Represent a numeric vector as a set of characters using :

Question

In R, you can define an arbitrary integer sequence using :, e.g.

a = c(1:3, 12:14)
print(a)
## 1 2 3 12 13 14

I'm looking for a way to do the inverse operation, e.g. given a vector of integers I want to produce a character (or character vector) that collapses the integer sequence(s) to the equivalent expressions using :, e.g.

some_function (a)
## "1:3" "12:14"

Bonus if the stride can be detected, e.g. c(1, 3, 5) becomes "1:2:5" or something like that.

Motivation: generate an integer sequence in R based on some data manipulation to identify database row selection, and pass the most concise representation of that sequence to an external program in the proper format.

You can play around with `deparse`, e.g. `deparse(c(1L, 2L, 3L))`. This is probably a dupe of https://stackoverflow.com/questions/34636461/function-to-summarize-vector-of-numbers-as-a-string — David Arenburg, Apr 01 '18 at 17:08
Woah, I had no idea `deparse` worked like that. Thanks for the tip and link @David, you are right my question is very similar to that older question... but now we have some answers for variable-stride sequences, so I think it's worth keeping the question up. — mikeck, Apr 02 '18 at 16:36

score 1 · Answer 1 · answered Apr 01 '18 at 17:47

Ah, nerd heaven. Here's a first shot. You could even use this for encoding within R.

Needs testing; code always prints the stride out.

encode_ranges <- function (x) {
  rle_diff <- list(
    start = x[1],
    rled   = rle(diff(x))
  )

  class(rle_diff) <- "rle_diff"
  rle_diff
}

decode_ranges <- function (x) {
  stopifnot(inherits(x, "rle_diff"))
  cumsum(c(x$start, inverse.rle(x$rled)))
}

format.rle_diff <- function (x, ...) {
  stopifnot(inherits(x, "rle_diff"))
  output <- character(length(x$rled$values))

  start <- x$start
  for (j in seq_along(x$rled$values)) {
    stride <- x$rled$values[j]
    len    <- x$rled$lengths[j]
    if (len == 1L) {
      start <- end + stride
      next
    }
    end       <- start + stride * x$rled$lengths[j]
    output[j] <- paste(start, end, stride, sep = ":")
  }

  output <- output[nchar(output) > 0]
  paste(output, collapse = ", ")
}

print.rle_diff <- function (x, ...) cat(format(x, ...))

encode_ranges(c(1:3, 12:14))
encode_ranges(c(1, 3, 5, 8:10, 14, 17, 20))

Onyambu · Accepted Answer · 2018-04-01T18:34:44.320

1

We can be able to take into consideration the rle of the differences and paste the range together taking into consideration the sequence distance.

fun=function(s){
  m=c(0,diff(s))
  b=rle(m)
  b$values[b$lengths==1&b$values!=1]=0
  l=cumsum(!inverse.rle(b))
  d=function(x)paste0(range(x[,1]),
                      collapse = paste0(":",unique(x[-1,-1]),":"))
  f=c(by(cbind(s,m),l,d))
  sub("::.*","",sub(":1:",":",f))
}   

fun(c(1,1:3,12:14,c(1,3,5)))
      1       2       3       4 
    "1"   "1:3" "12:14" "1:2:5" 
fun(c(1, 3, 5, 8:10, 14, 17, 20))
        1         2         3 
  "1:2:5"    "8:10" "14:3:20" 

fun(1)
  1 
"1"

edited Apr 01 '18 at 18:34

answered Apr 01 '18 at 17:48

Onyambu

67,392
3
24
53

This is great. I was wondering if `rle` cold be used in this case but I couldn't figure out how... I'm curious about how this will perform for larger sequences, but it gets the job done just how I want it! – mikeck Apr 02 '18 at 16:38

score 0 · Answer 3 · answered Apr 01 '18 at 16:54

0

We create a grouping variable with diff and cumsum, then use on the group by functions to paste the range of values

f1 <- function(vec) {
  unname(tapply(vec, cumsum(c(TRUE, diff(vec) != 1)), 
       FUN = function(x) paste(range(x), collapse=":")))
  }

f1(a) 
#[1] "1:3"   "12:14"

For the second case

b <- c(1, 3, 5)
un1 <- unique(diff(c(1, 3, 5)))
paste(b[1], un1, b[length(b)], sep=":")
#[1] "1:2:5"

answered Apr 01 '18 at 16:54

akrun

874,273
37
540
662

`f1` doesn't work for the trivial case `f1(1)`. Second example doesn't work for arbitrary combinations, e.g. `b = c(1,3,5,6,7,9)` yields the incorrect answer `1:2:9`. – mikeck Apr 02 '18 at 16:35

Represent a numeric vector as a set of characters using :

3 Answers3