Question

Copy and run the MATLAB program in this blog program to run 
for n = 13
      subplot(2,2,n-7)
      F = fft(eye(n));
      plot(F)
      axis square
      axis off
      title(['n = ' int2str(n)])
   end

Answer 1

The maximum number of subplots you can have in a matlab program is 4.

Depending on the number chosen, the plot will be plotted in that quadrant.

So fixing the error in the code -

for n = 13
      subplot(2,2,n-9)
      F = fft(eye(n));
      plot(F)
      axis square
      axis off
      title(['n = ' int2str(n)])
   end

you get a plot like this -

n=13

To make the graph occupy the entire space, you can remove the subplot line and your code will be like -

for n = 13
      F = fft(eye(n));
      plot(F)
      axis square
      axis off
      title(['n = ' int2str(n)])
   end

and the corresponding plot will be -

n=13