# [R] [FORGED] Spline function

Rolf Turner r@turner @end|ng |rom @uck|@nd@@c@nz
Wed Aug 15 00:40:58 CEST 2018

```On 15/08/18 03:48, Tania Morgado Garcia wrote:

>   Hello everyone. I'm new to R and I'm using spline functions. With the
> command splinefun (x, y) I get the function of interpolating the values x
> and y. Later, I can evaluate that function for values of x by obtaining the
> respective values of y. The point is that I need the inverse operation,
> with the function, for a value of Y I need to know the value of x. Could

Your question is ill-posed.  There could easily be multiple x values
corresponding to a single y value, unless the spline function is monotone.

If you can specify an interval which encloses the x value that you are
trying to and over which the spline function is monotone, then uniroot()
might provide what you want.

Something like:

spf <- splinefun(x,y)
uniroot(f=function(x){spf-y0},interval=c(a,b))

where y0 is the y value for which you want the corresponding x value,
where spf() is monotone on [a,b], and where there *exists* an x value
between a and b such that spf(x) = y0.

Good luck.

cheers,

Rolf Turner

--
Technical Editor ANZJS
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276

```