MATLAB: For fft(matrix) end

Question

a=zeros(n,n,k)
for i=1:k
  a(i)=fft(matrix);
end

I am get the error as

??? In an assignment  A(:) = B, the number of elements
in A and B
must be the same.

What can I do, Plz help

Answer 1

a(:,:,k) = fft(matrix);