function [Xk]=plot_dfs(xn)
% Plot Discrete-Time Fourier Series of a periodic sequence
% Ref:“Digital Signal with Matlab,” Ingle and Proakis, Thomas Learning, 2000. 
%【“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 1: 
%>> h=[1/3 1/3 1/3];   
%>> [Xk]=plot_dfs(h);
% Example 2:
%>> h=[1/3 1/3 1/3 0 0 0 0 0];   
%>> [Xk]=plot_dfs(h);
N=length(xn)
w=0:2*pi/N:(2*pi-2*pi/N);			% 0 <= w < 2*pi 間取 N 點
n=0:1:(N-1);
Xk=xn*exp(-j*n'*w);            
magXk = abs(Xk); angXk = angle(Xk);
figure;
subplot(2,1,1); stem(n,magXk);grid;
xlabel('n=0~(N-1)'); ylabel('|Xk|'); title('Magnitude Part')
subplot(2,1,2); stem(n,angXk);grid
xlabel('n=0~(N-1)'); ylabel('radians');title('Angle Part')