title: Parse JSON array to matrix using map
body: <p>I'm using swiftyJSON.</p>
  
  <p>The data looks like the following but not really, there are about 700,000 elements.</p>
  
  <pre><code>{
      "data": [
          [
              "one",
              "steak",
              -0.234
          ],
          [
              "cat",
              "pizza",
              1.9812
          ]
      ]
  }
  </code></pre>
  
  <p>Currently I'm using this</p>
  
  <pre><code>let jsonArray:[JSON] = json["data"].arrayValue
  for ( i=0; i&lt;jsonArray.count; i++ )//number of data points
  {
    A.append(jsonArray[i][0].string! )
    B.append(jsonArray[i][1].string! )
    X.append(jsonArray[i][2].float! )
  }
  </code></pre>
  
  <p>but it is slow, I'd like to use <code>map</code> to speed things up to create individual arrays like shown or a M x N matrix.  Or how would I put this into a struct?</p>