thankyou Berend.....
regards
eliza
> Subject: Re: [R] extracting column and regular interval in R
> From: b...@xs4all.nl
> Date: Tue, 18 Sep 2012 17:43:15 +0200
> CC: r-help@r-project.org
> To: eliza_bo...@hotmail.com
>
>
> On 18-09-2012, at 16:55, eliza botto wrote:
>
> >
> > Dear R users,
> > i have a matrix with 31 rows and 444 columns and i want to extract every
> > 37th column of that matrix starting from 1. more precisely i want to select
> > columns 1, 38,75, 112 and so on. then doing the same by starting from
> > column number 2(2,39,76,113.......).
> > i know that there is a manual way of doing it but i wanted to make it more
> > quickly as i have fairly large data to dealth with.
> >
>
> Indexing with
>
> c(1,seq_len((length(x)-1)/M)*M+1)
>
> is also an option.
> It appears to be faster than Michael's method.
>
> N <- 444
> a <- 1:N
>
> f1 <- function(x,M) {
> x[c(TRUE,rep(FALSE,M-1))]
> }
>
> f2 <- function(x,M) {
> x[c(1,seq_len((length(x)-1)/M)*M+1)]
> }
>
> M <- 37
> M
> z1 <- f1(a,M)
> z2 <- f2(a,M)
>
> > identical(z1,z2)
> [1] TRUE
>
>
> Nrep <- 100000
>
> library(rbenchmark)
> benchmark(f1(a,M), f2(a,M), replications=Nrep)
>
> > benchmark(f1(a,M), f2(a,M), replications=Nrep)
> test replications elapsed relative user.self sys.self user.child
> 1 f1(a, M) 100000 1.395 1.437 1.367 0.027 0
> 2 f2(a, M) 100000 0.971 1.000 0.959 0.011 0
>
> For N <- 4440 the timings are
>
> > benchmark(f1(a,M), f2(a,M), replications=Nrep)
> test replications elapsed relative user.self sys.self user.child
> 1 f1(a, M) 100000 4.636 2.483 4.569 0.065 0
> 2 f2(a, M) 100000 1.867 1.000 1.714 0.153 0
>
>
> Berend
>
>
>
>
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