[R] Double And average a vector
Petr Savicky
savicky at cs.cas.cz
Sat Mar 24 12:25:35 CET 2012
On Sat, Mar 24, 2012 at 03:49:28AM -0700, Alaios wrote:
> Dear all,
> I am reading with R some measurements vectors from an external device.
> The length of each vector (containing numbers) is not constant.
>
> I would like by doubling the numbers of elements I select to shrink the vector to a new one
>
> Let's call the vector I read from the device as oldvector
>
>
> and the shrinked version of it as newVersion
>
> I want to have
> newVersion[1]<- mean(oldvector[1])
> newVersion[2]<- mean(oldvector[2:3])
> newVersion[3]<- mean(oldvector[4:7])
> newVersion[4]<- mean(oldvector[8:15])
>
> as I said the length of oldvector is not the same sol it might be also the case that the last window will not find the corresponding number of elements.
>
> How I can handle that in R?
Hi.
You asked a similar question at
https://stat.ethz.ch/pipermail/r-help/2012-March/306188.html
The current formulation is more accurate. Let me recall one of
the previous solutions suitable for the current situation
oldvector <- 1:20
ilog <- floor(log2(1:length(oldvector)))
ilog
[1] 0 1 1 2 2 2 2 3 3 3 3 3 3 3 3 4 4 4 4 4
newVersion <- unname(tapply(oldvector, ilog, FUN = mean))
newVersion
[1] 1.0 2.5 5.5 11.5 18.0
Here, newVersion[1] is mean(oldvector[which(ilog == 0)]), where
which(ilog == 0) # [1] 1
newVersion[1] is mean(oldvector[which(ilog == 1)]), where
which(ilog == 1) # [1] 2 3
also
which(ilog == 2) # [1] 4 5 6 7
which(ilog == 3) # [1] 8 9 10 11 12 13 14 15
which(ilog == 4) # [1] 16 17 18 19 20
The last group ends at 20, so it is smaller than the previous one.
Hope this helps.
Petr Savicky.
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