title: 2D arrays using NSMutableArray
body: <p>I need to create a mutable two-dimensional array in Objective-C.</p>
  
  <p>For example I have:</p>
  
  <pre><code>NSMutableArray *sections;
  NSMutableArray *rows;
  </code></pre>
  
  <p>Each item in <code>sections</code> consists of an array <code>rows</code>. <code>rows</code> is an array that contains objects.</p>
  
  <p>And I want to do something like this:</p>
  
  <pre><code>[ sections[i] addObject: objectToAdd]; //I want to add a new row
  </code></pre>
  
  <p>In order have something like this:
  section 0, rows: obj1, obj2, obj3
  section 1, rows: obj4, obj5, obj6, obj 7...</p>
  
  <p>Is there a way to do that in Objective-C?</p>