Collapse consecutive runs of numbers to a string of ranges

Question

Let\'s say I have the following vector of numbers:

vec = c(1, 2, 3, 5, 7, 8, 9, 10, 11, 12)

I\'m looking for a function that will create a

Answer 1

I assume that the vector is sorted as in the example. If not use vec <- sort(vec) beforehand.

Edit note: @DavidArenburg spotted a mistake in my original answer where c(min(x), x) should actually be c(0, x). Since we know now that we always need to add a 0 in the first place, we can omit the first step of creating x and do it "on the fly". The original answer and additional options are now edited to reflect that (you can check the edit history for the original post). Thanks David!

A note on calls to unname: I used unname(sapply(...)) to ensure that the resulting vector is not named, otherwise it would be named 0:(n-1) where n equals the length of new_vec. As @Tensibai noted correctly in the comments, this doesn't matter if the final aim is to generate a length-1 character vector as produced by running toString(new_vec) since vector names will be omitted by toString anyway.

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One option (possibly not the shortest) would be:

new_vec <- unname(sapply(split(vec, c(0, cumsum(diff(vec) > 1))), function(y) {
  if(length(y) == 1) y else paste0(head(y, 1), "-", tail(y, 1))
}))

Result:

new_vec
#[1] "1-3"  "5"    "7-12"
toString(new_vec)
#[1] "1-3, 5, 7-12"

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Thanks to @Zelazny7 it can be shortened by using the range function:

new_vec <- unname(sapply(split(vec, c(0, cumsum(diff(vec) > 1))), function(y) {
    paste(unique(range(y)), collapse='-')
}))

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Thanks to @DavidArenburg it can be further shortened by using tapply instead of sapply + split:

new_vec <- unname(tapply(vec, c(0, cumsum(diff(vec) > 1)), function(y) {
  paste(unique(range(y)), collapse = "-")
}))