[R] Loop with variable index
Rolf Turner
r.turner at auckland.ac.nz
Wed Jan 30 21:14:24 CET 2008
On 31/01/2008, at 8:58 AM, cvandy wrote:
>
> I have a list of 20 values.
***NO***! You have (or should have) a *vector* of 20 values.
Vectors and lists are different concepts. Learn and understand
the difference, else the world will come to an end.
> The first time through a loop I want to find the
> mean and stnd.dev. of the first two values; the second time through
> the loop
> I want to find the mean and stnd. dev. of the first 3 values, etc.
> until
> the last time through the loop I want to find the mean and stnd.
> dev. of all
> 20 values, so I end up with 19 means and stnd. deviations.
Why (on earth) would you want to do this?
> How would I construct such a loop?
Let the vector be ``x''.
mns <- list()
sds <- list()
for(i in 2:20) {
mns[[i-1]] <- mean(x[1:i])
sds[[i-1]] <- sd(x[1:i])
}
mns <- unlist(mns)
sds <- unlist(sds)
will do what you want.
cheers,
Rolf Turner
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