[R] c(1:n, 1:(n-1), 1:(n-2), ... , 1)

[William Dunlap]

If you are interested in speed for long input vectors try the following,
which should give the same result as sequence().
mySequence <-function (nvec)
{
    nvec <- as.integer(nvec)
    seq_len(sum(nvec)) - rep(cumsum(c(0L, nvec[-length(nvec)])),
        nvec)
}

E.g.,
> n <- rpois(1e6, 3)
> system.time(mySequence(n))
   user  system elapsed
   0.07    0.00    0.07
> system.time(sequence(n))
   user  system elapsed
   0.88    0.00    0.87
> identical(mySequence(n), sequence(n))
[1] TRUE


Bill Dunlap
TIBCO Software
wdunlap tibco.com


On Thu, Sep 17, 2015 at 12:53 PM, Dan D <ddalthorp at usgs.gov> wrote:
> Very nice variety of solutions to create c(1:n, 1:(n-1), 1:(n-2), ... , 1)
>
> #Testing the methods with n=1000 (microbenchmark)
> n<-1000
>
> # by far the nicest-looking, easiest to follow, and fastest is Frank
> Schwidom's:
> # it also requires the minimum amount of memory (as do several of the
> others)
> # 2.73 milliseconds (1x)
> sequence(n:1)
>
> # not nearly as nice-looking but almost as fast:
> # 2.82 milliseconds (1.03x)
> do.call(c, lapply(n:1, function(n1) 1:n1))
>
> # an improvement on look but 5x slower than do.call is:
> # 13.3 milliseconds  (4.9x)
> unlist(lapply(n:1, seq))
>
> ## the others are uglier and way slower [uses a full (n+1) x (n+1) matrix]
> # 60.8 milliseconds (22.3x)
> outer( 1:(n+1), 1:(n+1), '-')[ outer( 1:n, 1:(n+1), '>')]
>
> # 71.8 milliseconds (26.3x) [uses a full (n x n) matrix]
> junk<-array(1:n,dim=c(n,n))
> junk[((lower.tri(t(junk),diag=T)))[n:1,]]
>
> # 421.3 milliseconds (154x)
> Reduce( function(x,y){c( 1:y, x)}, 1:n)
>
> # 3200 milliseconds (1170x)
> cc<-0; # establish result as numeric
> for(i in seq(n,1,-1)){ cc<-c(cc,seq(1,i)); str(cc); }; #generate array
>     cc<-cc[2:length(cc)]; #remove the leading 0
> } #
>
> # crashes:
> mklist <- function(n) {
>      if (n==1) return(1) else return( c(seq(1,n),mklist(n-1)) )
> }
>
>
>
> --
> View this message in context: http://r.789695.n4.nabble.com/c-1-n-1-n-1-1-n-2-1-tp4712390p4712399.html
> Sent from the R help mailing list archive at Nabble.com.
>
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