How to calculate conditional probability?

Given two vectors X and Y how can we calculate P(X/Y). I need it for the Naive Bayes Classification Algorithm. Note: X and Y can have two or more same elements. For example X=[1 1 2 3 4 4 2 3 5] Y=[ 3 3 4 1 2 2 2 7 8]

Thanks

[Daniel Shub]

You cannot. You need more than just independent observations of X and Y, you need paired observations of X and Y together, in other words something like a matrix.