[R] a question regarding updating formulas with coefficients

[Charles C. Berry]

On Wed, 11 Aug 2010, David Winsemius wrote:

>
> On Aug 11, 2010, at 6:03 PM, Jarrett Byrnes wrote:
>
>> I have formulae with coefficents that I would like to update.  However, I 
>> get some strange results.  For example, see the following:
>> 
>> For the formula y ~ d+ 3*r+t
>
> Did you really get meaningful results from that formula? Care to provide an 
> example? Maybe there is something more for me to learn.
>
>
>> I want to add a variable p, so
>> 
>> > update(y~d+0*r+t, .~.+p)
>
> In formulas the "*" operator is an interaction creator. so you told R to make 
> 0 + r + 0:r. Probably not what you thought you were doing. So what were you 
> trying to do anyway?
>
>> 
>> produces
>> 
>> y ~ d + t + p - 1
>
> Which at least explains why you got the -1 (which in R formulas is that same 
> as +0).
>> 
>> If the coefficient is not 0, but rather, something else - say, 3,
>
> What do you think you are accomplishing when you put a scalar coefficient in 
> the formula?
>

Maybe he wants

 	?I

??

Chuck

>> I get the following:
>> 
>> > update(y~d+3*r+t, .~.+p)
>> 
>> Error in terms.formula(tmp, simplify = TRUE) :
>>  invalid model formula in ExtractVars
>> > 
>> 
>> 
>> Is there a way to do this,
>
>> or a different call I should be trying?
>
> What you should be doing depends on what you want to happen.
>
> -- 
>
> David Winsemius, MD
> West Hartford, CT
>
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> and provide commented, minimal, self-contained, reproducible code.
>

Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	            UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901