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

[ken knoblauch]

ken knoblauch <ken.knoblauch <at> inserm.fr> writes:

> 
> 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
> 
Actually, combining Ista and my responses, 
a general response could be
something like

ifelse(v %% n, v, function(v){} )

where you have set n and define some
function

HTH,

Ken

-- 
Ken Knoblauch
Inserm U846
Stem-cell and Brain Research Institute
Department of Integrative Neurosciences
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