[R] Extracting data from matrices

[Dimitris Rizopoulos]

Dear Dan,

you could just use the following:

V <- cbind(V, 1:n)
W <- P[V]

I hope this helps.

Best,
Dimitris

----
Dimitris Rizopoulos
Doctoral Student
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/16/396887
Fax: +32/16/337015
Web: http://www.med.kuleuven.ac.be/biostat/
     http://www.student.kuleuven.ac.be/~m0390867/dimitris.htm


----- Original Message ----- 
From: "Daniel Bebber" <danbebber at forestecology.co.uk>
To: <r-help at stat.math.ethz.ch>
Sent: Wednesday, May 12, 2004 2:09 PM
Subject: [R] Extracting data from matrices


> 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]}
>
> Many thanks,
> Dan Bebber
> ____________________________
> Department of Plant Sciences
> University of Oxford
> South Parks Road
> Oxford OX1 3RB
> UK
> Tel. 01865 275060
> Fax. 01865 275074
>
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