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

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 at 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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