R help - Apr 2012 - Apply function to every 'nth' element of a vector

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.

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)

Thanks for your time,
Michael Bach

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
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

Michael Bach <phaebz <at> gmail.com> writes:
> 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.
> 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)
> Michael Bach
Hi Michael,

v^(2 - seq_along(v) %% 2)

[1]  1  4  3 16

Ken

-- 
Ken Knoblauch
Inserm U846
Stem-cell and Brain Research Institute
Department of Integrative Neurosciences
18 avenue du Doyen L?pine
69500 Bron
France
tel: +33 (0)4 72 91 34 77
fax: +33 (0)4 72 91 34 61
portable: +33 (0)6 84 10 64 10
http://www.sbri.fr/members/kenneth-knoblauch.html

On Apr 5, 2012, at 7:01 AM, Michael Bach 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.
>
> 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)
An alternate approach to the ifelse() solutions you have been offered  
is to use identical indexing on both sides of an assignment function.

 > v <- c(1, 2, 3, 4)
 > w <-v
 > w[seq(2, length(w), by =2)] <-w[seq(2, length(w), by =2)]^2
 > w
[1]  1  4  3 16

If you still wanted to create a function that would square each  
element "in place" you could do this with an indexing strategy:

  v <- c(1, 2, 3, 4)
  w <-v
  mysqr <- function (x) { eval.parent(substitute(x <- x^2))  }
  mysqr(w[seq(2, length(w), by =2)])
  w
#[1]  1  4  3 16

(Credit: http://www.statisticsblog.com/2011/10/waiting-in-line-waiting-on-r/ 
  and search on 'inc')
>
> Thanks for your time,
> Michael Bach
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
David Winsemius, MD
West Hartford, CT