Question: How do I filter a set?

Dear Colleagues,

I am not sure if there exist a simple way to handle the issues I am facing. I am trying to obtain numeric roots for a polynomial f(x,a). I know for sure that there can be many roots depending on the value of parameter a. However, I cannot say for sure how many roots are possible for each value of parameter a. Some of these roots are complex numbers. Also, I need to choose only those roots that have following properties:

1. They are real.

2. f(x*,a) i.e., function value at a root is positive. 

How do I solve f(x,a)=0 to store all roots in a set? Furthermore, how do I select and print roots that have the properties mentioned above? Is there a way to do filtering of a set specifying properties of the members of the set? Please suggest. Your help is highly appreciated.

Regards,

Omkar