From ted.harding at nessie.mcc.ac.uk  Mon Jul 16 16:14:40 2007
From: ted.harding at nessie.mcc.ac.uk ( (Ted Harding))
Date: Mon, 16 Jul 2007 15:14:40 +0100 (BST)
Subject: [Rd] formula(CO2)
In-Reply-To: <XFMail.070716145756.ted.harding@nessie.mcc.ac.uk>
Message-ID: <XFMail.070716151440.ted.harding@nessie.mcc.ac.uk>

On 16-Jul-07 13:57:56, Ted Harding wrote:
> On 16-Jul-07 13:28:50, Gabor Grothendieck wrote:
>> The formula attribute of the builtin CO2 dataset seems a bit strange:
>> 
>>> formula(CO2)
>> Plant ~ Type + Treatment + conc + uptake
>> 
>> What is one supposed to do with that?  Certainly its not suitable
>> for input to lm and none of the examples in ?CO2 use the above.
> 
> I think one is supposed to ignore it! (Or maybe be inspired to
> write a mail to the list ... ).
> 
> I couldn't find anything that looked like the above formula from
> str(CO2). But I did spot that the order of terms in the formula:
> Plant, Type, treatment, conc, uptake, is the same as the order
> of the "columns" in the dataframe.
> 
> So I tried:
> 
>   D<-data.frame(x=(1:10),y=(1:10))
> 
>   formula(D)
>   x ~ y
> 
> So, lo and behold, D has a formula!
> 
> Or does it? Maybe if you give formula() a dataframe, it simply
> constructs one from the "columns".

Now that I think about it, I can see a use for this phenomenon:

  > formula(CO2)
  Plant ~ Type + Treatment + conc + uptake
  > formula(CO2[,2:5])
  Type ~ Treatment + conc + uptake
  > formula(CO2[,3:5])
  Treatment ~ conc + uptake
  > formula(CO2[,4:5])
  conc ~ uptake
  > formula(CO2[,c(5,1,2,3,4)])
  uptake ~ Plant + Type + Treatment + conc


Could save a lot of typing!

Best wishes,
Ted.

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Date: 16-Jul-07                                       Time: 15:14:38
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[Rd] formula(CO2)
 (Ted Harding