Px=fliplr(cumsum(fliplr(C)));
PS. In future, don't whine, it isn't becoming... :)
PPS
You can avoid one function call by writing the inner index reversal explicitly--I've no idea if would make any performance difference for large vector sizes or not.
Px=fliplr(cumsum(C(end:-1:1)));
Matlab doesn't have postfix indexing expressions so can't avoid the outer call.
