The data.table package provides a convenient way to group by runs in data. Consider the following example data:
library(data.table)
(DT <- data.table(x = c(1, 1, 2, 2, 2, 1), y = 1:6))
# x y
# 1: 1 1
# 2: 1 2
# 3: 2 3
# 4: 2 4
# 5: 2 5
# 6: 1 6
The variable x has three runs: a run ...