[R] Recursive decreasing sequences

[Marc Schwartz]

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