function [X]=plot_dtft(x,n)
% Plot Discrete-Time Fourier Transform of a sequence
% 摘自“數位訊號處理—使用Matlab”, 余兆棠 陳順智 譯（Ingle and Proakis 原著），滄海圖書
% Example: 
%>> h=[1/3 1/3 1/3]; n=[0 1 2];  % 3-point running averager
%>> [X]=plot_dtft(h,n);
w=-2*pi: pi/100:2*pi;			% -2*pi ~ 2*pi 間取401點
k= -200:200;					% k= (100/pi)*w
% X=x*(exp(-j*pi/100)).^(n'*k); 或 X=x*exp(-j*(pi/100)*n'*k);
X=x*exp(-j*n'*w);            % 以上兩種寫法亦可
magX = abs(X); angX = angle(X);
figure;
subplot(2,1,1); plot(w/pi,magX);grid;
xlabel('frequency in pi units'); ylabel('|X|'); title('Magnitude Part')
subplot(2,1,2); plot(w/pi,angX);grid
xlabel('frequency in pi units'); ylabel('radians');title('Angle Part')