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

[Gavin Simpson]

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