[R] About classifier in RWeka

Achim Zeileis Achim.Zeileis at wu-wien.ac.at
Thu Jun 4 09:32:24 CEST 2009


On Wed, 3 Jun 2009, Huihua Lu wrote:

> Hi everyone,
>
> I have trouble to use RWeka, I tried: (w=weather dataset, all
> preditors are nominal)
>
>> m<-J48(play~., data=w)
>> e<-evaluate_Weka_classifier(m,cost = matrix(c(0,2,1,0),
> + ncol = 2),numFolds = 10, complexity = TRUE,seed = 123,
> + class = TRUE)
> it gives me exactly what I want, but when I tried the same classifier
> on the other published data: (iris dataset has all numeric preditors)
>
>> m<-J48(Species~., data=iris)
>> e <- evaluate_Weka_classifier(m,
> + cost = matrix(c(0,2,1,0), ncol = 2),
> + numFolds = 10, complexity=TRUE, class=TRUE)
> it threw me the error as below:
>
> Error in .jnew("weka/classifiers/Evaluation", instances, costMatrix) :
>  Failed to create object of class `weka/classifiers/Evaluation'
> In addition: Warning message:
> In .jnew("weka/classifiers/Evaluation", instances, costMatrix) :
>  NewObject
> ("weka/classifiers/Evaluation","(Lweka/core/Instances;Lweka/classifiers
> /CostMatrix;)V",...) failed
>
> I know there are something wrong in the function of
> evaluate_Weka_classifier, but I am stuck. Any one can help me out?
> Is it because the attributes of predictors?  then, how can I set up
> the parameters in evaluat_Weka_classifier function? Thanks very much!

For me, it throws one more error, that you didn't include:

Exception in thread "main" java.lang.Exception: Cost matrix not compatible 
with data!
         at weka.classifiers.Evaluation.<init>(Unknown Source)

Your response has three levels, hence the cost matrix needs to be 3x3.

e <- evaluate_Weka_classifier(m,
   cost = matrix(c(0, 1, 2, 2, 0, 1, 1, 1, 0), ncol = 3),
   numFolds = 10, complexity=TRUE, class=TRUE)

works correctly but throws a warning. This warning is not critical, 
though, and should be fixed in the next RWeka version.

Best,
Z

> lu
>
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