[R] finding x values to meet a y
Martin Maechler
maechler at stat.math.ethz.ch
Tue Aug 8 10:05:21 CEST 2006
>>>>> "AOlinto" == Antonio Olinto <aolinto_r at 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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