title: Error in if/while (condition) {: missing Value where TRUE/FALSE needed
body: <p>I received this error message:</p>
  
  <pre><code>Error in if (condition) { : missing value where TRUE/FALSE needed
  </code></pre>
  
  <p>or</p>
  
  <pre><code>Error in while (condition) { : missing value where TRUE/FALSE needed
  </code></pre>
  
  <p>What does it mean, and how do I prevent it?</p>