title: R - return position of element in matrix?
body: <p>Given a matrix:</p>
  
  <pre><code>      [,1] [,2]
  [1,]    0  0.0
  [2,]   -1  0.8
  </code></pre>
  
  <p>What is the quickest way in R to iterate over the matrix and return the position of all non-zero entries as an index?</p>