Find start and end positions/indices of runs/consecutive values

Question

Problem: Given an atomic vector, find the start and end indices of runs in the vector. Example vector with runs: ``` x = rev(rep(6:10, 1:5)) # [1] 10 10 10 10 10 9 9 9 9 8 8 8 7 7 6 ``` Output from `rle()`: ``` rle(x) # Run Length Encoding # lengths: int [1:5] 5 4 3 2 1 # values : int [1:5] 10 9 8 7 6 ``` Desired output: ``` # start end # 1 1 5 # 2 6 9 # 3 10 12 # 4 13 14 # 5 15 15 ``` The base `rle` class doesn't appear to provide this functionality, but the class `Rle` and function `rle2` do. However, given how minor the functionality is, sticking to base R seems more sensible than installing and loading additional packages. There are examples of code snippets (here, here and on SO) which solve the slightly different problem of finding start and end indices for runs which satisfy some condition. I wanted something that would be more general, could be performed in one line, and didn't involve the assignment of temporary variables or values. Answering my own question because I was fru…

Accepted Answer

Core logic:

```
# Example vector and rle object
x = rev(rep(6:10, 1:5))
rle_x = rle(x)

# Compute endpoints of run
end = cumsum(rle_x$lengths)
start = c(1, lag(end)[-1] + 1)

# Display results
data.frame(start, end)
#   start end
# 1     1   5
# 2     6   9
# 3    10  12
# 4    13  14
# 5    15  15
```

Tidyverse/`dplyr` way (data frame-centric):

```
library(dplyr)

rle(x) %>%
  unclass() %>%
  as.data.frame() %>%
  mutate(end = cumsum(lengths),
         start = c(1, dplyr::lag(end)[-1] + 1)) %>%
  magrittr::extract(c(1,2,4,3)) # To re-order start before end for display
```

Because the `start` and `end` vectors are the same length as the `values` component of the `rle` object, solving the related problem of identifying endpoints for runs meeting some condition is straightforward: `filter` or subset the `start` and `end` vectors using the condition on the run values.

Find start and end positions/indices of runs/consecutive values

Solution

Problem