On Sep 13, 2012, at 11:53 AM, William Dunlap wrote:
> Your method would not work for, e.g., "a:d". You could look at the "factors"
> attribute
> of a terms object and select out those columns with non-zero entries for the
> variables
> in the interaction of interest. E.g.,
>> fm <- attr(terms(~a*b*c*d), "factors")
>> fm
> a b c d a:b a:c b:c a:d b:d c:d a:b:c a:b:d a:c:d b:c:d a:b:c:d
> a 1 0 0 0 1 1 0 1 0 0 1 1 1 0 1
> b 0 1 0 0 1 0 1 0 1 0 1 1 0 1 1
> c 0 0 1 0 0 1 1 0 0 1 1 0 1 1 1
> d 0 0 0 1 0 0 0 1 1 1 0 1 1 1 1
>> colnames(fm)[fm["b",]==0 | fm["c",]==0]
> [1] "a" "b" "c" "d" "a:b" "a:c" "a:d" "b:d" "c:d"
> "a:b:d" "a:c:d"
>
It's probably a black mark against my abilities to do logic manipulations, but
it made a lot more sense when I wrote it (admittedly the same meaning) as :
colnames(fm)[ !(fm["b",]==1 & fm["c",]==1) ]
Here's a grepping method that only requires that the order be a.d in any term:
> as.formula(paste("~", paste(
grep("a.+d", attr(terms(~a*b*c*d), "term.labels" ) ,
invert=TRUE, value=TRUE), collapse="+") ) )
~a + b + c + d + a:b + a:c + b:c + b:d + c:d + a:b:c + b:c:d
I think that if you are working with a*b*c*d that the order will always be
a-before-d.
--
David.
> Bill Dunlap
> Spotfire, TIBCO Software
> wdunlap tibco.com
>
>
>> -----Original Message-----
>> From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On
>> Behalf
>> Of David Winsemius
>> Sent: Thursday, September 13, 2012 11:28 AM
>> To: Bert Gunter
>> Cc: Alexander Shenkin; r-help@r-project.org
>> Subject: Re: [R] remove all terms with interaction factor in formula
>>
>>
>> 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 <ashen...@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
>>>>
>>>> ______________________________________________
>>>> R-help@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.
>>>
>>>
>>>
>>> --
>>>
>>> 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@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
>>
>> ______________________________________________
>> R-help@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
______________________________________________
R-help@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.
