代码：
conv_time = zeros(1,150); fft_time = zeros(1, 150);
%
% Nmax = 2048;
for L = 1:150
tc = 0; tf = 0;
N = 2*L-1; nu = ceil(log10(N)/log10(2)); N = 2^nu;
for I = 1:100
h = randn(1,L); x = rand(1,L);
t0 = clock; y1 = conv(h,x); t1 = etime(clock,t0); tc = tc + t1;
t0 = clock; y2 = ifft(fft(h,N) .* fft(x,N)); t2 = etime(clock,t0);
tf = tf + t2;
end
%
conv_time(L)=tc/100; fft_time(L)=tf/100;
endn=1:150; figure('NumberTitle', 'off', 'Name', 'Exameple5.23 ')
set(gcf,'Color','white');
plot(n(25:150), conv_time(25:150), n(25:150), fft_time(25:150));
xlabel('sequence length N'); ylabel('Time in Sec.'); title('comparison of convolution times');
运行结果：
我的机子结果如下图：
书中的例子，运行结果如下图：
