title: Error using xlswrite
body: <p>So I have this piece of code.</p>
  
  <p>Forward = </p>
  
  <pre><code>ForwardDifferentiation(a, eq, deltaX);
      Backward = BackwardDifferentiation(a, eq, deltaX); 
      Central = CentralDifferentiation(a, eq, deltaX); 
      SecDeriv = second(a, eq, deltaX); 
      ThirdDeriv = third(a, eq, deltaX);
      FourthDeriv = fourth(a, eq, deltaX); 
  
      fprintf('y'' using Forward Difference @x=%g is: %.5f', a, Forward);
      fprintf('\ny'' using Backward Difference @x=%g is: %.5f', a, Backward);
      fprintf('\ny'' using Central Difference @x=%g is: %.5f', a, Central);
      fprintf('\ny" using Second Derivative @x=%g is: %.5f', a, SecDeriv);
      fprintf('\ny"'' using Third Derivative @x=%g is: %.5f', a, ThirdDeriv);
      fprintf('\ny"" using Fourth Derivative @x=%g is: %.5f', a, FourthDeriv);
  
      filename = 'NumericalDifferentiation.xlsx';
      A = {'y''(Forward Diff)', 'y''(Backward Diff)', 'y''(Central Diff)', ...
      'y"', 'y"''', 'y""'; Forward, Backward, Central, SecDeriv, ...
      ThirdDeriv, FourthDeriv};
      xlswrite(filename,A) % Writing an xls file
  </code></pre>
  
  <p>But errors always occur about the xlswrite:</p>
  
  <p>Error using xlswrite (line 220)
  The file C:\Program Files\MATLAB\R2013a\bin\MP2a\NumericalDifferentiation.xlsx is not
  writable.  It may be locked by another process. </p>
  
  <p>and</p>
  
  <p>Error in NumDif (line 66)
          xlswrite(filename,A) % Writing an xls file</p>
  
  <p>How could I fix this?</p>