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"
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. ______________________________________________ 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.
