[R] Compressing a sequence

[Ben Bolker]

   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
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