Limit functions allowed in student answers

[PhilippImhof]

The original code of the Formulas question plugin only allowed the following functions to be used by students in the answer type "Numerical formula":

abs, acos, asin, atan, ceil, cos, exp, fact, floor, ln, log10, sin, sqrt, tan

No functions accepting more than one argument could be used, e. g. atan2 was not allowed. Also, the function log was not allowed, in order to avoid ambiguity as to whether the student meant the natural or common logarithm.

With the newer parser method, the list of possible functions has grown bigger. However, for the moment, it might be better to stay with the original functions plus some more, but to still disallow functions with more than one argument. The following functions can be used in the answer type "Numerical formula":

abs, acos, acosh, asin, asinh, atan, atan2, atanh, ceil, cos, cosh, exp, fact, floor, ln, log, log10, ncr, npr, sin, sinh, sqrt, tan, tanh

The intention behind that limitation is that once multi-argument functions are allowed, it cannot be taken back. However, having multi-argument functions means that the comma becomes a meaningful character in student responses and that will make it much more complicated to allow the use of the comma as a decimal point.

When allowing multi-argument functions, it might be necessary to force students to use the semi-colon as the argument separator, or to find other (better) solutions. Keeping those functions our for the moment leaves time to think that through.

[PhilippImhof]

As log is no longer (or still not) allowed, we add lg as a synonym for log10, because it is frequently used in certain countries and it is an official notation according to ISO 80000-2.

[coveralls]

Pull Request Test Coverage Report for Build 14559089128

Details

• 5 of 5 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 95.81%

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Totals
Change from base Build 14533404972:	0.003%
Covered Lines:	3773
Relevant Lines:	3938

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💛 - Coveralls