[R] finding x values to meet a y

[Martin Maechler]

>>>>> "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