R help - May 2010 - Avoiding for-loop for splitting vector into subvectors based on positions

Dear all,

I'm trying to optimize code and want to avoid for-loops as much as possible.
I'm applying a calculation on subvectors from a big one, and I get the
subvectors by using a vector of starting positions:

x <- 1:10
pos <- c(1,4,7)
n <- length(x)

I try to do something like this :
pos2 <- c(pos, n+1)

out <- c()
for(i in 1:n){
     tmp <- x[pos2[i]:pos2[i+1]]
     out <- c(out, length(tmp))
}

Never mind the length function, I apply a far more complicated one. It's
about the use of the indices in the for-loop. I didn't see any way of doing
that with an apply, unless there is a very convenient way of splitting my
vector in a list of the subvectors or so.

Anybody an idea?
Cheers
-- 
Joris Meys
Statistical Consultant

Ghent University
Faculty of Bioscience Engineering
Department of Applied mathematics, biometrics and process control

Coupure Links 653
B-9000 Gent

tel : +32 9 264 59 87
Joris.Meys@Ugent.be
-------------------------------
Disclaimer : http://helpdesk.ugent.be/e-maildisclaimer.php

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Try this:
> x <- 1:10
> pos <- c(1,4,7)
> pat <- rep(seq_along(pos), times=diff(c(pos, length(x) + 1)))
> split(x, pat)
$`1`
[1] 1 2 3
$`2`
[1] 4 5 6
$`3`
[1]  7  8  9 10



On Tue, May 4, 2010 at 11:29 AM, Joris Meys <jorismeys@gmail.com> wrote:
> Dear all,
>
> I'm trying to optimize code and want to avoid for-loops as much as
> possible.
> I'm applying a calculation on subvectors from a big one, and I get the
> subvectors by using a vector of starting positions:
>
> x <- 1:10
> pos <- c(1,4,7)
> n <- length(x)
>
> I try to do something like this :
> pos2 <- c(pos, n+1)
>
> out <- c()
> for(i in 1:n){
>     tmp <- x[pos2[i]:pos2[i+1]]
>     out <- c(out, length(tmp))
> }
>
> Never mind the length function, I apply a far more complicated one.
It's
> about the use of the indices in the for-loop. I didn't see any way of
doing
> that with an apply, unless there is a very convenient way of splitting my
> vector in a list of the subvectors or so.
>
> Anybody an idea?
> Cheers
> --
> Joris Meys
> Statistical Consultant
>
> Ghent University
> Faculty of Bioscience Engineering
> Department of Applied mathematics, biometrics and process control
>
> Coupure Links 653
> B-9000 Gent
>
> tel : +32 9 264 59 87
> Joris.Meys@Ugent.be
> -------------------------------
> Disclaimer : http://helpdesk.ugent.be/e-maildisclaimer.php
>
>        [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
>
http://www.R-project.org/posting-guide.html<http://www.r-project.org/posting-guide.html>
> and provide commented, minimal, self-contained, reproducible code.
>


-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem that you are trying to solve?

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