# [R] Fastest way to extract rows of smaller matrix many times by index to make larger matrix? and multiply columsn of matrix by vector

Jeff Newmiller jdnewm|| @end|ng |rom dcn@d@v|@@c@@u@
Tue Sep 14 09:49:00 CEST 2021

```That is about as fast as it can be done. However you may be able to avoid doing it at all if you fold V2 into a matrix instead. Did you mean to use matrix multiplication in your calculation of M3?

On September 13, 2021 11:48:48 PM PDT, nevil amos <nevil.amos using gmail.com> wrote:
>Hi is there a faster way to "extract" rows of a matrix many times to for a
>longer matrix based in a vector or for indices than M[ V, ]
>
>I need to "expand" ( rather than subset)  a matrix M of 10-100,000 rows x
>~50 columns to produce a matrix with a greater number (10^6-10^8) of rows
>using a vector V containing the 10^6 -10^8 values that are the indices of
>the rows of M. the output matrix M2 is then multiplied by another vector V2
>With the same length as V.
>
>Is there a faster way to achieve these calculations (which are by far the
>slowest portion of a function looped 1000s of times? than the standard  M2
><- M[ V, ] and  M3<-M2*V2, the two calculations are taking a similar time,
>Matrix M also changes for each loop.
>
>
>M<-matrix(runif(50*10000,0,100),nrow=10000,ncol=50)
>x = 10^7
>V<-sample(1:10000,x,replace=T)
>V2<-(sample(c(1,NA),x,replace=T))
>print<-(microbenchmark(
>M2<-M[V,],
>M3<-M2*V2,
>times=5,unit = "ms"))
>
>
>
>thanks for any suggestions
>
>Nevil Amos
>
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>
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