title: how to convert all <NotAvailable> to <NA>
body: <p>I have this df</p>
  <pre><code>PoolQC          Fence           MiscFeature
  &lt;chr&gt;           &lt;chr&gt;           &lt;chhr&gt;
  &lt;NOT AVAILABLE&gt; &lt;NOT AVAILABLE&gt; &lt;NOT AVAILABLE&gt;     
  &lt;NOT AVAILABLE&gt; &lt;NOT AVAILABLE&gt; &lt;NOT AVAILABLE&gt;     
  &lt;NOT AVAILABLE&gt; &lt;NOT AVAILABLE&gt; &lt;NOT AVAILABLE&gt;     
  &lt;NOT AVAILABLE&gt; &lt;NOT AVAILABLE&gt; &lt;NOT AVAILABLE&gt;     
  &lt;NOT AVAILABLE&gt; &lt;NOT AVAILABLE&gt; &lt;NOT AVAILABLE&gt;     
  &lt;NOT AVAILABLE&gt; MnPrv           Shed        
  &lt;NOT AVAILABLE&gt; &lt;NOT AVAILABLE&gt; &lt;NOT AVAILABLE&gt;     
  &lt;NOT AVAILABLE&gt; &lt;NOT AVAILABLE&gt; Shed        
  &lt;NOT AVAILABLE&gt; &lt;NOT AVAILABLE&gt; &lt;NOT AVAILABLE&gt;     
  &lt;NOT AVAILABLE&gt; &lt;NOT AVAILABLE&gt; &lt;NOT AVAILABLE&gt;
  </code></pre>
  <p>how do I convert this all NOT AVAILABLE to NA, so that if I run this code</p>
  <pre><code>df %&gt;% 
    is.na() %&gt;% 
    colSums() %&gt;% 
    sort(decreasing = TRUE)
  </code></pre>
  <p>it can detect the NA value</p>
  <p>or, can I convert it by csv files reading?</p>
  <pre><code>df = read.csv(&quot;C:/Users/x.csv&quot;, sep = &quot;;&quot;)
  </code></pre>