title: Weird behaviour by ordering a data frame
body: <p>I have the following data frame that I want to order by the fifth column ("Distance").
  When I try `</p>
  
  <pre><code>df.order &lt;- df[order(df[, 5]), ]
  </code></pre>
  
  <p>I always get the following error message.</p>
  
  <pre><code>Error in order(df[, 5]) : unimplemented type 'list' in 'orderVector1'`
  </code></pre>
  
  <p>I don't know why R consider my data frame as a list. Running <code>is.data.frame(df)</code> returns <code>TRUE</code>. I have to admit that <code>is.list(df)</code> also returns <code>TRUE</code>. Is is possible to force my data frame to be only a data frame and not a list?
  Thanks for your help.</p>
  
  <pre><code>structure(list(ID = list(1, 2, 3, 4, 5, 6, 7, 8, 9, 10), 
                 Latitude = list(50.7368, 50.7368, 50.7368, 50.7369, 50.7369, 50.737, 50.737, 50.7371, 50.7371, 50.7371), 
                 Longitude = list(6.0873, 6.0873, 6.0873, 6.0872, 6.0872, 6.0872, 6.0872, 6.0872, 6.0872, 6.0872), 
                 Elevation = list(269.26, 268.99, 268.73, 268.69, 268.14, 267.87, 267.61, 267.31, 267.21, 267.02), 
                 Distance = list(119.4396, 119.4396, 119.4396, 121.199, 121.199, 117.5658, 117.5658, 114.9003, 114.9003, 114.9003), 
                 RxPower = list(-52.6695443922406, -52.269130891243, -52.9735258244422, -52.2116571930007, -51.7784534281727, -52.7703448813654, -51.6558862949081, -52.2892907635308, -51.8322993596551, -52.4971436682333)), 
            .Names = c("ID", "Latitude", "Longitude", "Elevation", "Distance", "RxPower"),
            row.names = c(NA, 10L), class = "data.frame")
  </code></pre>