R reshape2 'Aggregation function missing: defaulting to length' [duplicate]

Answer 1

Thanks to @akrun who pointed it out.

Well, there's a high chance that your data has duplicate row that look either like this:

student    test    score
Adam      Exam1     80
Adam      Exam1     85
Adam      Exam2     90
John      Exam1     70
John      Exam2     60

Or like this:

student   class     test    score
Adam      Biology   Exam1     80
Adam      Theology  Exam1     85
Adam      Theology  Exam2     90
John      Biology   Exam1     70
John      Theology  Exam2     60

When you cast it like this: dcast(data, student + class ~ test, value.var='score')