to realize is that R does vectorized computation in `Fortran'
order.  Here's a perhaps more illuminating example:

> tr <- rep(1:2, 3)
> tr
[1] 1 2 1 2 1 2
> ex1 <- as.data.frame(matrix(1:12, 2, 6))
> ex1
V1 V2 V3 V4 V5 V6
1  1  3  5  7  9 11
2  2  4  6  8 10 12
> tr * ex1[1,]
V1 V2 V3 V4 V5 V6
1  1  6  5 14  9 22
> (tr * ex1)[1,]
V1 V2 V3 V4 V5 V6
1  1  3  5  7  9 11

When you do (tr * ex1)[1,], R multiplies tr and ex1 element-by-element,
stacking _columns_ of ex1 to match tr, and recyle the elements of tr to
match the length of the `stacked' ex1.

Andy

> From: Wayne Jones
>
> 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.
>
>
> Can anyone shed any light on my predicament??
>
>
>
> > version
>          _
> platform i386-pc-mingw32
> arch     i386
> os       mingw32
> system   i386, mingw32
> status
> major    1
> minor    9.1
> year     2004
> month    06
> day      21
> language R
>
>
>
> Regards
>
> Wayne Jones
>
>
>
>
>
>
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