Re: [R] Efficient way to find consecutive integers in vector?

From: Marc Schwartz <marc_schwartz_at_comcast.net>
Date: Thu, 20 Dec 2007 16:33:54 -0600

On Thu, 2007-12-20 at 22:43 +0100, Johannes Graumann wrote:
> Hi all,
>
> Does anybody have a magic trick handy to isolate directly consecutive
> integers from something like this:
> c(1,2,3,4,7,8,9,10,12,13)
>
> The result should be, that groups 1-4, 7-10 and 12-13 are consecutive
> integers ...
>
> Thanks for any hints, Joh

Not fully tested, but here is one possible approach:

> Vec

 [1] 1 2 3 4 7 8 9 10 12 13

Breaks <- c(0, which(diff(Vec) != 1), length(Vec))

> Breaks

[1] 0 4 8 10

> sapply(seq(length(Breaks) - 1),

         function(i) Vec[(Breaks[i] + 1):Breaks[i+1]]) [[1]]
[1] 1 2 3 4

[[2]]
[1] 7 8 9 10

[[3]]
[1] 12 13

For a quick test, I tried it on another vector:

set.seed(1)
Vec <- sort(sample(20, 15))

> Vec

 [1] 1 2 3 4 5 6 8 9 10 11 14 15 16 19 20

Breaks <- c(0, which(diff(Vec) != 1), length(Vec))

> Breaks

[1] 0 6 10 13 15

> sapply(seq(length(Breaks) - 1),

         function(i) Vec[(Breaks[i] + 1):Breaks[i+1]]) [[1]]
[1] 1 2 3 4 5 6

[[2]]
[1] 8 9 10 11

[[3]]
[1] 14 15 16

[[4]]
[1] 19 20

HTH, Marc Schwartz



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