MATLAB: Errors appear when doing FFT on data imported from text file

error fixingfft

trying to determine the frequency estimate after doing FFT on data imported from a text file.
 load spot_num.txt
 ssn = spot_num(:,3);
 ssn = ssn-mean(ssn);
 x=ssn;
 xdft = fft(x);
 xdft = xdft(1:length(x)/2+1);
 freq = 0:Fs/length(x):Fs/2;
 [maxval,idx] = max(abs(xdft));
 freq(idx)
it returns:
 Error in ==> Untitled3 at 7
 freq = 0:Fs/length(x):Fs/2;
so whats wrong with that?

Best Answer

Your sampling interval is 1 month, so your DFT bins will be in cycles/month. So you can just set Fs = 1, but then keep in mind that the frequencies will be in cycles per month, so your highest frequency of 1/2 is 1 cycle/2 months.
Also, the code I had given you earlier was assuming you have an even number of samples. If your signal length is odd, you have to make a small adjustment.
   Fs = 1;
   xdft = fft(x);
   xdft = xdft(1:length(x)/2+1);
   freq = 0:Fs/length(x):Fs/2;
   [maxval,idx] = max(abs(xdft));
   freq(idx)
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