Re: [R] randomForest

From: Martin Maechler <maechler_at_stat.math.ethz.ch>
Date: Mon 11 Jul 2005 - 16:48:36 EST

>>>>> "Duncan" == Duncan Murdoch <murdoch@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



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