R in a sandbox/jail
I've not tried an automated system like you describe but I have tried to make test scripts available that compare the output of their programs to correct output. That way the students can check their progress as they work. In general, I find this approach doesn't work well for grading because in order to specify a homework to the point where the students' output matches your output, the homework ends up being 10 pages long (not unlike a legal document). For example, in a number of instances I'd say "the function should return a number..." and then I'd compare the returned result to the actual answer. But a student would 'print' the number instead, not knowing the difference between 'return+autoprint' and 'print'. I found it difficult to write a test script that gives partial credit for getting the answer right but the concept wrong. Perhaps that's a problem with the way I grade things, but I found it difficult to write an acceptable test script. -roger
Barry Rowlingson wrote:
Someone recently suggested building a system for automatically testing student's R programs. They would upload them to our Virtual Learning Environment, which would then run the code on some inputs and see if it got the right output. If it does, the student scores points for that course. My first thought was "you want to run unchecked, student-submitted code on a server that has access to students' grades?". Can this be done securely? The idea might be to run R in a chroot-jail, freshly generated for each run. The jail would not be able to access anything outside of it, and once the R session has finished the calling process can pick up the output from within the jail. Maybe that's overkill. Perhaps if you run the user's code as an ordinary user and store the answers/results in a directory only root can read that would work (given no local root exploits). Other precautions could include limiting the runtime or cputime for the R session. It might be necessary to limit network access too. Anyone done anything like this? Personally I think there are too many other problems with automated systems like this, particularly that just because a program produces the correct output that makes it a good one. Sure, at the production stage that's a requirement, but I'd rather students learnt to program well than to program correctly - since correctness follows goodness but goodness does not follow correctness. But that's an argument for another day! Barry
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Roger D. Peng | http://www.biostat.jhsph.edu/~rpeng/