Re: Extracting Elements from a matrix

["Richard A. Becker" <rab@research.att.com>]

Try 

  mat[cbind(seq(along=pos),pos)]

  Rick Becker

> Dear Splus experts,
> 
> Does anyone has an idea how to extract some elements from a matrix in an
> EFFICIENT way? The following example clarifies  my question.
> 
> Given a  5x3 matrix  mat <- matrix(1:15, nrow=5, byrow=F)  and a leng=
> th
> 5 -vector  of positions  pos <- c(2, 1, 3, 3, 1), the goal is to extract
> the vector c(6, 2, 13, 14, 5) which  corresponds to the column pos[j]
> (j in 1:3) and row i  ( i in 1:5) without using for-loops and other
> memory consuming functions.
> I've solve it via lists. However the slowness of the command executions
> doesn't make it useful for  monte carlo simulation.
> 
> Thanks in advance.
> PI
> 
> S-PLUS : Copyright (c) 1988, 2002 Insightful Corp.
> S : Copyright Lucent Technologies, Inc.
> Version 6.1.2 Release 2 for Linux 2.2.12 : 2002=20
> Working data will be in .Data
> 
> 
> Dr. Pierre Ilouga
> Screening Operations/Discovery Informatics
> EVOTEC AG
> Schnackenburgallee 114
> 22525 Hamburg
> Germany
> 
> pierre.ilouga@evotec.com
> www.evotec.com
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> 
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