title: Replace all NA with FALSE in selected columns in R
body: <p>I have a question similar to <a href="https://stackoverflow.com/questions/6684695/r-replace-na-with-item-from-vector">this one</a>, but my dataset is a bit bigger: 50 columns with 1 column as UID and other columns carrying either <code>TRUE</code> or <code>NA</code>, I want to change all the <code>NA</code> to <code>FALSE</code>, but I don't want to use explicit loop.</p>
  <p>Can <code>plyr</code> do the trick? Thanks.</p>
  <h3>UPDATE #1</h3>
  <p>Thanks for quick reply, but what if my dataset is like below:</p>
  <pre><code>df &lt;- data.frame(
    id = c(rep(1:19),NA),
    x1 = sample(c(NA,TRUE), 20, replace = TRUE),
    x2 = sample(c(NA,TRUE), 20, replace = TRUE)
  )
  </code></pre>
  <p>I only want <code>X1</code> and <code>X2</code> to be processed, how can this be done?</p>