title: Extract values at row r and column n-1 in matrix in R
body: <p>I'd like to return a list of some values within my matrix. Essentially I would be looking to extract values at </p>
  
  <pre><code>[2,1]
  [5,4]
  [8,7]
  and so on. 
  </code></pre>
  
  <p>Basically, every third row beginning at row <code>n=2</code>, and the corresponding value in column <code>n-1</code>. </p>
  
  <p>It seems simple, but I can't wrap my head around it. </p>
  
  <p>My matrix is <code>141x141</code>. This output should yield 47 values.</p>