On 14/08/2018 11:48 AM, 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?
Others have pointed out uniroot(). One other possibility: maybe you don't need both the function and its inverse, or an approximate inverse is good enough. In either of those cases, just swap x and y in the call to splinefun(), and you'll get a new function mapping y values to the corresponding x values. (You'll get nonsense or an error in cases where this mapping is not unique.) It won't match the inverse of the original spline interpolator except at observed (x,y) pairs, but will usually be close, especially if the functions are pretty smooth.
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