[R] creating a reverse geometric sequence
Duncan Murdoch
murdoch.duncan at gmail.com
Sun May 23 20:12:18 CEST 2010
Erik Iverson wrote:
> Hello,
>
> Can anyone think of a non-iterative way to generate a decreasing geometric
> sequence in R?
>
> For example, for a hypothetical function dg, I would like:
>
> > dg(20)
> [1] 20 10 5 2 1
>
> where I am using integer division by 2 to get each subsequent value in the
> sequence.
>
>
> There is of course:
>
> dg <- function(x) {
> res <- integer()
> while(x >= 1) {
> res <- c(res, x)
> x <- x %/% 2
> }
> res
> }
>
> > dg(20)
> [1] 20 10 5 2 1
>
> This implementation of 'dg' uses an interative 'while' loop. I'm simply
> wondering if there is a way to vectorize this process?
>
Something like this should work, at least for integer bases:
base <- 2
len <- ceiling(log(x, base))
floor(x/base^(seq_len(len)-1))
Duncan Murdoch
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