Confusion Matrix in Mathematica

A confusion matrix is an effective visualization technique to diagnose how a classification algorithm is behaving. You can create one easily in Mathematica given a list of 2-tuples, where the 2-tuple is of the form {ground truth category, predicted category}.

ConfusionMatrix[gndpre_List] :=
 With[
  {categories = Union[Flatten[gndpre]]},
  Partition[
   Flatten[
    Table[
     Count[gndpre, {row, col}],
     {row, categories},
     {col, categories}
     ]
    ],
   Length[categories]
   ]
  ]