Question

I have a state column in a dataframe and I want to create two new columns:
One that looks ahead to the next stage change and one that looks back to the previous state change. So the resulting dataframe will look like below:

state coming  previous
  a     a-b     NA
  a     a-b     NA
  a     a-b     NA
  a     a-b     NA
  b     b-c     a-b
  b     b-c     a-b
  b     b-c     a-b
  c     c-a     b-c
  c     c-a     b-c
  c     c-a     b-c
  a     NA      c-a
  a     NA      c-a

Or maybe even better, but now you just create two transition columns:

state trans1   trans2
  a     a-b     NA
  a     a-b     NA
  a     a-b     NA
  a     a-b     NA
  b     a-b     b-c 
  b     a-b     b-c
  b     a-b     b-c
  c     c-a     b-c
  c     c-a     b-c
  c     c-a     b-c
  a     c-a     NA 
  a     c-a     NA

[Edit]
changed states named “1” to “c” because it was confusing

Answer 1

Let’s give that dataframe a name, say ‘inp’. Use the rle function to construct the sequence of “states”:

> rle(inp$state)
Run Length Encoding
  lengths: int [1:4] 4 3 3 2
  values : chr [1:4] "a" "b" "1" "a"

runinp <- rle(inp$state)$values
paste( runinp[-length(runinp)], runinp[-1], sep="-")
# [1] "a-b" "b-1" "1-a"
inp$coming <- rep( c( paste( runinp[-length(runinp)], runinp[-1], sep="-"), NA), 
                    rle(inp$state)$lengths )
inp$coming
# [1] "a-b" "a-b" "a-b" "a-b" "b-1" "b-1" "b-1" "1-a" "1-a" "1-a" NA    NA   

inp$previous <- 
 rep( c( NA_character_, paste(runinp[-1], runinp[-length(runinp)], sep="-")), 
      rle(inp$state)$lengths )
inp$previous
 [1] NA    NA    NA    NA    "b-a" "b-a" "b-a" "1-b" "1-b" "1-b" "a-1" "a-1"

(I was able to overcome my difficulty with understanding your first request, but had persistent difficulty with the second part.)