[R] Extracting data from matrices

[Uwe Ligges]

Daniel Bebber wrote:

> Dear R list
> 
> I have an m * n matrix P and a vector V of length n containing indices for
> rows in P.
> For each of the m columns I want to extract the value in the row specified
> by V, and put these values into a new vector W of length n.
> At present I am doing this with a for.... loop, but I imagine there is a faster
> way that doesn’t involve loops.
> If anyone knows the way I would be most grateful.
> 
> Below is the code I am using at present-
> 
> for (i in 1:n){
> W[i]<-P[V[i],i]}

So, nrow(P)=n and ncol(P)=m ? What happens if you have less columns than 
rows to be selected? Anyway:

W <- P[V + nrow(P) * (seq(along = V) - 1)]

Uwe Ligges



> Many thanks,
> Dan Bebber
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