define a formula

[Stefan]

Unless you absolutely want to write a function for that (for learning purposes maybe) you don't have to:
1. You could use the mean(L,p) function where L is the list of values and p the list of probabilities.
2. Or if you write L and p as vectors you could just calculate the dot product of the vectors L and p: [L1, L2, L3,...]*[p1, p2, p3...]

If p1, p2, etc are not probabilities in range 0 to 1, but frequencies instead (e.g. 3 times, 7 times etc) for method 2 you should also divide by sum(p): E(X)=L*p/sum(p)