[R] Compressing a sequence

[Ben Bolker]

   And some more from 2013: 
https://stackoverflow.com/questions/14868406/collapse-continuous-integer-runs-to-strings-of-ranges

   Can be as short as:

condense <- function(x)
   unname(tapply(x, c(0, cumsum(diff(x) != 1)), FUN = function(y)
     paste(unique(range(y)), collapse = "-")
   ))

z <- c(1, 3, 4, 5, 7, 8, 12, 13, 14, 15, 20)

condense(z) |> paste(collapse = ", ")

"1, 3-5, 7-8, 12-15, 20"

On 2025-02-21 9:16 p.m., Ben Bolker wrote:
>    There are some answers from 2016 here:
> 
> https://stackoverflow.com/questions/34636461/collapse-consecutive-runs- 
> of-numbers-to-a-string-of-ranges
> 
> On 2025-02-21 7:59 p.m., Steven Ellis wrote:
>> Hi Dennis,
>>
>> A quick Claude request:
>>
>> "using r I have a sequence like:        1, 3, 4, 5, 7, 8, 12, 13, 14, 15,
>> 20I would like to display it as:        1, 3-5, 7-8, 12-15, 20"
>>
>> yielded:
>>
>> condense_sequence <- function(nums) {
>>    if (length(nums) == 0) return("")
>>    if (length(nums) == 1) return(as.character(nums))
>>
>>    # Sort the numbers just in case they're not in order
>>    nums <- sort(unique(nums))
>>
>>    # Initialize variables
>>    ranges <- vector("character")
>>    start <- nums[1]
>>    prev <- nums[1]
>>
>>    for (i in 2:length(nums)) {
>>      if (nums[i] != prev + 1) {
>>        # End of a sequence
>>        if (start == prev) {
>>          ranges <- c(ranges, as.character(start))
>>        } else {
>>          ranges <- c(ranges, paste(start, prev, sep="-"))
>>        }
>>        start <- nums[i]
>>      }
>>      prev <- nums[i]
>>    }
>>
>>    # Handle the last number or range
>>    if (start == prev) {
>>      ranges <- c(ranges, as.character(start))
>>    } else {
>>      ranges <- c(ranges, paste(start, prev, sep="-"))
>>    }
>>
>>    # Join all ranges with commas
>>    paste(ranges, collapse=", ")
>> }
>>
>> # Your sequence
>> nums <- c(1, 3, 4, 5, 7, 8, 12, 13, 14, 15, 20)
>>
>> # Apply the function
>> result <- condense_sequence(nums)
>> print(result)
>> # Output: "1, 3-5, 7-8, 12-15, 20"
>>
>> Which appears to work well, though you may have other thoughts in mind /
>> edge cases this code does not cover.
>>
>> Best,
>> Steven
>>
>> On Fri, Feb 21, 2025 at 7:47 PM Dennis Fisher <fisher using plessthan.com> 
>> wrote:
>>
>>> R 4.4.0
>>> OS X
>>>
>>> Colleagues
>>>
>>> I have a sequence like:
>>>          1, 3, 4, 5, 7, 8, 12, 13, 14, 15, 20
>>>
>>> I would like to display it as:
>>>          1, 3-5, 7-8, 12-15, 20
>>>
>>> Any simple ways to accomplish this?
>>>
>>> Dennis
>>>
>>>
>>> Dennis Fisher MD
>>> P < (The "P Less Than" Company)
>>> Phone / Fax: 1-866-PLessThan (1-866-753-7784)
>>> www.PLessThan.com
>>>
>>>
>>>          [[alternative HTML version deleted]]
>>>
>>> ______________________________________________
>>> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> PLEASE do read the posting guide
>>> https://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>>>
>>
>>     [[alternative HTML version deleted]]
>>
>> ______________________________________________
>> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide https://www.R-project.org/posting- 
>> guide.html
>> and provide commented, minimal, self-contained, reproducible code.
> 

-- 
Dr. Benjamin Bolker
Professor, Mathematics & Statistics and Biology, McMaster University
Director, School of Computational Science and Engineering
 > E-mail is sent at my convenience; I don't expect replies outside of 
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