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\section{Conclusion}
We have used AST patterns as features to describe program structure. By encoding only relations between particular nodes, each pattern can match many programs. AST patterns thus function as a sort of “regular expressions” for trees.
We presented a method for automatically extracting AST patterns from student programs. Our patterns encode relations between data objects in a program, with each pattern connecting either two instances of the same variable, a variable and a value, or two values. We consider such patterns as the atomic syntactic relations in a program, and use them as machine-learning features.
We explained how to induce rules for classifying correct and incorrect programs based on AST patterns. Since the goal of our research is to generate hints, we adapted the CN2 algorithm to produce rules useful for that purpose. We induce rules in two passes: we first learn the rules for incorrect programs, and then use programs not covered by any such rule to learn the rules for correct programs.
Evaluation shows that our patterns are useful for classifying Prolog programs. Other programming languages will likely require different patterns. For example, in commonly taught imperative languages such as Python or Java, each variable can take on different values and appear in many places. Further research is needed to determine the kinds of patterns useful in such situations.
We showed how to generate hints based on rules by highlighting buggy patterns or pointing out what patterns are missing. Evaluation on past student data shows that useful hints can be provided for many incorrect submissions this way. The quality of feedback could be improved by annotating rules with explanations in natural language. Since patterns and rules are easily interpretable, they can also help when manually authoring tutoring systems, by indicating the common errors and the typical solution strategies for each problem.
In the future we will attempt to improve rule accuracy for certain problems, such as \code{union}. This will likely necessitate new kinds of patterns, for example to handle the cut operator. Adapting our methods to handle Python programs will give us some insight into what kinds of patterns could be useful in different situations. Finally, we will implement hint generation in an online programming tutor CodeQ, and evaluate the effect of automatic feedback on students’ problem-solving.
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