title: How to reshape a vector to make a matrix?
body: <p>Here is what I have:</p>
  
  <pre><code>[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
  </code></pre>
  
  <p>And here is what I want to get:</p>
  
  <pre><code>[
    1,  2,  3,  4,
    5,  6,  7,  8,
    9, 10, 11, 12
  ] 
  </code></pre>
  
  <p>The number of rows and columns (3 and 4 in example) is already known.</p>
  
  <p>How would I do that?</p>