title: Replacing single value in df by name
body: <p>I would like to replace a single value in a dataframe by name:</p>
  <pre><code>df &lt;- data.frame(samples= c('sample_2', 'sample_2', 'sample_3', 'sample_3', 'sample_4', 'sample_4'), size=rnorm(3), group= c('group_1', 'group_1','group_1','group_2','group_2','group_2'))
  </code></pre>
  <p>Is there a simple way to replace the value in the field 'size' in line 4 using a combination of the 'samples' and 'group' names (e.g. using dplyr)? I don't want to use numerical indexing of rows and columns).</p>