I am not sure why you want to use a loop or fprintf. The simplest, but potentially not square enough solution for a filled square
N = 5;
x = repmat(char(42), N, N);
disp(x);
*****
*****
*****
*****
*****
For an unfilled square you can do
N = 5;
x = repmat(char(42), N, N);
x(2:end-1, 2:end-1) = char(32);
disp(x);
*****
* *
* *
* *
*****
On my system I get a squarer square with
x = repmat([char(42), char(32)], N, N)
* * * * *
* * * * *
* * * * *
* * * * *
* * * * *
Depending on how perfect you want, maybe you could use a thin space or a wide space.
For a filled diamond
a = [fliplr(tril(true(N))), tril(true(N)); triu(true(N)), fliplr(triu(true(N)))];
x = repmat(char(32), 2*N, 2*N);
x(a) = char(42);
disp(x)
**
****
******
********
**********
**********
********
******
****
**
For an empty diamond
a = [fliplr(triu(tril(true(N)))), triu(tril(true(N))); tril(triu(true(N))), fliplr(tril(triu(true(N))))];
x = repmat(char(32), 2*N, 2*N);
x(a) = char(42);
disp(x)
**
* *
* *
* *
* *
* *
* *
* *
* *
**
If you really need to use fprintf
y = mat2cell(x(:), ones(numel(x), 1), 1);
fprintf([repmat('%s', 1, 2*N), '\n'], y{:})
