# Automated Classification and Feedback Generation for Programming Assignments

Online-learning platforms (e.g., Coursera, Khan Academy, edX) are very popular and massive open online courses (MOOCs) have attracted thousands of students. We study the following reserach problem : in the traditional classroom setting students are given feedback (hints) by an instructor; in MOOCs this is not possible, since there typically is only a small number of instructors. For introductory programming assignments, students require feedback on different aspects of their code:

1. Correctness: guidance for writing a correct program.
2. Performance: help for improving the performance of an already correct program.

We have developed a tool that classifies student programs based on their high-level idea (which we also call algorithmic strategy); this classification enables us to automatically give feedback on correctness and performance: By recognizing the high-level idea of the student we can say “good job, your code is efficient!” or give a suggestion how to improve the code. Similarly, by recognizing the high-level idea of the student we can compare the student’s code to a correct program with the same high-level idea, and based on the differences suggest how to repair the program. Examples below illustrate these ideas:

Assignment: Decide if two strings (s and t) are anagrams (two strings are anagrams if they can be permuted to become equal).
 ```bool Puzzle(string s, string t) { var sa = s.ToCharArray(); var ta = t.ToCharArray(); Array.Sort(sa); Array.Sort(ta); return sa.SequenceEqual(ta); }``` ```bool Puzzle(string s, string t) { return s.All(c => s.Where(c2 => c2 == c) .Count() == t.Where(c2 =>c2 == c) .Count()); }``` ```bool Puzzle(string s, string t) { if (s.Length != t.Length) return false; char[] sa = s.ToCharArray(); char[] ta = t.ToCharArray(); for (int j=0; j < sa.Length; j++) { for (int i=0; i < sa.Length - 1;i++) { if (sa[i] < sa[i+1]){ char temp=sa[i]; sa[i]=sa[i+1]; sa[i+1]=temp; } if (ta[i] < ta[i+1]){ char temp=ta[i]; ta[i] = ta[i+1]; ta[i+1] = temp; } } } for (int k = 0; k < sa.Length; k++) { if (sa[k] != ta[k]) return false; } return true; }``` ```bool Puzzle(string s, string t) { if (s.Length != t.Length) return false; foreach (Char ch in s.ToCharArray()){ if (countChars(s, ch) != countChars(t, ch)){ return false; } } return true; } int countChars(String s, Char c){ int number = 0; foreach (Char ch in s.ToCharArray()){ if (ch == c){ number++; } } return number; }``` Feedback: “Instead of sorting, compare the number of characters in strings”. Feedback: “Count the characters in a pre-processing phase, instead of in a loop”.

These four examples show correct, but inefficient programs (all have complexity O(n2), while there exists a solution with complexity O(n) — do you know what it is?). The programs on the right use a different high-level idea (aka algorithmic strategy) than the programs on the left (counting character occurences instead of sorting). Our tool classifies programs based on the used algorithm, and despite their syntactic differences gives the same feedback for the programs on the left resp. on the right.

Assignment: Read two integers 0 <= n <= m, and count the Fibonacci numbers in the closed interval [n,m].
 ```int main() { long a=0, b=1,c=0,cnt=0; long int n,m; scanf("%ld%ld",&n,&m); while (c<=m) { c=a+b; if ((c>=n)&&(c<=m)) cnt=cnt+1; a=b; b=c; } printf("%d",cnt); }``` ```int main() { long int n,m,f,n1=2,n2=3; int c=0; scanf("%ld %ld",&n,&m); while(f<=m) { f=n1+n2; n1=f; n2=n1; if(f>=n && f<=m) c++; } printf("%d",c); }``` Correct program Incorrect program; feedback: "1. Assign 0 to n2 and f, and 1 to n1 before the loop; 2. Move the statement n2=n1; before n1=f; in the loop."
Our tool computes the smallest number of changes that transform the incorrect program on the right into the the correct program on the left, resulting in the feedback shown above. In fact, our tool computes such a repair with regard to every correct program in the database and reports only the repair that is best (smallest number of edits) with regard to all correct programs.

Classification and feedback generation is a very active area of research, and our goal is to develop techniques and tools to improve the state-of-the-art in automated programming education. If you find these results exciting, we can offer you a variety of topics for bachelor/master theses, projects for practical course work, and student jobs.

For more information, also check our recent paper on this topic: Feedback Generation for Performance Problems in Introductory Programming Assignments.

## 3 Forsyte papers accepted at CAV 2019

Papers co-authored by Jure Kukovec, Marijana Lazić, and Josef Widder were accepted to be presented at the 31st International Conference on Computer-Aided Verification. […]

## Deadline for Applications: Helmut Veith Stipend for Female Master´s Students in CS

Motivated female students in the field of computer science (CS) who plan to pursue (or pursue) one of the master‘s programs in Computer Science at the Vienna University of Technology – TU Wien taught in English are invited to apply for the annually awarded Helmut Veith Stipend. The Helmut Veith Stipend is dedicated to the […]

## FORSYTE’s 2018 paper awards

FORSYTE has had a quite successful year: Adrian Rebola Pardo and his co-authors received the IJCAR best paper award for their paper Extended Resolution Simulates DRAT, Mitra Tabaei Befrouei and her co-authors received an OOPSLA 2018 Distinguished Paper award for their paper Randomized Testing of Distributed Systems with Probabilistic Guarantees, and Thomas Pani received the […]

## OOPSLA Distinguished Paper Award

Mitra Tabaei Befrouei and her co-authors from MPI-SWS Burcu Kulahcioglu Ozkan, Rupak Majumdar, and Filip Niksic, received an OOPSLA’18 Distinguished Paper Award for their contribution “Randomized Testing of Distributed Systems with Probabilistic Guarantees” (Open Access article). Congratulations!

## Zvonimir Rakamiric visiting FORSYTE

Prof. Zvonimir Rakamiric from the School of Computing at the University of Utah is spending his sabbatical with the FORSYTE group at TU Wien. He is generously sponsored by the Wolfgang Pauli Institute and a Pauli Fellow.