title: Matlab: how to implement a dynamic vector
body: <p>I am refering to an example like this 
  I have a function to analize the elements of a vector, 'input'. If these elements have a special property I store their values in a vector, 'output'.
  The problem is that at the begging I don´t know the number of elements it will need to store in 'output'so I don´t know its size.
  I have a loop, inside I go around the vector, 'input' through an index. When I consider special some element of this vector capture the values of 'input' and It  be stored in a vector 'ouput' through a sentence like this:</p>
  
  <pre><code>For i=1:N %Where N denotes the number of elements of 'input'
  ...
  output(j) = input(i);
  ...
  end
  </code></pre>
  
  <p>The problem is that I get an Error if I don´t previously "declare" 'output'. I don´t like to "declare" 'output' before reach the loop as output = input, because it store values from input in which I am not interested and I should think some way to remove all values I stored it that don´t are relevant to me.
  Does anyone illuminate me about this issue?
  Thank you.</p>