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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