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Abstract: How well do code-writing tasks measure students’ knowledge of programming patterns and anti-patterns? How can we assess this knowledge more accurately? To explore these questions, we surveyed 328 intermediate CS students and measured their performance on different types of tasks, including writing code, editing someone else’s code, and, if applicable, revising their own alternatively-structured code. Our tasks targeted returning a Boolean expression and using unique code within an if and else.We found that code writing sometimes under-estimated student knowledge. For tasks targeting returning a Boolean expression, over 55% of students who initially wrote with non-expert structure successfully revised to expert structure when prompted - even though the prompt did not include guidance on how to improve their code. Further, over 25% of students who initially wrote non-expert code could properly edit someone else’s non-expert code to expert structure. These results show that non-expert code is not a reliable indicator of deep misconceptions about the structure of expert code. Finally, although code writing is correlated with code editing, the relationship is weak: a model with code writing as the sole predictor of code editing explains less than 15% of the variance. Model accuracy improves when we include additional predictors that reflect other facets of knowledge, namely the identification of expert code and selection of expert code as more readable than non-expert code. Together, these results indicate that a combination of code writing, revising, editing, and identification tasks can provide a more accurate assessment of student knowledge of programming patterns than code writing alone. 

How can we teach students to use more readable code structures? How common is it for students to choose less readable (but still functional) alternatives? We explore these questions for a specific anti-pattern: using sequential if statements when conditions are exclusive (rather than using else-if or else). We created and validated an automated detector to identify this anti-pattern in student's code. Running the detector on 1,764 homework submissions (from 270 students in a CS2 class on data structures and algorithms) showed that this anti-pattern was common and varied by assignment: across 12 assignments, 3% to 50% of submissions used sequential ifs for exclusive cases. However, using this anti-pattern did not preclude using else-ifs: across assignments, up to 34% of the submissions used both forms. Further, students used sequential if statements in surprising ways, such as checking a condition and then the negation of that condition, indicating a more novice level of understanding than expected for an intermediate course. Hand-inspection of the detector-flagged cases suggests that sequential ifs for exclusive cases may be a code smell that can indicate larger problems with logic and abstraction.