how to compute posterior probability?

I am implementing fusion rule i.e. sum rule for combining classifiers and making decision.

Problem: Classify input pattern Z into one of m possible classes (c1, …,cm) based on evidence provided by R classifiers

Let xi be the feature vector for the ith classifier derived from Z; xi´s are independent

Assign Z → cj, if g(cj) ≥ g(ck), 1 ≤ k ≤ m, k ≠ j

sum rule: g(C_r )= P(C_r│x_i )

Now want to compute posterior probability P(C_r│x_i ) for sum rule.

how to calculate P(C_r│x_i ) in matlab .... is their any code. please help me.