matrix sum with extending dimensions

If there are two matrices like a=[1,0;0,1] b=[2,0;0,2], is there any command by which I get a 3x3 matrix as c=[1,0,0;0,3,0;0,0,2]?

I just add the last element in a with first element in b and then get a new matrix with extending dimension.

Thank you,

Rick

Answer 1

[Azzi Abdelmalek]

a=[1,0;0,1]
b=[2,0;0,2]
ad=diag(a),
bd=diag(b),
dc=[ad(1:end-1) ;ad(end)+bd(1) ; bd(2:end)]
c=diag(dc)