Missing documentation for syntax (parser)

[creativecreatorormaybenot]

I think it is awesome that functions like Log are supported 🙌

Problem

However, I cannot seem to figure out how to use something like log_e(x).

I would expect this to use e as the base and be equivalent to ln(x), however, it throws during parsing.

FormatException: The input String is not a correct expression

[creativecreatorormaybenot]

And btw, @fkleon what is this supposed to represent?

math-expressions/test/parser_test_set.dart

Line 157 in ea54ff7

inputStrings.add('log(10,100)');

Afaic, log only takes 1 argument. I tried log(e,x) just out of curiosity, however, it did not work.

[fkleon]

Hi @creativecreatorormaybenot - thanks for your questions and reports, always great to get feedback from users!

I agree that the parser syntax documentation is mostly missing, I'll add some specific documentation to the appropriate expression classes. As you've discovered the unit tests are the source of truth at the moment.
Historically the parser was just an afterthought mainly for testing purposes, but people seem to use it a lot! There's also a new parser implementation in the pipeline that will hopefully fix some of the oddities and issues.

Regarding your specific question, math-expressions implements two logarithm functions:

• Log is the generic logarithm function with arbitrary base and argument. The parser does not support constants such as π (Pi) or e (Euler's Number), so you can only approximate the natural algorithm with this, e.g. log(2.71828, x) (read: logarithm base 2.71828 of x)
• Ln is the natural logarithm function (log base e). This can be expressed as ln(x) (read: logarithm base e of x)

Hope that answers your question, I'll leave the ticket open though for the documentation improvements.

[fkleon]

This is not explicitly documented yet, but starting from version 2.0.0 the parser should be compatible with thetoString() representation of any expression. So in addition to the tests, the toString() implementation of an expression defines the expected syntax.

[creativecreatorormaybenot]

@fkleon These news on the parser are absolutely amazing! Thank you ✨

We actually also have some unit tests for expression equality (using your package for evaluation) and I wanted to add a test for ln(x) = log_e(x).

I also realized how log worked after a while (that e, was unsupported) and came up with ln(x) / ln(10) = log(10, x) 😃