title: How to delete a single value within a data.frame
body: <p>I would like to delete a single value of a cell within a data.frame. The value is a factor (numeric)</p>
  
  <p>I tried to access the value like this:</p>
  
  <pre><code>which(colnames(df) == "variable.name")
  which(rownames(df) == "row.name")
  
  df[i, j] &lt;- NULL
  </code></pre>
  
  <p>I was also trying to produca a NA by &lt;- is.na, this didn't work as well. How would I do this? </p>