On May 23, 2010, at 1:43 PM, 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.
> dg <- function(ratio, len) (ratio)^( 0:(len-1) )
> 20*dg(.5, 20)
[1] 2.000000e+01 1.000000e+01 5.000000e+00 2.500000e+00 1.250000e+00
6.250000e-01
[7] 3.125000e-01 1.562500e-01 7.812500e-02 3.906250e-02 1.953125e-02
9.765625e-03
[13] 4.882812e-03 2.441406e-03 1.220703e-03 6.103516e-04 3.051758e-04
1.525879e-04
[19] 7.629395e-05 3.814697e-05
> (20*dg(.5, 20))[1:19] / (20*dg(.5, 20))[2:20]
[1] 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
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?
Thanks,
Erik
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David Winsemius, MD
West Hartford, CT
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