title: Copy value in the neighboring cell R
body: <p>I have a big dataset df; its short version looks like this:</p>
  
  <pre><code>Time    Block   Accuracy    AccuracyFinal
  142     Run2    NA          incorrect
  234     Run2    incorrect   NA
  121     Run2    NA          NA
  143     Run2    NA          incorrect
  122     Run2    incorrect   NA
  </code></pre>
  
  <p>I want to copy the value <em>incorrect</em> in the neighboring cell on the left in df$Accuracy, so it would look like this:</p>
  
  <pre><code>Time    Block   Accuracy    AccuracyFinal
  142     Run2    incorrect   incorrect
  234     Run2    incorrect   NA
  121     Run2    NA          NA
  143     Run2    incorrect   incorrect
  122     Run2    incorrect   NA
  </code></pre>
  
  <p>What is the most efficient way to do it?</p>