r - Collapse continuous integer runs to strings of ranges

Question

I have some data in a list that I need to look for continuous runs of integers (My brain thinkrle but don't know how to use it here).

It's easier to look at the data set and explain what I'm after.

Here's the data view:

$greg
 [1]  7  8  9 10 11 20 21 22 23 24 30 31 32 33 49

$researcher
[1] 42 43 44 45 46 47 48

$sally
 [1] 25 26 27 28 29 37 38 39 40 41

$sam
 [1]  1  2  3  4  5  6 16 17 18 19 34 35 36

$teacher
[1] 12 13 14 15

Desired output:

$greg
 [1]  7:11, 20:24, 30:33, 49

$researcher
 [1] 42:48

$sally
 [1] 25:29, 37:41

$sam
 [1]  1:6, 16:19 34:36

$teacher
 [1] 12:15

Use base packages how can I replace continuous span with a colon between highest and lowest and commas in between non the non continuous parts? Note that the data goes from a list of integer vectors to a list of character vectors.

MWE data:

z <- structure(list(greg = c(7L, 8L, 9L, 10L, 11L, 20L, 21L, 22L, 
    23L, 24L, 30L, 31L, 32L, 33L, 49L), researcher = 42:48, sally = c(25L, 
    26L, 27L, 28L, 29L, 37L, 38L, 39L, 40L, 41L), sam = c(1L, 2L, 
    3L, 4L, 5L, 6L, 16L, 17L, 18L, 19L, 34L, 35L, 36L), teacher = 12:15), .Names = c("greg", 
    "researcher", "sally", "sam", "teacher"))

Answer 1

I think diff is the solution. You might need some additional fiddling to deal with the singletons, but:

lapply(z, function(x) {
  diffs <- c(1, diff(x))
  start_indexes <- c(1, which(diffs > 1))
  end_indexes <- c(start_indexes - 1, length(x))
  coloned <- paste(x[start_indexes], x[end_indexes], sep=":")
  paste0(coloned, collapse=", ")
})

$greg
[1] "7:11, 20:24, 30:33, 49:49"

$researcher
[1] "42:48"

$sally
[1] "25:29, 37:41"

$sam
[1] "1:6, 16:19, 34:36"

$teacher
[1] "12:15"