title: replace NA values with sequence
body: <p>I am trying to fill in some NA values of a string but count the NA values that are being filled.</p>
  <p>For example I have the following string:</p>
  <pre><code>string = c(&quot;A&quot;, &quot;B&quot;, &quot;C&quot;, NA, NA, &quot;D&quot;, &quot;E&quot;, NA, &quot;F&quot;, &quot;G&quot;, NA, NA)
  </code></pre>
  <p>Expected output:</p>
  <pre><code>&quot;A&quot;, &quot;B&quot;, &quot;C&quot;, &quot;NA_VALUE_1&quot;, &quot;NA_VALUE_2&quot;, &quot;D&quot;, &quot;E&quot;, &quot;NA_VALUE_3&quot;, &quot;F&quot;, &quot;G&quot;, &quot;NA_VALUE_4&quot;, &quot;NA_VALUE_5&quot;
  </code></pre>
  <p>So, I want to paste the sequence <code>paste(&quot;NA_VALUE_&quot;, valueHere)</code> where <code>valueHere</code> increases.</p>