# [R] Strange Matrix Multiplication Behaviour

Arne Henningsen arnehe at gmx.de
Mon Oct 4 17:35:28 CEST 2004

```Hi Wayne,

are you aware that you are NOT doing matrix multiplication? A matrix
multiplication is done by %*%, e.g.
> tr %*% t(as.matrix(ex1)

Or do you want a normal scalar multiplication with the elements of each row of
ex1 to be multiplied with the corresponding element of tr? However, R does
the multiplication not row-wise, but column-wise. Thus, the results are not
as you expected. To do a row-wise multiplication you can transform the
data.frame:

> tr <- as.vector(10*c(1:6))
> tr
[1] 10 20 30 40 50 60
> ex1 <- as.data.frame(matrix(1:12,nrow=2,byrow=TRUE))
> ex1
V1 V2 V3 V4 V5 V6
1  1  2  3  4  5  6
2  7  8  9 10 11 12
> tr * ex1
V1  V2  V3  V4  V5  V6
1  10  60 150  40 150 300
2 140 320 540 200 440 720
> t( tr * t(ex1) )
V1  V2  V3  V4  V5  V6
1 10  40  90 160 250 360
2 70 160 270 400 550 720
> t( tr * t(ex1) )[1,]
V1  V2  V3  V4  V5  V6
10  40  90 160 250 360
> t( tr * t(ex1[1,]) )
V1 V2 V3  V4  V5  V6
1 10 40 90 160 250 360

I hope that this makes it clear.

Best wishes,
Arne

On Monday 04 October 2004 16:10, 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.
>
>
> 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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--
Arne Henningsen
Department of Agricultural Economics
University of Kiel
Olshausenstr. 40
D-24098 Kiel (Germany)
Tel: +49-431-880 4445
Fax: +49-431-880 1397
ahenningsen at agric-econ.uni-kiel.de
http://www.uni-kiel.de/agrarpol/ahenningsen/

```