a=[2,3,6,7;4,2,8,6;3,6,9,2;4,7,9,3]
a1=a([1:3],[2:3])
a2=a(2,3)
zeros(3,4)
ones(5)
eye(3)
rand(4)
randn(3)
% 删除行列
a(:,[3])=[]
% 增加元素和列
a(5,6)=45
a(:,4)=[90,89,34,56,98]