I think the problem here is you still need the original String#to_f 
method, which you have not duplicated. You're better off creating a new 
method for this; perhaps "to_f_truncate" which then calls the unmodified 
to_f and modifies the results.

It's also a very bad idea to modify such an essential core class like 
that. Better to make your own subclass and use that, or make a method 
within a specific scope so you don't destroy the behaviour of everything 
sharing the same runtime environment. You'd probably be surprised how 
often to_f is used in the underlying components of your program.

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