function [c,l]=DFT(x,N,str)

%generation a function with following uses:
%compute dft;output the amplitude and angle of sample signal in frequency field;
%plot the digital signal in both time and frequency fields
%str:sample frequence
%x:sampled signal
%N:length of signal x
%c:dft signal
%l:angle frequency

n=0:N-1;
k=-200:200;
w=(pi/100)*k;
l=w;
c=x*(exp(-j*pi/100).^(n'*k));       %計算DFT[x(n)]
magX=abs(c);                        %The Ampalitatude
angX=angle(c);                      %The Angle
ymax=max(magX);

subplot(2,2,1);
t=max(x);
n=0:N-1;
stem(n,x,'.');
xlabel('n');
ylabel('xa(n)');
title('信號的原形');
text((0.3*N),(0.8*t),str);
hold on
n=0:N-1;
m=zeros(N);
plot(n,m);

subplot(2,2,3);
plot(w/pi,magX);
axis([-2 2 0 ymax]);
xlabel('w/pi');
ylabel('|Y(jw)|');
title('上圖信號的傅式變換(|jw|)');