text search in r

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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David Winsemius, MD
Alameda, CA, USA