[R] Subsetting a matrix without for-loop
Petr Pikal
petr.pikal at precheza.cz
Tue Jan 31 08:38:42 CET 2006
Hi
> matrix(colSums(embed(A,3)[1:3,]),3,6, byrow=T)[3:1,]
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] 6 33 60 87 114 141
[2,] 9 36 63 90 117 144
[3,] 12 39 66 93 120 147
will do it. However I am not sure if it is quicker than your for
loop.
HTH
Petr
On 30 Jan 2006 at 16:29, Camarda, Carlo Giovanni wrote:
Date sent: Mon, 30 Jan 2006 16:29:38 +0100
From: "Camarda, Carlo Giovanni" <Camarda at demogr.mpg.de>
To: <r-help at stat.math.ethz.ch>
Subject: [R] Subsetting a matrix without for-loop
> Dear R-users,
> I'm struggling in R in order to "squeeze" a matrix without using a
> for-loop. Although my case is a bit more complex, the following
> example should help you to understand what I would like to do, but
> without the slow for-loop. Thanks in advance, Carlo Giovanni Camarda
>
>
> A <- matrix(1:54, ncol=6) # my original matrix
> A.new <- matrix(nrow=3, ncol=6) # a new matrix which I'll fill
> # for-loop
> for(i in 1:nrow(A.new)){
> B <- A[i:(i+2), ] # selecting the rows
> C <- apply(B,2,sum) # summing by columns
> A.new[i,] <- C # inserting in the new matrix
> }
>
>
> +++++
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Petr Pikal
petr.pikal at precheza.cz
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