I am trying to reinvent wheels of fft. But the recursive function of mine seems to be not working due to N

The following is my code.

return_arr = zeros(N,1);

will create Size inputs must be scalar error.

since N is array. Why did N becomes array?

N =16;

x = zeros(N,1); % original equation to be DFT

for n = 0:(N-1)
   x(n+1) = sin((n*x_length*pi/N)) + 5*sin(2*(n)*x_length*pi/N) + 3*sin(4*pi*(n)*x_length*pi/N); % original function

end

for k = 0:(N-1)
    test_fft = my_fft(N, x, k);
end 


function return_arr = my_fft(N, arr, k)
return_arr = zeros(N,1);
if N > 1
    arr_odd = arr(1:2:end);
    arr_even = arr(2:2:end);
    
    
    temp = 2*pi*(k)/N;
    return_arr(k+1, 1) = my_fft(arr_even, N/2, k) + (cos(temp) + 1i*sin(temp)) * my_fft(arr_odd, N/2, k);
    
    
else
    temp = 2*pi*(k)/N;
    return_arr(1, 1) = arr * (cos(temp) + 1i*sin(temp));
end
end

Answer 1

[Steven Lord]

Why did N becomes array?

Because in your recursive call:

    return_arr(k+1, 1) = my_fft(arr_even, N/2, k) + (cos(temp) + 1i*sin(temp)) * my_fft(arr_odd, N/2, k);

arr_even and arr_odd are non-scalar but you pass them in as the first input to your function.