title: How can I change a dataframe of factors so that the dataframe can be boxplotted?
body: <p>I have a dataframe of which the columns contain a variable amount of numbers and a variable amount of NA's. The dataframe looks like this:</p>
  
  <pre><code>    V1 V2 V3 V4 V5 V6
  1    0 11  4  0  0 10
  2    0 17  3  0  2  2
  3   NA  0  4  0  1  9
  4   NA 12 NA  1  1  0
  &lt;snip&gt;
  743 NA NA NA NA  8 NA
  744 NA NA NA NA  0 NA
  </code></pre>
  
  <p>I want to make a boxplot out of this, but when I do </p>
  
  <pre><code>boxplot(dataframe)
  </code></pre>
  
  <p>I get the error</p>
  
  <pre><code>adding class "factor" to an invalid object
  </code></pre>
  
  <p>When I do</p>
  
  <pre><code>lapply(dataframe,class)
  </code></pre>
  
  <p>I get the folowing output:</p>
  
  <pre><code>$V1
  [1] "factor"
  $V2
  [1] "factor"
  &lt;snip&gt;
  $V6
  [1] "factor"
  </code></pre>
  
  <p>So how can I change my dataframe so that the columns are seen as numeric?</p>