Re: [R] Dynamic Dictionary Data Type?

From: Kjetil Brinchmann Halvorsen <kjetil_at_acelerate.com>
Date: Fri 03 Jun 2005 - 04:14:59 EST

Prof Brian Ripley wrote:

> On Thu, 2 Jun 2005, hadley wickham wrote:
>
>>> An environment is a hash table, and copying occurs only on
>>> modification,
>>> when any language would have to copy in this context.
>>
>
> Caveat: the default is new.env(hash=FALSE), so an environment is a
> hash table in one sense but not necessarily so in the strict sense.
>
>> Yes, I'm aware that copying only occurs on modification. However, it
>> was my understanding that
>>
>> a <- list(a =1)
>> a$b <- 4
>>
>> would create a new copy of a,
>
>
> Thomas was talking about environments, not lists (and $ works
> differently for them).
>
>> whereas in Java say
>>
>> HashMap a = new HashMap();
>> a.put("a", 1);
>> a.put("b", 2);
>>
>> wouldn't create a copy of a. (please bear in mind my java syntax is
>> very rusty!)
>>
>> Caching data implies updating at each step, thus possibly creating n
>> copies of the list. Is that wrong?
>
>
> It depends what you mean by `copy'. If you expand a hash table you at
> some point need to re-arrange the hashing of the entries. That's what
> will happen with an R environment or an R list too. The possible
> difference is that you might expect the table to have some room for
> expansion, and in your list example you did not give any.
>
> R's operations on lists make more copies than are strictly necessary,
> but it is not clear that this is more costly than the housekeeping
> that would otherwise be necessary. In a$b <- 4, the wrapper VECSXP is
> recreated and the pointers copied across, but the list elements are
> not copied. For
> a$a <- 4 it is probable that no copying is done (although if a2 <- a
> had been done previously, the pending recursive copy would then be done).
> (It is also possible that I have overlooked something in the rather
> complex code used to do these subassignments.)
>
The original poster asked for caching of results. here is an example using new.env():

memo <- function(fun) {

   mem <- new.env()
   function(x) {

       if (exists(as.character(x), envir=mem)) get(as.character(x), envir=mem, inherits=FALSE) else {

       val <- fun(x)
       assign(as.character(x), value=val, envir=mem)
       val }

   }
}

 > fib <- memo(function(n) if(n<=1) 1 else fib(n-1)+fib(n-2))  > system.time( fib(300) )
[1] 0.01 0.00 0.02 NA NA

ls(get("mem", env=environment(fib)))

   *output supressed*

To compare:
system.time( {fib2 <- function(n)if(n<=1)1 else fib2(n-1)+fib2(n-2);fib2(30)})

[1] 8.07 0.08 12.75 NA NA

(there is (at least) one problem with this solution: if the global workspace contains a
 variable `6`, it gives error. Why?)

Kjetil

-- 

Kjetil Halvorsen.

Peace is the most effective weapon of mass construction.
               --  Mahdi Elmandjra




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Received on Fri Jun 03 04:26:16 2005

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