[R] Nesting Functions

[Duncan Murdoch]

On 1/27/2006 9:07 PM, Manuel Morales wrote:
> On Thu, 2006-01-26 at 21:55 -0500, Duncan Murdoch wrote:
>> On 1/26/2006 9:45 PM, Manuel Morales wrote:
>>> Dear list members,
>>>
>>> I'm looking for a way to write "nested" functions similar to the
>>> function "Nest" or "NestList" in Mathematica.
>>>
>>> E.g.,
>>>
>>> f<-function(x) x+2*x
>>>
>>> f(f(f(2)))
>>>
>>> might instead be written as nest(f, 2, 3)
>>>
>>> read as, nest function f 3 times with 2 as the initial value.
>> It's easy enough using a for loop:
>>
>> nest <- function(f, initial, reps) {
>>     result <- initial
>>     for (i in seq(len=reps)) result <- f(result)
>>     result
>> }
>>
>> Duncan Murdoch
> That works, thanks! But what if I want to apply the function to a set of
> vectors.
> 
> init.values<-c(3,10,20)
> rep.values<-c(0,1,2)
> 
> nest(f,init.values,rep.values) fails because only the first value is
> used in a for loop. The following works, but it's clunky and doesn't
> scale with variation in the number of reps.
> 
> nest.vectorize<-function(f, initial, reps)
>     ifelse(reps==0,initial,
>     ifelse(reps==1,f(initial),f(f(initial))))
> 
> nest.vectorize(f,init.values,rep.values)
> 
> Any suggestions?

Something like this would work:

nest <- function(f, initial, reps) {
      result <- initial
      for (i in seq(len=max(reps))) result <- ifelse(i <= reps,
					            f(result), result)
      result
  }