[R] text search in r

[David Winsemius]

On Aug 2, 2012, at 2:27 PM, rnewbie565 wrote:

> I am trying to count the number of times that the characters in a  
> string
> change
> For example-
> new=c(AAAABBBBBABBBABB)

Presumably you meant to quote that string.

> I want to find the number of times that B changes to A and the  
> number of
> times that A changes to B. I tried the grep command but I only  
> figured out
> the positions of when B changes to A when I only need the number of  
> times it
> occurs.

 >  new=c('AAAABBBBBABBBABB')
 > unlist(strsplit(new,""))
  [1] "A" "A" "A" "A" "B" "B" "B" "B" "B" "A" "B" "B" "B" "A" "B" "B"
 > rle(unlist(strsplit(new,"")))
Run Length Encoding
   lengths: int [1:6] 4 5 1 3 1 2
   values : chr [1:6] "A" "B" "A" "B" "A" "B"

 > paste0( rle(unlist(strsplit(new,"")))$values, collapse="")
[1] "ABABAB"
 > gregexpr("AB", .Last.value)
[[1]]
[1] 1 3 5
attr(,"match.length")
[1] 2 2 2
attr(,"useBytes")
[1] TRUE


So the length of that list value could be used for the AB transitions.  
Similarly for the BA ones

 > strrle <- paste0( rle(unlist(strsplit(new,"")))$values, collapse="")
 > length( gregexpr("BA", strrle)[[1]] )
[1] 2

>
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>
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David Winsemius, MD
Alameda, CA, USA