Hi all, Just wonder: 1)When substracting array2 from array1 to get the difference elements between two arrays(array1-array2), must array1.size >= array2? 2)Is there mentod in Array class that can return the element only unique to either of two arrays? For example, array1=[1,2,3],array2=[1,4,7] the return array is [2,3,4,7] Thanks, Li -- Posted via http://www.ruby-forum.com/.