# Re: [R] finding x values to meet a y

From: Martin Maechler <maechler_at_stat.math.ethz.ch>
Date: Tue 08 Aug 2006 - 18:05:21 EST

>>>>> "AOlinto" == Antonio Olinto <aolinto_r@bignet.com.br> >>>>> on Mon, 7 Aug 2006 19:49:42 -0300 writes:

AOlinto> Hi,
AOlinto> I'd like to find which values of x will give me a y.

AOlinto> In other words, in the example of a gaussian curve, I want to find the values of     AOlinto> x that will give me a density, let's say, of 0.02.

AOlinto> curve(((1/(sqrt(2*pi)*10))*exp(-((x-50)^2)/(2*10^2))),xlim=c(0,100))

In other words, you are trying to *invert a function* numerically, i.e.,

find x such that f(x) = y0

Now, this is trivially the same as finding the "root" (or "zero") of the function f~(x) = f(x) - y0

==> Use *the* R root finding function :

uniroot() [or polyroot() if f(.) is a polynomial]

Martin Maechler, ETH Zurich

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