[R] Interpolate x from y

[Patrizio Frederic]

Greg,
it seems an obvious behavior to me

y=c(2,2,2,3,3,3,1)
x=1:length(y)
plot(x,y)
lines(x,approxfun(x,y)(x)) # for every x it exists one only value of y

plot(y,x)
lines(sort(y),approxfun(y,x)(sort(y))) # for some y it exists more
than one value of x!

approxfun return a function. By definition a function maps one value
of domain into one only value of codomain. Would you expect that one y
value returns more than one x? I don't
Be more specific and maybe we can help you

Patrizio

2009/3/25 Greg <chaoborid at gmail.com>:
> Is it possible to interpolate a value for x with knowledge of y?
>
> For example, approx(x, y, xout) will give me y's given a set of x's,
> which is opposite to what I'm after.  I've tried switching x and y,
> e.g., approx(y, x, xout), but in a real data set it is possible to
> have more than one y for a given x causing approx() to remove
> coordinates.
>
> Thanks for your help,
>
> Greg.
>
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