Number of character set repetitions

Question

How to count character set repetitions in a vector? Imagine the following vector consisting of "A" and "B" :

 x <- c("A", "A", "A", "B", "B", "A", "A", "B", "A") 

In this example, the first set is the sequence "A" and "B" from index 1 to 5, the second set is the sequence "A" and "B" from index 6 to 8, and then the third set is the last single "A" :

 x <- c("A", "A", "A", "B", "B", # set 1 "A", "A", "B", # set 2 "A") # set 3 

How to set up a counter for each set of variables? I need a vector like this:

 c(1, 1, 1, 1, 1, 2, 2, 2, 3) 

thanks

Answer 1

Alternative 1.

 cumsum(c(TRUE, diff(match(x, c("A", "B"))) == -1)) # [1] 1 1 1 1 1 2 2 2 3 

Step by step:

 match(x, c("A", "B")) # [1] 1 1 1 2 2 1 1 2 1 diff(match(x, c("A", "B"))) # [1] 0 0 1 0 -1 0 1 -1 diff(match(x, c("A", "B"))) == -1 # [1] FALSE FALSE FALSE FALSE TRUE FALSE FALSE TRUE c(TRUE, diff(match(x, c("A", "B"))) == -1) # [1] TRUE FALSE FALSE FALSE FALSE TRUE FALSE FALSE TRUE 

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Alternative 2.

Using data.table::rleid :

 library(data.table) cumsum(c(TRUE, diff(rleid(x) %% 2) == 1)) # [1] 1 1 1 1 1 2 2 2 3 

Step by step:

 rleid(x) # [1] 1 1 1 2 2 3 3 4 5 rleid(x) %% 2 # [1] 1 1 1 0 0 1 1 0 1 diff(rleid(x) %% 2) # [1] 0 0 -1 0 1 0 -1 1 diff(rleid(x) %% 2) == 1 # [1] FALSE FALSE FALSE FALSE TRUE FALSE FALSE TRUE c(TRUE, diff(rleid(x) %% 2) == 1) # [1] TRUE FALSE FALSE FALSE FALSE TRUE FALSE FALSE TRUE 

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Answer 2

Use rle :

 x <- c("A", "A", "A", "B", "B", "A", "A", "B", "A") tmp <- rle(x) #Run Length Encoding # lengths: int [1:5] 3 2 2 1 1 # values : chr [1:5] "A" "B" "A" "B" "A" 

Now change the values:

 tmp$values <- ave(rep(1L, length(tmp$values)), tmp$values, FUN = cumsum) 

and invert the path length:

 y <- inverse.rle(tmp) #[1] 1 1 1 1 1 2 2 2 3 

Answer 3

We can only use base R methods

 x1 <- split(x, cumsum(c(TRUE, x[-1]!= x[-length(x)]))) x2 <- sapply(x1, `[`, 1) as.numeric(rep(ave(x2, x2, FUN = seq_along), lengths(x1))) #[1] 1 1 1 1 1 2 2 2 3