The traditional method of automatically assessing programming exercises in Computer Science uses a black-box approach where a set of test data is inputed to both students and teachers programs and their outputs compared. This approach is useful for grading but inadequate for detecting and correcting students errors. In this paper we present several cases where we were able to develop matching algorithms to compare answers with solutions and pinpoint differences between them. In some cases the matching is based on the actual structure of answers and solutions. In other cases we use execution side-effects to collect a structure that can be compared using a matching algorithm. This approach is currently being used in Ganesh - a web environment for learning Computer Science.

Keywords: Program assessment; Computer assisted instruction