title: Setting Eigen Matrix/Vector by index
body: <p>How exactly do we set the value of an Eigen Vector or Matrix by index. I'm trying to do something similar to:</p>
  
  <pre><code>// Assume row major
  matrix[i][j] = value
  // or
  vector[i] = value
  </code></pre>
  
  <p>I might have missed it, but could not find anything in the <a href="https://eigen.tuxfamily.org/dox/AsciiQuickReference.txt" rel="nofollow noreferrer">quick reference guide</a>.</p>