[R] [FORGED] Spline function

[Rolf Turner]

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
> you please help me?

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

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Technical Editor ANZJS
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