[R] matching a sequence in a vector?

[Redding, Matthew]

Thankyou all for your great, and creative solutions.

There is definately more than one way to skin a cat.

A colleague alerted me to another solution:

seq.strt <- which( sapply( 1:(length(exmpl)-length(patrn)+1), function(i) isTRUE( all.equal( patrn, exmpl[ i + 0:(length(patrn)-1) ] ) ) ) ) 

Apparently this came from a post in the help archive that my searches missed.

I think the solutions you have put up are more readable.

Kind regards

Matt



>-----Original Message-----
>From: r-help-bounces at r-project.org 
>[mailto:r-help-bounces at r-project.org] On Behalf Of Berend Hasselman
>Sent: Thursday, 16 February 2012 1:35 AM
>To: Martin Morgan
>Cc: r-help at r-project.org
>Subject: Re: [R] matching a sequence in a vector?
>
>
>On 15-02-2012, at 15:27, Martin Morgan wrote:
>
>> 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...
>
>Clever.
>However it is quite slow.
>
>Berend
>
>______________________________________________
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>PLEASE do read the posting guide 
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>and provide commented, minimal, self-contained, reproducible code.
>
>
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