[R] Question about rpart(sth~.,database)

[Grześ]

Gavin Simpson wrote:
> 
> Grześ wrote:
>> I have a standard database - HouseVotes84 
>> For example:
>>        Class   V1 V2 V3   V4 V5 V6 V7 V8 V9 V10  V11 V12 V13 V14 V15  V16
>> 1 republican    n  y  n    y  y  y  n  n  n   y <NA>   y   y   y   n    y
>> 2 republican    n  y  n    y  y  y  n  n  n   n    n   y   y   y   n <NA>
>> 3 democrat <NA>  y  y <NA>  y  y  n  n  n   n    y   n   y   y   n    n
>>      .
>>      .
>>      .
>> end I build a tree like this: 
>>> hv.tree1=rpart(Class~.,HouseVotes84)
>> everything is ok! My question is:
>> What exactly mean "Class~.,"? 
> 
> It means include all remaining variables in HouseVotes84 on the rhs of 
> the formula, i.e. as variables that should be used to predict the Class 
> variable.
> 
>> 
>> Why when I use "Class~.," - then I get the best solution but when I use
>> as a
>> parameter like this:
>>> hv.tree2=rpart(V2~.,HouseVotes84)
> 
> Why does this surprise you? You are now trying to predict the variable 
> V2 (y/n) from Class and all remaining variables.
> 
>> I also get solution but not such good like before.  
> 
> They are solutions to two different problems.
> 
> If you want to predict Class, then you need
> 
> Class ~ ., data = HouseVotes84
> 
> or, to specify exactly which variables to use as predictors of Class, 
> state them explicitly:
> 
> Class ~ V1 + V3 + V4, data = HouseVotes84
> 
> I think you should look at the documentation that comes with R (An 
> Introduction to R) or some of the contributed help documents on the R 
> Website to read up on model formulae and how to represent models using 
> this notation.
> 
> HTH
> 
> G
> 
> ______________________________________________
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> and provide commented, minimal, self-contained, reproducible code.
> 
> 

Thank you very much!

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