Re: Extracting Elements from a matrix

["Greg Snow" <Greg.Snow@intermountainmail.org>]

Is this what you are looking for?

 

> mat <- matrix(1:15, nrow=5, byrow=F)
> mat
     [,1] [,2] [,3]
[1,]    1    6   11
[2,]    2    7   12
[3,]    3    8   13
[4,]    4    9   14
[5,]    5   10   15
> pos <- c(2, 1, 3, 3, 1)
> mat[ cbind(1:5, pos) ]
[1]  6  2 13 14  5

 

The key is to subscript using a matrix (not multiple vectors) with the 1st column representing all the rows and the 2nd the columns from those rows.

 

Hope this helps,

 

--
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow@intermountainmail.org
(801) 408-8111

 

---

From: s-news-owner@lists.biostat.wustl.edu [mailto:s-news-owner@lists.biostat.wustl.edu] On Behalf Of Ilouga, Pierre
Sent: Wednesday, November 07, 2007 10:12 AM
To: s-news@lists.biostat.wustl.edu
Subject: [S] Extracting Elements from a matrix

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 length 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

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