title: Delete rows with negative values
body: <p>In R I am trying to delete rows within a dataframe (ants) which have a negative value under the column heading Turbidity. I have tried</p>
  
  <pre><code>ants&lt;-ants[ants$Turbidity&lt;0,]
  </code></pre>
  
  <p>but it returns the following error:</p>
  
  <pre><code>Warning message:
  In Ops.factor(ants$Turbidity, 0) : &lt; not meaningful for factors
  </code></pre>
  
  <p>Any ideas why this may be? Perhaps I need to make the negative values 
  NA before I then delete all NAs?</p>
  
  <p>Any ideas much appreciated, thank you!</p>
  
  <p>@Joris: result is</p>
  
  <pre><code>str(ants$Turbidity)
  </code></pre>
  
  <p>num [1:291] 0 0 -0.1 -0.2 -0.2 -0.5 0.1 -0.4 0 -0.2 ...</p>