title: cor shows only NA or 1 for correlations - Why?
body: <p>I'm running <code>cor()</code> on a <code>data.frame</code>with all numeric values and I'm getting this as the result:</p>
  
  <pre><code>       price exprice...
  price      1      NA
  exprice   NA       1
  ...
  </code></pre>
  
  <p>So it's either <code>1</code> or <code>NA</code> for each value in the resulting table.  Why are the <code>NA</code>s showing up instead of valid correlations?</p>