title: How to convert 0 across specified columns to NAs
body: <p>I have a dataframe:</p>
  <pre><code>dat &lt;- data.frame(col1 = sample(0:3, 10, replace = TRUE),
                    col2 = sample(0:3, 10, replace = TRUE),
                    col3 = sample(0:3, 10, replace = TRUE),
                    col4 = sample(0:3, 10, replace = TRUE))
  </code></pre>
  <p>I want to convert the <code>0</code>s in <code>col1</code>, <code>col2</code> and <code>col4</code> to <code>NA</code>s. How can I do that? Most examples do the other way around.</p>
  <p>Thanks!</p>