title: creating 2 dimensional array of unspecified (varying) size
body: <p>Newbie question: I want to dynamically create an integer 2D array <code>M[i,j]</code>, whose sizes (in both dimensions) are unknown beforehand. Moreover, for each index <code>i</code>, the size of the i-th row may vary. </p>
  
  <p>Question 1: How do I declare such an array (do I even have to)? I have tried <code>Array[]</code>, <code>Array(Int64,1...)</code>, and <code>Array((Int,Int),0)</code>as in <a href="https://stackoverflow.com/questions/19419124/initialize-an-empty-array-of-tuples-in-julia?rq=1">this hint</a> and others.</p>
  
  <p>Question 2: once created, how to I populate the array in a smart and concise way? Say my i-th row is suppose to be equal to a given 1-dimensional <code>B</code>, I would like to write</p>
  
  <pre><code>A[i] = B
  </code></pre>
  
  <p>or </p>
  
  <pre><code>A[i,:] = B
  </code></pre>
  
  <p>or even </p>
  
  <pre><code>A[i,1:n] = B
  </code></pre>
  
  <p>where <code>n</code> is the size of <code>B</code>. All of these give me a <code>BoundsError()</code>. Slicemight do the trick, but I cannot make it agree with my declaration.</p>