Publishing Software Carpentry lesson on GAP

5 minute read


Following the 2nd CoDiMa training school, I have published the Software Carpentry lesson on GAP via Zenodo: see 10.5281/zenodo.167362. The lesson is based on the problem of determining an average order of an element of a finite group, and finding examples of groups for which the average order of their elements is an integer. First I have heard about this problem when Steve Linton used it in a talk in order to quickly demonstrate some GAP features to a general scientific audience. I have tried to expand on it in my talk in Newcastle in May 2015 (see the blog post  here),  and decided to proceed with it.

Indeed, the problem of determining an average order of the element of the group  is simple enough to not to distract learners too much from the intended learning outcomes of the lesson. An undergraduate algebra course is sufficient for its  understanding. Moreover, those not familiar with the group theory still should be able to follow the lesson just by grasping the idea that there is a mathematical structure called group, and we need to find an average value of a certain numerical parameter associated with each element of it. On the other hand, those with sufficient theoretical background will hopefully enjoy seeing how the initial naive implementation is being refined several times during the lesson, and how theoretical insights are giving much more significant advances than minor code optimisations or just getting more cores.

The lesson starts with formulating the problem of finding examples of groups  such that the average order of their element is an integer. It first explains how to work with the GAP command prompt, demonstrates some basic language constructions and explains how to find necessary information in the GAP help system. At this point using the command line we establish a rough prototype of the code to compute an average order of a group, and tried several examples, none of them yielding an integer.

Next, it discusses that the command line usage is good only for rapid prototyping, and explains how to create GAP functions, place them into a file, and read that file into GAP. After that, the initial implementation is used to create a regression test: GAP runs it by comparing the actual output with the reference output, and will fail the test in case of any discrepancies. Testing the code is a topic that usually escapes beginners’ attention, and I am really excited about managing to cover it as a part of the introductory GAP lesson. I explain the “make it right, than make it fast” paradigm, and show how to create and run regression tests in GAP after the first naive implementation is available. To demonstrate test failures, I deliberately mixed up function names to break the test, and it was a real pleasure when the audience pointed that out before I even managed to re-run the test and demonstrate that it failed.

Having the improved and tested implementation, we start systematic search for  finite groups with an integer average order of an element using the GAP Small Groups Library which contains, among others, all 423 164 062 groups of order at most 2000 except 1024.  At the same time, the lesson introduces modular programming and shows how one can design a system of functions to perform the search in a way that one could re-use  most of the code and only develop a new function to test a single group to deal  with another search problem. Then the first interesting example (a group of order 105 such that the average order of its elements is 17) is discovered! The next obstacle is to check all 56092 groups of order 256, however, a short theoretical observation shows that we can exclude groups of prime power order from the search, as they will never have an integer average order of an element. After modifying the code to skip such orders, the search continues, and then another example (a group of order 357) is found. Discovering another known group with this property is left as one of the exercises.

The lesson finishes with explaining how the knowledge about the object can be stored in it. For example, once the average order of an element of a group is calculated and stored in the group, it can be next time retrieved at zero cost,  avoiding redundant calculations.

Of course, it is not possible to cover everything in a several hours long course, but it fits really well into the week-long CoDiMa training school like  this. It prepares the audience to hear  about more advanced topics during the rest of the week: debugging and profiling;  advanced GAP programming; GAP type system; distributed parallel calculations;  examples of some algorithms and their implementations, etc. Also, staying for  the whole week of the school, everyone has plenty of opportunities to ask  further questions to instructors.

What next? So far I am only aware that it has been taught twice (by myself) at two annual  CoDiMa training schools in computational discrete mathematics.  I can surely teach it myself, but is it written clearly enough to be taught by others? Is it possible for the reader to follow it for self-studying? Is there any introductory material missing, or is there an interest in having more advanced lesson(s) on some other aspects of the GAP system? If you would like to contribute to its further development, issues and pull requests to its repository on GitHub are most welcome! Also, we invite collaborators interested in developing a lesson on SageMath: please look at this repository and add a comment to this issue  if you’re interested in contributing.