On Aug 26, 2010, at 10:35 AM, Niels Richard Hansen wrote: > > > On 26/08/10 09.30, Dimitris Rizopoulos wrote: >> I think you need an I(), i.e., >> >> form <- ~ I(a > 1) - 1 > > Yes, it solves the concrete problem, but does not really answer > the question. Let me rephrase. Should the use of terms like (a>1) > be discouraged in R and replaced by I(a>1) systematically, or is the > behavior below unexpected?
I'd say "yes", and "maybe". I think the only constructs that are guaranteed to protect arithmetic expressions are function calls, including I(). So it is more or less coincidental that ~ (a>1) - 1 works in the first place; when updating it, you probably should "expect the unexpected". There could still be a bug -- it looks somewhat wrong that a>1 isn't retained as a call to "<" with arguments a and 1, parentheses or not, but the model formula expansion code involves deep and dark magic, and I'd just avoid going too near it. -- Peter Dalgaard Center for Statistics, Copenhagen Business School Solbjerg Plads 3, 2000 Frederiksberg, Denmark Phone: (+45)38153501 Email: pd....@cbs.dk Priv: pda...@gmail.com ______________________________________________ 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.
