[R] Replacing several rows of a matrix at once

[Peter Ehlers]

On 2010-11-28 19:53, Bryan Hanson wrote:
> Hello Folks.  This must be a silly question with a (not) obvious (to me)
> answer.
>
> Consider this:
>
> tmp<- matrix(1:200, nrow = 20)
> vec<- 300:309
>
> tmp[9,]<- vec # replacing one row works fine
>
> p<- c(3, 11, 17)
> tmp[p,]<- vec
> # replacing multple rows pastes the values down a column and recycles vec.
>
> What I want to do is replace multiple rows simultaneously at once.  I
> suppose I can write a function, but this seems pretty fundamental so I feel
> I must be missing some obvious alternative.  I'm feeling like I'm in the
> Inferno!

Since matrices in R use column-major order, transpose first:

  ttmp <- t(tmp)
  ttmp[, p] <- vec
  (ans <- t(ttmp))

Peter Ehlers

>
> TIA.  Bryan
> *************
> Bryan Hanson
> Professor of Chemistry&  Biochemistry
> DePauw University, Greencastle IN USA
>
>> sessionInfo()
> R version 2.12.0 (2010-10-15)
> Platform: x86_64-apple-darwin9.8.0/x86_64 (64-bit)
>
> locale:
> [1] en_US.UTF-8/en_US.UTF-8/C/C/en_US.UTF-8/en_US.UTF-8
>
> attached base packages:
>   [1] splines   datasets  tools     grid      grDevices graphics  utils
> stats
>   [9] methods   base
>
> other attached packages:
>   [1] survival_2.35-8    gridExtra_0.7      GGally_0.2.2       xtable_1.5-6
>   [5] mvbutils_2.5.1     ggplot2_0.8.8      proto_0.3-8        reshape_0.8.3
>   [9] ChemoSpec_1.46     seriation_1.0-2    colorspace_1.0-1   TSP_1.0-1
> [13] R.utils_1.5.3      R.oo_1.7.4         R.methodsS3_1.2.1  rgl_0.92.794
> [17] lattice_0.19-13    mvoutlier_1.4      plyr_1.2.1
> RColorBrewer_1.0-2
> [21] chemometrics_1.0   som_0.3-5          robustbase_0.5-0-1 rpart_3.1-46
> [25] pls_2.1-0          pcaPP_1.8-3        mvtnorm_0.9-92     nnet_7.3-1
> [29] mclust_3.4.6       MASS_7.3-8         lars_0.9-7         gclus_1.3
> [33] cluster_1.13.1     e1071_1.5-24       class_7.3-2
>
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