Warnings when using upToConstant where C0 is already defined

[dlglin]

The code that is triggered by setting the upToConstant flag in an answer checker is hard-coded to add C0 as the parameter, which can cause issues if that variable is already present in the context.

The scenario originally arose when calling $ans->cmp(upToConstant=>1) on the same object more than once. Consider the MWE:

DOCUMENT();

loadMacros("PGstandard.pl","PGML.pl","PGcourse.pl");

$ans = Compute("x");

BEGIN_PGML
[_]{$ans->cmp(upToConstant=>1)}

[_]{$ans->cmp(upToConstant=>1)}
END_PGML

ENDDOCUMENT();

This leads to the (non-fatal) warning "Variable 'C0' already exists". In this case it doesn't break anything since C0 is being redefined for the same purpose.

The more dangerous scenario is when the problem author has already declared C0 as a variable in the problem. e.g.

DOCUMENT();

loadMacros("PGstandard.pl","PGML.pl","PGcourse.pl");

Context()->variables->add("C0"=>"Real");
$ans = Compute("x+C0");

BEGIN_PGML
[`x+C0=`][_]{$ans->cmp(upToConstant=>1)}
END_PGML

ENDDOCUMENT();

In this case C0 ends up redefined as a parameter rather than a real-valued variable, which means that answers are not checked correctly. Any answer including C0 gets the message "Variable 'C0' is not defined in this context", where answers like x+C0 or x+C0+3 should be marked correct.

It seems like the fix would be to modify

pg/lib/Value/AnswerChecker.pm

Lines 1914 to 1915 in 0440842

	$context->variables->add('C0' => 'Parameter');
	my $f = $self->Package("Formula")->new('C0') + $self;

to first check if C0 exists in the context, and if so find a new variable name to use instead. How hard would this be to do?

[dpvc]

How hard would this be to do?

It should not be too hard to do. You have the correct location in the file. A loop like

my $i = 0;
while ($context->variables->get("C$i")) {
  $i++;
}
$context->variables->add("C$i" => 'Parameter'); 
 my $f = $self->Package("Formula")->new("C$i") + $self; 

should do the trick (though I haven't tried this).