Approximate experimental data from multiple cells

I have several cell arrays with probe data from .mat-file.

I need to find resonance frequencies and q-factors for each of the 1000 inputs, and then estimate (approximate) them. However, the code below provides only 1 value of fres instead of 1000, and no plot for (i, fres) is shown.

• FStart - cell array (1000*1)
• FEnd - cell array (1000*1)

• Amp - multiarray (1000*100)

  for i = 1:1:1000
      f1 = FStart(i):10:FEnd(i);     
      grid on  
      y1 = plot(f1,Amp(i,:)); 
      [maxValue, maxIndex] = max(Amp(i,:));  %find maximum value of amplitude for each i
      [Q_Value, Q_Index] = max(0.5*Amp(i,:));  %also tried 0.5*max() and /2
      fres = f1(maxIndex);  %by index of max amplitude value find resonance frequency
      plot(i,fres)     %plot resonance frequency for each i
      hold on
  end
  

NB: the last FStart value is less than the first FreqEnd value

Futhermore, I try to esteem the Q factor as: Max(frequency on level = 1/2*MaxAmplitude)-Min(frequency on level = 1/2*MaxAmplitude)

fmin = min(f2(Q_Index))
fmax = max(f2(Q_Index))

But it shows fmin = fmax

Could you please tell, what's the problem here?

Apart from the comment to your answer which clearly shows where the problem is, you may wan to remove the plot from the script by storing fres as a vector:

for i = 1:1000
    f1 = FStart(i):10:FEnd(i);     
    grid on  
    y1 = plot(f1,Amp(i,:)); 
    [maxValue, maxIndex] = max(Amp(i,:));  %find maximum value of amplitude for each i
    [Q_Value, Q_Index] = max(0.5*Amp(i,:));  %also tried 0.5*max() and /2
    fres(i) = f1(maxIndex);  %by index of max amplitude value find resonance frequency    
end
plot(1:1000,fres,'ko-','LineWidth',2)