title: How do I use a vector of indices to extract rows from a matrix?
body: <p>I have two matrices, <code>M1</code> (6400x6) and <code>M2</code> (315x3)</p>
  
  <p><code>M2</code> is a subset of <code>M1</code>. I did some data imputation on <code>M2</code> and now I would like to use <code>M2</code>'s row indices to extract the corresponding rows from the <code>M1</code>. How can I approach this?</p>
  
  <p>Thanks</p>