Make data optional during construction

lib/types/matrix.js

@@ -1,16 +1,36 @@
 var matrix = require('../numbers/matrix');
 
 var Matrix = function (data) {
-  if (!data) {
-    throw new Error('must provide valid data');
+  if (data) {
+    this.data = data;
+
+    //cache row and colCount so .length
+    //isn't constantly recalculatd.
+    this.colCount = this.data[0].length;
+    this.rowCount = this.data.length;
   }
+}
 
-  this.data = data;
+/**
+ * Fill a matrix with an array of arrays.
+ *
+ * @param {Array} Array of array of numbers.
+ * @return {Matrix} Updated/Filled matrix.
+ */
+Matrix.prototype.fill = function(aoa) {
+  for (var i = 0, rowCount = aoa.length, colCount = aoa[0].length; i < rowCount; i++) {
+    if (aoa[i].length != colCount) {
+        throw new IllegalArgumentException("All rows must have the same length.");
+    }
+    for (var j = 0; j < colCount; j++) {
+        this.data[i][j] = aoa[i][j];    
+    }
+  }
 
-  //cache row and colCount so .length
-  //isn't constantly recalculatd.
-  this.colCount = this.data[0].length;
-  this.rowCount = this.data.length;
+  this.colCount = colCount;
+  this.rowCount = rowCount;
+
+  return this;
 }
 
 /**