title: How to build a Javascript Array Delta
body: <p>I have an array of arrays where the position of the value is important in that this data is used to ultimately layout a grid.</p>
  
  <pre><code>[[a,b,c,d],[a,b,c,d][a,b,c,d],[a,b,c,d]] // Original data
  </code></pre>
  
  <p>Based on an action, values within this "dataset" can change, but the size will never change.</p>
  
  <pre><code>[[a,b,c,d],[a,b,b,b][a,c,c,c],[a,b,c,d]] // Modified data
  </code></pre>
  
  <p>What I'd like to do is to return a dataset that only contains the delta values:</p>
  
  <pre><code>[[null,null,null,null,],[null,null,b,b],[null,c,null,c],[null,null,null,null,]]
  </code></pre>
  
  <p>Now, I do always know the max X and Y of the dataset, and could simply loop through the original dataset, comparing it's value to the corresponding value and build a new array, but it seems like there could be a more efficient way to accomplish this.</p>
  
  <p>That said, my js-fu is minimal at best, and that's why I'm brining the problem here. Are there any language provided methods for accomplishing this? Suggested approaches? etc?</p>