title: How in MATLAB to set all but every 3rd element to 0?
body: <p>My vector is <code>[a(1) a(2) a(3) a(4) a(5) a(6) ... a(3k)]</code>.</p>
  
  <p>I want to get a vector <code>[0 0 a(3) 0 0 a(6) ... a(3k)]</code>.</p>
  
  <p>What's a <strong>short</strong> way to do that in MATLAB?</p>