I see, so predict is mono-directional: only gives x with a know y but not the other way round. Thank you
On Tue, Jan 26, 2021 at 10:20 AM Jeff Newmiller <jdnew...@dcn.davis.ca.us> wrote: > > model2 <- lm( x~y ) > predict(model2, data.frame(y=26)) > > model2 is however not the inverse of model... if you need that then you need > to handle that some other way than using predict, such as an invertible > monotonic spline (or in this case a little algebra). > > On January 26, 2021 1:11:39 AM PST, Luigi Marongiu <marongiu.lu...@gmail.com> > wrote: > >Hello, > >I have a series of x/y and a model. I can interpolate a new value of x > >using this model, but I get funny results if I give the y and look for > >the correspondent x: > >``` > >> x = 1:10 > >> y = 2*x+15 > >> model <- lm(y~x) > >> predict(model, data.frame(x=7.5)) > > 1 > >30 > >> predict(model, data.frame(y=26)) > > 1 2 3 4 5 6 7 8 9 10 > >17 19 21 23 25 27 29 31 33 35 > >Warning message: > >'newdata' had 1 row but variables found have 10 rows > >> data.frame(x=7.5) > > x > >1 7.5 > >> data.frame(y=26) > > y > >1 26 > >``` > >what is the correct syntax? > >Thank you > >Luigi > > > >______________________________________________ > >R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see > >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. > > -- > Sent from my phone. Please excuse my brevity. -- Best regards, Luigi ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see 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.
