title: Check if value is in data frame
body: <p>I'm trying to check if a specific value is anywhere in a data frame.</p>
  
  <p>I know the <code>%in%</code> operator should allow me to do this, but it doesn't seem to work the way I would expect when applying to a whole data frame:</p>
  
  <pre><code>A = data.frame(B=c(1,2,3,4), C=c(5,6,7,8))
  1 %in% A
  
  [1] FALSE
  </code></pre>
  
  <p>But if I apply this to the specific column the value is in it works the way I expect:</p>
  
  <pre><code>1 %in% A$C
  
  [1] TRUE
  </code></pre>
  
  <p>What is the proper way of checking if a value is anywhere in a data frame?</p>