# Re: [R] how to store recursive results

From: Thomas Lumley <tlumley_at_u.washington.edu>
Date: Fri 22 Sep 2006 - 16:19:34 GMT

On Fri, 22 Sep 2006, X.H Chen wrote:

> Hi Gabor,
>
> Thanks for pointing out this for me. However, what I try to get is how to
> construct such form a function f that
>
> ret<-f(...),
>
> where ret contains the each recursive result from f, and meantime f consists
> of no <<- operator. Do you have any idea how to implemet this. Thanks a lot
>

It depends on the situation. You can always pass the results back in a list or vector, eg

cumfactorial<-function(n){

``` 	           if (n==0)
1
else c(1, n*cumfactorial(n-1))
```
}

If you want to get the results out then you have to either accumulate and return them like this or use <<-, since return() and <<- are the only ways to get results out of a function. As long as you don't use <<- to assign to variables outside the function it is a perfectly reasonable thing to do

If you were doing something like a Fibonacci sequence then assigning would be preferable

```     fib<-function(n){
memo<-new.env(hash=TRUE)
fibrec<-function(m){
if (m<=2) return(1)
vm<-paste("v",m,sep="")
if(exists(vm,envir=memo,inherits=FALSE))
return(get(vm,envir=memo,inherits=FALSE))
rval<-fibrec(m-1)+fibrec(m-2)
assign(vm,rval,envir=memo)
rval
}
fibrec(n)
sapply(ls(envir=memo),get, envir=memo)
}

```

since the memoization converts the algorithm from exponential time to linear time.

R-help@stat.math.ethz.ch 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. Received on Sat Sep 23 02:27:38 2006

Archive maintained by Robert King, hosted by the discipline of statistics at the University of Newcastle, Australia.
Archive generated by hypermail 2.1.8, at Fri 22 Sep 2006 - 17:30:19 GMT.

Mailing list information is available at https://stat.ethz.ch/mailman/listinfo/r-help. Please read the posting guide before posting to the list.