matlab FFT()执行时间

Nmax = 2048;
fft_time = zeros(1,Nmax);
for n = 1:1:Nmax
    x = rand(1,n);              %Uniformly distributed pseudorandom numbers
    t = clock;fft(x);fft_time(n)= etime(clock,t);
end
n = [1:1:Nmax];
plot(n,fft_time,'.')
xlabel('N');
ylabel('Time in sec');
title('FFT execution times');