[R] lapply() reccursively

[Kaveh Vakili]

Thanks,  this is the closest to what i want (gives the same result as cumprod()) ...but using this function seems actually slower than the loop (is it normal ?):

a1<-runif(100000)
cadd<-function(x) Reduce("*", x, accumulate = TRUE)
looop<-function(a1){
j<-length(a1)
for(i in 2:j){
a1[i]<-a1[i-1]*a1[i]
}
a1
}
> 
> system.time(cadd(a1))
   user  system elapsed 
  1.344   0.004   1.353 
> system.time(cumprod(a1))
   user  system elapsed 
  0.004   0.000   0.002 
> system.time(looop(a1))
   user  system elapsed 
  0.772   0.000   0.775 
> 


>On Tue, 13 Oct 2009, Kaveh Vakili wrote:
>
>>
>> Hi all,
>>
>> I was wondering whether it is possible to use the lapply() function
>> to alter the value of the input, something in the spirit of :
>>
>> a1<-runif(100)
>> a2<-function(i){
>> a1[i]<-a1[i-1]*a1[i];a1[i]
>> }
>> a3<-lapply(2:100,a2)
>>
>> Something akin to a for() loop, but using the lapply() infrastructure.
>> I haven't been able to get rapply() to do this.
>
>Maybe you want to check out
>
> 	?Reduce
>
>For the example above, something like
>
>a3 <-  Reduce( "*", a1, accumulate = TRUE )
>
>
>HTH,
>
>Chuck
>
>>
>> The reason is that the "real" a2 function is a difficult function that only needs to be evaluated if the value of a1[i-1] meets some criteria.
>>
>> Thanks in advance,
>>
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>> and provide commented, minimal, self-contained, reproducible code.
>>
>
>Charles C. Berry                            (858) 534-2098
>                                             Dept of Family/Preventive Medicine
>E mailto:cberry at tajo.ucsd.edu	            UC San Diego
>http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901
>
>
>
>