Sequence reduction in R

Question

Assume you have a vector like so:

v <- c(1,1,1,2,2,2,2,1,1,3,3,3,3)

How can it be best reduced to a data.frame like this?

v.df <- data.frame(value=c(1,2,1,3),repetitions=c(3,4,2,4))

In a procedural language I might just iterate through a loop and build the data.frame as I go, but with a large dataset in R such an approach is inefficient. Any advice?

Answer 1

with(rle(v), data.frame(values, lengths))

should get you what you need.

values lengths
     1       3
     2       4
     1       2
     3       4

Answer 2

or more simply

data.frame(rle(v)[])