Do NOT use eval for trivial code like this.
That approach is slow, complex, inefficient, and not recommended. Rather than messing around with fieldnames like that, the simple and efficient MATLAB solution would be to just use indexing into a cell array or structure array, e.g.:
C = cell(1,N);
for k = 1:N
...
C{k} = Xtmp;
end
Or if you really want to use a structure:
C = struct('X',cell(1,N));
for k = 1:N
...
C(k).X = Xtmp;
end
