[R] Obtaining the value of x at a given value of y in a smooth.spline object

Greg Snow Greg.Snow at imail.org
Wed Aug 12 22:25:35 CEST 2009

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.snow at imail.org

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Kavitha Venkatesan
> Sent: Wednesday, August 12, 2009 11:43 AM
> To: r-help at 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]]
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