CODEWITHSUNDEEP
r
Ben
Ben

Reputation: 1522

Find the inverse function in R

Is there a function in R that generates the inverse of a given function? To be more specific: I have a polynomial of a third order and I need the inverse of it. It's strictly monotonously.

I read a few times that uniroot and/or polyroot can help. But how? Uniroot yields the root of a function and polyroot the zeros of a function. How can I use that for the inverse? Maybe a dumb question but I don't get it..

Upvotes: 1

Views: 5093

Answers (2)

Lee
Lee

Reputation: 63

Sorry for the late reply, but you could try using this function: 

inverse = function(fn, interval = NULL, lower = min(interval), upper = max(interval), ...){
    Vectorize(function(y){
        uniroot(f=function(x){fn(x)-y}, lower=lower, upper=upper, ...)$root
    })
}

I've seen variants of this a few times, but never with Vectorize built in. I put the function above together to hopefully be a little more user friendly, e.g.:

x = 1:10
y = sqrt(x)
sqrt.inv = inverse(sqrt, lower=1, upper=10)
sqrt.inv(y)
# [1]  1  2  3  4  5  6  7  8  9 10

Hope that helps!

Upvotes: 5

Ben
Ben

Reputation: 1522

The package investr is able to apply an inverse regression.

Upvotes: 1

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