matching a sequence in a vector?

On 02/14/2012 11:45 PM, Petr Savicky wrote:

On Wed, Feb 15, 2012 at 02:17:35PM +1000, Redding, Matthew wrote:

Hi All,


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

match(exmpl, patrn) returns indexes that differ by 1 if the sequence 
patrn occurs

  n = length(patrn)
  r = rle(diff(match(exmpl, patrn)) == 1)

we're looking for a run of TRUE's of length 3, and can find their ends 
(of the runs of diffs) as cumsum(r$length)

  cumsum(r$length)[r$values & r$length == (n - 1)] - (n - 2)

Seems like there could be edge cases that I'm missing...