how to get f(x)=___ from a piecwise function

From actual real-world readings, I have two vectors:

x<- c(-100.4, 32.0, 99.8, 200.2, 300.6, 399.8, 500.0, 600.0, 699.6, 799.6, 
899.8)
y<- c(0.4, 0.0, 0.2, -0.2, -0.6, 0.2, 0.0, 0.0, 0.4, 0.4, 0.2)

which, in the usual way constitute a  continuous piecewise function.

What I want to do is find an easy method to get at f(x) for some x I have 
NOT specified in the above vector.  For example I want f(356).

I have already put the time and effort in to write a program to compute this 
by breaking the function into the various pieces and computing the slopes of 
the individual lines etc. etc.

I am just looking to find an easier method.

Thank you for your help.
Eric