# [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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