Creating a Matrix from a Sum

Dear All,
I need some help to efficiently write an array manipulation. I suspect 
this could be a one-liner.
Assume that you have a histogram of observations that you store in a 
vector x.
Let us say that its i-th entry, x_i, corresponds to the number of 
observations in the i-th channel, for i=1,2...N. (or 0,1...N-1, please 
let me know if there is any potential 0/1 pitfall in the following).
Now, for any two channels i and j, I want to calculate the probability 
of having an observation  in any other channel k, where  k>=max(i,j).
That is to say

P(i,j)=sum_{k=max(i,j)}^N x_k /C,
where C=sum_{i=1}^N x_i is just a normalization factor.

Does anyone know a good way of writing this in a code to get the P(i,j) 
matrix?
Many thanks

Lorenzo