Hi Jinsong and Thierry, >> (x1 + x2 + x3) ^2 will give you the main effects and the interactions.
Although it wasn't specifically requested it is perhaps important to note that (...)^2 doesn't expand to give _all_ interaction terms, only interactions to the second order, so the interaction term x1:x2:x3 will be missing, To get these, do (x1 + x2 + x3)^3 Regards, Mark. ONKELINX, Thierry wrote: > > (x1 + x2 + x3) ^2 will give you the main effects and the interactions. > So this will shorten your equation to > > ft <- lm(y ~ (x1 + x2 + x3) ^2 + I(x1 * x1)+ I(x2 * x2) + I(x3 * x3), > mydata) > > HTH, > > Thierry > > ------------------------------------------------------------------------ > ---- > ir. Thierry Onkelinx > Instituut voor natuur- en bosonderzoek / Research Institute for Nature > and Forest > Cel biometrie, methodologie en kwaliteitszorg / Section biometrics, > methodology and quality assurance > Gaverstraat 4 > 9500 Geraardsbergen > Belgium > tel. + 32 54/436 185 > [EMAIL PROTECTED] > www.inbo.be > > To call in the statistician after the experiment is done may be no more > than asking him to perform a post-mortem examination: he may be able to > say what the experiment died of. > ~ Sir Ronald Aylmer Fisher > > The plural of anecdote is not data. > ~ Roger Brinner > > The combination of some data and an aching desire for an answer does not > ensure that a reasonable answer can be extracted from a given body of > data. > ~ John Tukey > > -----Oorspronkelijk bericht----- > Van: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] > Namens Jinsong Zhao > Verzonden: zondag 27 juli 2008 9:39 > Aan: r-help@r-project.org > Onderwerp: [R] A easy way to write formula > > Hi > > I have a data frame, including x1, x2, x3, and y. I use lm() to fit > second-order linear model, like the following: > > ft <- lm(y ~ x1 + x2 + x3 + I(x1 * x1) + I(x1 * x2) + I(x1 * x3) + I(x2 > * x2) + I(x2 * x3) + I(x3 * x3), mydata) > > if the independent variable number is large, the formula will be very > long. Is there a easy way to write formula like the above one? I have > read the R introduction, however, I don't find a easy way. > > Any hints will be appreciated. Thanks, > > Jinsong > > ______________________________________________ > 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. > > -- View this message in context: http://www.nabble.com/A-easy-way-to-write-formula-tp18674075p18674632.html Sent from the R help mailing list archive at Nabble.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.