Grouping of R dataframe by connected values

Question

I didn\'t find a solution for this common grouping problem in R:

This is my original dataset

ID  State
1   A
2   A
3   B
4   B
5   B
6   A
7   A
8            


        

Answer 1

Here is a method that uses the rle function in base R for the data set you provided.

# get the run length encoding
temp <- rle(df$State)

# construct the data.frame
newDF <- data.frame(State=temp$values,
                    min.ID=c(1, head(cumsum(temp$lengths) + 1, -1)),
                    max.ID=cumsum(temp$lengths))

which returns

newDF
  State min.ID max.ID
1     A      1      2
2     B      3      5
3     A      6      8
4     C      9     10

Note that rle requires a character vector rather than a factor, so I use the as.is argument below.

---

As @cryo111 notes in the comments below, the data set might be unordered timestamps that do not correspond to the lengths calculated in rle. For this method to work, you would need to first convert the timestamps to a date-time format, with a function like as.POSIXct, use df <- df[order(df$ID),], and then employ a slight alteration of the method above:

# get the run length encoding
temp <- rle(df$State)

# construct the data.frame
newDF <- data.frame(State=temp$values,
                    min.ID=df$ID[c(1, head(cumsum(temp$lengths) + 1, -1))],
                    max.ID=df$ID[cumsum(temp$lengths)])

data

df <- read.table(header=TRUE, as.is=TRUE, text="ID  State
1   A
2   A
3   B
4   B
5   B
6   A
7   A
8   A
9   C
10  C")