Nmax=2048;
fft_time=zeros(1,Nmax);
for n=1:1:Nmax
x=rand(1,n);
t=clock;
fft(x);
fft_time(n)=etime(clock,t);
end
n=[1:1:Nmax];
plot(n,fft_time,'.')
xlabel('N');ylabel('Time is Sec.')
title('FFT execution times')