[R] Strange Matrix Multiplication Behaviour

[Liaw, Andy]

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
> 
> 
> 
> 
> 
> 
> KSS Ltd
> Seventh Floor  St James's Buildings  79 Oxford Street  
> Manchester  M1 6SS  England
> Company Registration Number 2800886
> Tel: +44 (0) 161 228 0040	Fax: +44 (0) 161 236 6305
> mailto:kssg at kssg.com		http://www.kssg.com
> 
> 
> The information in this Internet email is confidential and 
> m...{{dropped}}
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! 
> http://www.R-project.org/posting-guide.html
> 
>