Generate expandable code for a particular matrix

I have devleoped the code to build a matrix of combinations of 1 and 2 in the diagonal (see below). I would like to have to code to be flexible so that is would do this for a matrix of size N instead of just 3.

A = zeros(3);

for i = 1:2

for j = 1:2

for k = 1:2

A(1,1) = i;

A(2,2) = j;

A(3,3) = k;

A

end

end

end

Answer 1

[Voss]

N = 5;
% The idea is to count up in binary from 0 to 2^N-1,
% but use 1s and 2s instead of 0s and 1s, then place 
% the elements along the diagonal of A. (This 
% generates the various diagonals of A in the same 
% order as your code does.)
A = zeros(N);
for ii = 0:2^N-1
    A(1:N+1:end) = dec2bin(ii,N)-'0'+1
end
%                  dec2bin(ii,N) ........ convert ii to a binary char vector N bits long, e.g.,
%                                         dec2bin(6,3) is '110', dec2bin(14,5) is '01110'
%                               -'0' .... then subtract the character '0' to get a vector of 
%                                         doubles, with 0 where the char vector had '0' and 1 
%                                         where it had '1'
%                                   +1 .. add 1 to make it 1 where the char vector was '0' 
%                                         and 2 where it was '1'
%   A(1:N+1:end) = ...................... store the result along the diagonal of A (linear 
%                                         index 1:N+1:end)