R sequence function (was: Re: [R] Recursive decreasing sequences)

On Fri, 2006-10-20 at 12:51 -0700, Julian Burgos wrote:

Hello fellow R's,

I'm sure there must be an easy way to do this.  But after digging in the
documentation and thinking about it for a while I couldn't figure it
out.  I need to get a decreasing recursive vector in.  I mean something
like this: if starting at 2, and ending at 6, the vector should be

 2 3 4 5 6 3 4 5 6 4 5 6 5 6 6

An easy way would be to do this

    x <- integer(0)
    for (i in 5) x <- c(x, i:5)

But I need to create really long vectors (where the ending value is in
the order of 6500) , and using loops is way to slow.  I'm looking for a
vectorized method.  Any help will be welcomed.

How about this:

Range <- c(2:6)

unlist(sapply(seq(along = Range), function(x) Range[x]:max(Range)))

 [1] 2 3 4 5 6 3 4 5 6 4 5 6 5 6 6


Then:

Range2 <- 2:6500

str(Range2)

 int [1:6499] 2 3 4 5 6 7 8 9 10 11 ...


system.time(res <- unlist(sapply(seq(along = Range2),
                                function(x) Range2[x]:max(Range2))))
[1] 0.492 0.136 0.647 0.000 0.000

str(res)

 int [1:21121750] 2 3 4 5 6 7 8 9 10 11 ...


HTH,

Marc Schwartz