title: Merge problems in R: Error in fix.by(by.x, x) : 'by' must specify uniquely valid columns
body: <p>I have two tables, both of which are aggregate outputs. I'd like to merge them together - they were both aggregated off the same data, so they have the same column names, but for the sake of edification I thought I'd try the specific by.x and by.y methods in the examples of ?merge.</p>
  
  <pre><code>merge(medA,countA, 
        by = c("manager_id", "manager_name", "grouping"))
  ## this works
  
  merge(medA,countA, 
    by.x = c(medA$manager_id, medA$manager_name, medA$grouping), 
    by.y = c(countA$manager_id, countA$manager_name, countA$grouping))
  ## this doesn't? 
  ## Error in fix.by(by.x, x) : 'by' must specify uniquely valid columns
  </code></pre>
  
  <p>It isn't particularly pressing: I can just use the merge that works. But I'm honestly confused why specific by.x and by.y doesn't work. I've googled around for it, but I can't seem to find similar problems. Any thoughts you guys have would be much appreciated.</p>