[R] remove all terms with interaction factor in formula

[David Winsemius]

On Sep 13, 2012, at 11:00 AM, Bert Gunter wrote:

> ~ a*b*d + a*c*d

That seemed pretty clear and obvious, but I started wondering how to tell the machine to do it. Here is another idea:

> grep("b:c", attr(terms(~a*b*c*d), "term.labels" ) ,invert=TRUE, value=TRUE)
 [1] "a"     "b"     "c"     "d"     "a:b"   "a:c"   "a:d"   "b:d"   "c:d"   "a:b:d" "a:c:d"

(Although I realize it's no longer a formula and might need to be reassembled with `paste` and  `as.formula`.)

-- 
David.

> -- Bert
> On Thu, Sep 13, 2012 at 10:49 AM, Alexander Shenkin <ashenkin at ufl.edu> wrote:
>> Hi Folks,
>> 
>> I'm trying to find a way to remove all terms in a formula that contain a
>> particular interaction.
>> 
>> For example, in the formula below, I'd like to remove all terms that
>> contain the b:c interaction.
>> 
>>> attributes(terms( ~ a*b*c*d))$term.labels
>> [1] "a"       "b"       "c"       "d"       "a:b"     "a:c"
>> [7] "b:c"     "a:d"     "b:d"     "c:d"     "a:b:c"   "a:b:d"
>> [13] "a:c:d"   "b:c:d"   "a:b:c:d"
>> 
>> My eventual use is to fit models with the reduced formulas.
>> 
>> For example:
>>> my_df = data.frame( iv = runif(100), a=runif(100), b=runif(100),
>> c=runif(100), d=runif(100))
>>> lm(iv ~ a*b*c*d, data=my_df)
>> 
>> I can remove particular terms with update(), but I don't see a way to
>> remove all terms that contain a given combination of factors.
>> 
>> Any help would be greatly appreciated.  Thanks!
>> 
>> Allie
>> 
>> ______________________________________________
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>> and provide commented, minimal, self-contained, reproducible code.
> 
> 
> 
> -- 
> 
> Bert Gunter
> Genentech Nonclinical Biostatistics
> 
> Internal Contact Info:
> Phone: 467-7374
> Website:
> http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

David Winsemius, MD
Alameda, CA, USA