A few functions that may help are splinefun or approxfun along with uniroot.
________________________________________ From: Kavitha Venkatesan [kavitha.venkate...@gmail.com] Sent: Wednesday, August 12, 2009 3:43 PM To: Greg Snow Cc: r-help@r-project.org Subject: Re: [R] Obtaining the value of x at a given value of y in a smooth.spline object I do want a programmatic solution. I was thinking of the linear interpolation based on the 2 y-values bracketing the given y as well, I think I will pursue this. Thanks a lot for your suggestion, Kavitha On Wed, Aug 12, 2009 at 4:25 PM, Greg Snow<greg.s...@imail.org> wrote: > Part of the problem is that there could in theory be multiple x values that > result in the same y value. > > One approach if you are happy with something interactive rather than > programatical is to use the TkSpline function in the TeachingDemos package to > fit the spline function and drag the x-value until you find the y value that > you want. > > You can also look at the return from smooth.spline, find the y that is > closest to your desired value and then find the corresponding x, or find the > 2 y-values that bracket your choice and linearly interpolate the > corresponding x values. > > Hope this helps, > > -- > Gregory (Greg) L. Snow Ph.D. > Statistical Data Center > Intermountain Healthcare > greg.s...@imail.org > 801.408.8111 > > >> -----Original Message----- >> From: r-help-boun...@r-project.org [mailto:r-help-boun...@r- >> project.org] On Behalf Of Kavitha Venkatesan >> Sent: Wednesday, August 12, 2009 11:43 AM >> To: r-help@r-project.org >> Subject: [R] Obtaining the value of x at a given value of y in a >> smooth.spline object >> >> I have some data fit to a smooth.spline object as follows: (x=vector of >> data >> for the predictor variable, y=vector of data for the response variable) >> >> fit <- smooth.spline(x,y) >> >> Now, given a spline fit point y_new, I want to be able to find out what >> value of x_new yielded this fit value. How to do so? >> (This problem is the inverse of the predict.smooth.spline function, >> which >> takes x_new as input and yields the corresponding y_new fit value) >> >> Any insight is much appreciated! >> >> Thanks, >> Kavitha >> >> [[alternative HTML version deleted]] >> >> ______________________________________________ >> 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.
