[R] Extracting from a matrix w/o for-loop

[Liaw, Andy]

Yes, by matrix indexing:

R> M1[M2]
[1]  1  2 13 15  8
R> v
[1]  1  2 13 15  8

Andy 

From: Camarda, Carlo Giovanni
> 
> Dear R-users,
> 
> likely there is a simple solution for this problem, but I currently
> cannot see it.
> 
> I basically would like to get from a matrix values in particular
> positions which are the rows of another matrix, without using a
> for-loop.
> 
> In other words: is there any way to avoid the for-loop in the 
> following
> simple example:
> 
> M1 <- matrix(1:20, ncol=2)
> M2 <- rbind(c(1,1), c(2,1), c(3,2), c(5,2), c(8,1))
> v <- numeric(nrow(M2))
> for(i in 1:length(v)){
>     v[i] <- M1[M2[i,1], M2[i,2]]
> }
> 
> Any suggestion would be welcome,
> 
> Ciao,
> Carlo Giovanni Camarda
> 
> ===========================================
> Camarda Carlo Giovanni
> PhD-Student
> Max Planck Institute for Demographic Research
> Konrad-Zuse-Strasse 1
> 18057 Rostock, Germany
> Tel:  +49 (0)381 2081 172
> Fax: +49 (0)381 2081 472
> camarda at demogr.mpg.de
> ===========================================
> 
> 
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