[R] Apply function to every 'nth' element of a vector

[Ista Zahn]

Good morning Michael,

On Thu, Apr 5, 2012 at 7:01 AM, Michael Bach <phaebz at gmail.com> wrote:
> Dear R users,
>
> how do I e.g. square each second element of a vector with an even
> number of elements? Or more generally to apply a function to every
> 'nth' element of a vector. I looked into the apply functions, but
> found no hint.

Use indexing, or ifelse.

>
> For example:
>
> v <- c(1, 2, 3, 4)
> mysquare <- function (x) { return (x*x) }
> w <- applyfun(v, mysquare, 2)
>
> then w should be c(1, 4, 3, 16)

Here is one way:

 w <- ifelse(v %in% seq(1, length(v), 2), v, v^2)

Best,
Ista

>
> Thanks for your time,
> Michael Bach
>
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