[R] matching a sequence in a vector?

Gabor Grothendieck ggrothendieck at gmail.com
Thu Feb 16 02:12:32 CET 2012

On Tue, Feb 14, 2012 at 11:17 PM, Redding, Matthew
<Matthew.Redding at deedi.qld.gov.au> wrote:
> I've been trawling through the documentation and listserv archives on this topic -- but
> as yet have not found a solution.  I'm sure this is pretty simple with R, but I cannot work out how without
> resorting to ugly nested loops.
> As far as I can tell, grep, match, and %in% are not the correct tools.
> Question:
> given these vectors --
> patrn <- c(1,2,3,4)
> exmpl <- c(3,3,4,2,3,1,2,3,4,8,8,23,1,2,3,4,4,34,4,3,2,1,1,2,3,4)
> how do I get the desired answer by finding the occurence of the pattern and returning the starting indices:
> 6, 13, 23

Here is a one-liner:

which(rollapply(exmpl, 4, identical, patrn, fill = FALSE, align = "left"))

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