## WeBWorK Problems

### contextLimitedPowers

by Gavin LaRose -
Number of replies: 1
Hi all,

I'm trying to use the LimitedPowers Context, and am finding that it is requiring a different incantation than I expect. I expected that the following would work: loadModules("parserLimitedContext.pl");
Context("LimitedContext::NoBaseE()"); or, alternately, the same thing omitting the () at the end of NoBaseE. However, I'm finding that doesn't work (it fails with an unknown context error), and that the following does loadModules("parserLimitedContext.pl");
$c = Context("Numeric")->copy(); LimitedPowers::NoBaseE($c );
Context(\$c); If anyone can explain what I'm missing (that is, why the latter works and the former doesn't), I'd be much appreciative.

Thanks,
Gavin

### Re: contextLimitedPowers

by Davide Cervone -
Gavin:

Your problem is that LimtedPowers::NoBaseE() is not the name of a context, but rather a function that modifies contexts. Calling it causes the current context to be modified, so that

    Context("Numeric");
LimitedPowers::NoBaseE();

would modify the Numeric context to not allow e to a power. The reason for this is so that you can modify any context (e.g., Numeric, or Complex, or even Vector) rather than having to have separate contexts for each. Also, the function can take parameters that control the limitations. For example
    LimitedPowers::OnlyIntegers(
minPower => -5, maxPower => 5,
message => "integer constants between -5 and 5",
);

allows integer powers only and then only between -5 and 5, and it provides a custom message to issue when the conditions aren't met.

Rather than modifying the current context, you can pass LimitedPowers::NoBaseE() and its brethren a context to operate on. That is how come your second example works. You are passing it a (copy) of the Numeric context, modifying that to not allow powers of e, and then setting it as the current context. That is actually equivalent to my first example above, since setting the context to Numeric already gets you a copy.

Anyway, hope that clear things up.

Davide