Add a parserLogb.pl macro for logarithms with base b.

[drgrice1]

This is similar to the parserRoot.pl macro. To enable the usage of the logb function in the current context call Parser::Logb->Enable.

Then you can call $n = logb(3, 5) to get the value of the logarithm with base 3 evaluated at 5, or $ans = Compute("logb(3, 5)") to use as an answer.

You can also call Parser::Logb->EnableComplex to allow negative bases and evaluation at negative numbers. Note that attempting to use a negative base or evaluate at a negative number would otherwise produce an error message.

An answer in this context will automatically have the MathQuill option logsChangeBase => 0 set. To facilitate this without the need to have numerous special cases for contexts in the ENDDOCUMENT method of PG.pl, there is a new mathQuillOpts context flag. The default value for this flag is a reference to a hash with no keys. Any context can set keys in this hash, and those will be tranferred to the MathQuill options for any answer rule in the context. This is now used by the the parserRoot.pl macro to set rootsAreExponents => 0, and by the new parserLogb.pl macro to set logsChangeBase => 0.

To facilitate students entering a logarithm with a base, if the logsChangeBase option is 0 (or not set), then a subscript button is added to the MathQuill toolbar.

Note that MathQuill is already set up to output the text logb(b, x) if log_b(x) is entered.

[Alex-Jordan]

I find that I have to set

Context()->flags->set(mathQuillOpts => {});

before I run Parser::Logb->Enable;, or else I get an error like:

Can't use an undefined value as a HASH reference at line 92 of [PG]/macros/parsers/parserLogb.pl
   from within Parser::Logb::Enable called at line 6 of undefined

Here is the file I am testing with:

DOCUMENT();

loadMacros(qw(PGstandard.pl PGML.pl parserLogb.pl));

#Context()->flags->set(mathQuillOpts => {});
Parser::Logb->Enable;
$n = logb(3, 5);

BEGIN_PGML
Enter a value for [`[$n]`].

[_]{$n}{5}
END_PGML

ENDDOCUMENT();

[Alex-Jordan]

Something came up today where a person used lg(), meaning the logarithm base 2. So it comes to mind when looking atht his. We have log(), log10(), and ln(). Is it appropriate to add lg()? And if so, is it appropriate to include that as part of this macro?

[drgrice1]

I tested with your sample code, and I do not see any errors or warnings about undefined HASH references. The mathQuillOpts context flag should always be defined to be a reference to an empty hash. It is set in lib/Parser/Context/Default.pm.

I am not sure that I agree to adding lg for a logarithm with base 2. That notation seems to be primarily a computer science notation, and it seems to be rather heavily disputed as to whether it is the logarithm with base 2 or base 10 (see for instance https://en.wikipedia.org/wiki/Binary_logarithm#Notation). This macro is designed to handle logarithms with explicit bases given. Special notations like log, ln, log10 and something like lg are not the intent for this macro.

[drgrice1]

Ahh, you may need to restart the webwork2 app to get the changes in this pull request to take effect (if you are not using morbo). The change in Default.pm occurs when the safe compartment is compiled, and that happens when the webwork2 app is first started. That may be why you got the error about the undefined value used as a HASH reference.

[Alex-Jordan]

Yeah, oops. I had not restarted webwork2. This looks good and works in all the ways.