[R] a sequence that wraps around

[William Dunlap]

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Jack Tanner
> Sent: Wednesday, September 16, 2009 7:08 AM
> To: r-help at stat.math.ethz.ch
> Subject: [R] a sequence that wraps around
> 
> I'd like to have something like seq() where I can pass in a 
> length of the
> desired sequence and a right limit so that the sequence goes 
> up to the limit and
> then starts again from 1.
> 
> # works now
> seq(from=2, length.out=3)
> [1] 2 3 4
> 
> # what I want
> seq(from=2, length.out=3, rlimit=3)
> [1] 2 3 1
> 
> # additional examples of what I want
> seq(from=2, length.out=4, rlimit=3)
> [1] 2 3 1 2
> seq(from=2, length.out=4, rlimit=4)
> [1] 2 3 4 1
> seq(from=2, length.out=3, rlimit=2)
> [1] 2 1 2
> 
> I can write this procedurally, but it seems like there ought 
> to be a cleaner R
> way of doing it. Thanks in advance for any suggestions.

The remainder (modulus) operator, x%%n, wraps to the range 0..n-1
but you can write a function that wraps to 1..n as
   mod1 <- function(x, n) (x-1L)%%n + 1L
Then pass the output of seq through that:
   > mod1(-2:10, 3)
    [1] 1 2 3 1 2 3 1 2 3 1 2 3 1

Bill Dunlap
TIBCO Software Inc - Spotfire Division
wdunlap tibco.com  

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