[R] randomForest

Weiwei Shi helprhelp at gmail.com
Mon Jul 11 16:41:41 CEST 2005

```Thanks.
Many people pointed that out. (It was due to that I only knew lappy by
that time :).

On 7/11/05, Martin Maechler <maechler at stat.math.ethz.ch> wrote:
> >>>>> "Duncan" == Duncan Murdoch <murdoch at stats.uwo.ca>
> >>>>>     on Thu, 07 Jul 2005 15:44:38 -0400 writes:
>
>     Duncan> On 7/7/2005 3:38 PM, Weiwei Shi wrote:
>     >> Hi there:
>     >> I have a question on random foresst:
>     >>
>     >> recently i helped a friend with her random forest and i came with this problem:
>     >> her dataset has 6 classes and since the sample size is pretty small:
>     >> 264 and the class distr is like this (Diag is the response variable)
>
>     >> sample.size <- lapply(1:6, function(i) sum(Diag==i))
>     >>> sample.size
>     >> [[1]]
>     >> [1] 36
>
>     ....
>
> and later you get problems because you didn't know that a *list*
> such as 'sample.size' should be made into a so called
> *atomic vector* {and there's a function  is.atomic(.) ! to test for it}
> and Duncan and others told you about unlist().
>
> Now there are two things I'd want to add:
>
> 1) If you had used
>
>       s.size <- table(Diag)
>
>    you had used a faster and simpler expression with the same result.
>    Though in general (when there can be zero counts), to give the
>    same result, you'd need
>
>       s.size <- table(factor(Diag, levels = 1:6))
>
>    Still a bit preferable to the lapply(.) IMO
>
>
> 2)  You should get into the habit of using
>      sapply(.)   rather than  lapply(.).
>
>     sapply() originally was exactly devised for the above task:
>     and stands for ``[s]implified lapply''.
>
>     It always returns an ``unlisted'' result when appropriate.
>
> Regards,
> Martin Maechler, ETH Zurich
>
>

--
Weiwei Shi, Ph.D

"Did you always know?"
"No, I did not. But I believed..."
---Matrix III

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