R help - Aug 2010 - a question regarding updating formulas with coefficients

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 I want to add a variable p, so
> update(y~d+0*r+t, .~.+p)
produces

y ~ d + t + p - 1

If the coefficient is not 0, but rather, something else - say, 3, 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?

-Jarrett

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?
> 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

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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> PLEASE do read the posting guide
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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