title: Rweka Error in model.frame.default(formula = class ~ ., data = rtrain) : object is not a matrix
body: <p>I'm new in using <code>Rweka</code> and <code>R</code>.
  When I'm using <code>KNN</code> to train the data like writing the following codes.</p>
  
  <pre><code>library(RWeka)
  trainfile='/poker-hand-training-true.arff'
  rtrain &lt;- as.data.frame(read.arff(file=trainfile))
  classifier &lt;- IBk(class ~., data = rtrain)
  </code></pre>
  
  <p>Here is the error log:</p>
  
  <pre><code>&gt; classifier &lt;- IBk(class ~., data = rtrain)
  Error in model.frame.default(formula = class ~ ., data = rtrain) : 
    object is not a matrix
  </code></pre>
  
  <p>And I've also test the class of the <code>train data</code>. Here it is:</p>
  
  <pre><code>&gt; rtrain
        V1 V2 V3 V4 V5 V6 V7 V8 V9 V10 L
  1      1 10  1 11  1 13  1 12  1   1 9
  2      2 11  2 13  2 10  2 12  2   1 9
  3      3 12  3 11  3 13  3 10  3   1 9
  4      4 10  4 11  4  1  4 13  4  12 9
  5      4  1  4 13  4 12  4 11  4  10 9
  6      1  2  1  4  1  5  1  3  1   6 8
  7      1  9  1 12  1 10  1 11  1  13 8
  8      2  1  2  2  2  3  2  4  2   5 8
  9      3  5  3  6  3  9  3  7  3   8 8
  10     4  1  4  4  4  2  4  3  4   5 8
  11     1  1  2  1  3  9  1  5  2   3 1
  &gt; class(rtrain)
  [1] "data.frame"
  </code></pre>