** Using WeBWorK **

It seems to me that it ought to be possible to use existing answer evaluators in new ones. However, my limited brain power is insufficient to accomplish this. I tried the simplest thing I could think of, which is the following.

sub newEvaluator {<br />    my $cAns = shift();<br />    my %opts = @_;<br />    my $evaluator = new AnswerEvaluator();<br />    $evaluator->install_evaluator(~~&fun_cmp, %opts);<br />    return $evaluator;<br />}

And then called my new evaluator with ANS(newEvaluator($answer)). But when I do this, I get the error message

Error in /cgi-bin/webwork-cgi/cgi-scripts/processProblem8.pl
Can't locate object method "error_flag" via package AnswerEvaluator 
at /opt/www/webwork/system/lib//PGtranslator.pm line 939.

Which seems to say that my simple-minded approach is, well, too simple-minded. If anyone could tell me how to do this correctly, I'd be appropriately grateful.

Thanks,
Gavin

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It should work more or less this way, but it doesn't quite yet. Give me a few hours to give you an example of using one answer evaluator inside another and I'll post it here. See you later tonight (or tomorrow morning)

--Mike

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Here's the source

##DESCRIPTION
##  A very simple first problem
##ENDDESCRIPTION

##KEYWORDS('algebra')

DOCUMENT();        
# This should be the first executable line in the problem.

loadMacros(
"PG.pl",
"PGbasicmacros.pl",
"PGchoicemacros.pl",
"PGanswermacros.pl",
"PGauxiliaryFunctions.pl"
);

TEXT(beginproblem());
$showPartialCorrectAnswers = 1;

$a = random(-10,-1,1);
$b = random(1,11,1);
$c = random(1,11,1);
$d = random(1,11,1);

BEGIN_TEXT
This problem demonstrates how you enter numerical answers into WeBWorK. $PAR
Evaluate the expression \(3($a )($b -$c -2($d ))\):
 \{ ans_rule(10) \}
$BR

END_TEXT


$ans = 3*($a)*($b-$c-2*($d));

# Here are the important lines where we build a new 
# answer evaluator using the old.  
# The new answer evaluator doesn't do much. Just adds
# the message 'experimental'.

$old_ans_eval = num_cmp($ans);

# notice that num_cmp() is a function which PRODUCES an answer evaluator 
# or technically a pointer to the answer evaluator object.  I prefer to think
# of $old_ans_eval as actually containing the answer evaluator object.

$new_eval = new AnswerEvaluator;

# now we have a new, empty answer evaluator

$new_eval->install_evaluator(sub {my $rh_ans=shift; <br />                                  $rh_ans = $old_ans_eval->evaluate($rh_ans->{student_ans});
                                  $rh_ans->{ans_message}='experimental'; 
                                  $rh_ans;} 
);

# We have added a new filter to the new answer evaluator.
# The structure of a filter is that it is supposed to be a subroutine 
# which accepts an answerHash object and returns an answerHash object.
# The first problem is that an AnswerEvaluator is not subroutine 
# (it's more complicated) so one can't use
# &$old_ans_eval($answer_hash) to process an answer hash, you need to say
# $old_ans_eval->evaluate($answer_hash).  
# For this routine we wrap a subroutine around the old answer evaluator 
# so that it will behave correctly.
# [Note:  The evaluate routine in AnswerEvaluator should (IMHO) be modified 
# so it can accept filters which are answer evaluators as well as ones
# which are ordinary subroutines. This won't be too hard, only a few hours work. ] 

# Secondly, even that might not work -- most answer evaluators, the ones built 
# with all of the AnswerEvaluator tools accept either
# a string input, OR an AnswerHash input.  Some of the older answer evalatuors, 
# may not do this reliably 
# -- there has not yet been a lot of reuse of answer evaluators so 
# there has not been much testing in this area.
# That appears to be the case with num_cmp type evaluators, so 
# instead of feeding it an answerHash
# we find the student answer inside the AnswerHash and feed it that. It will 
# take some work, but 
# eventually we should be able to refit all answer evaluators 
# so that they will accept either a 
# string or an AnswerHash and can therefore be used one inside the other.
# At the moment the evaluators produced by str_cmp aren't 
# even AnswerEvaluator objects, they are subroutines!


# The final thing the filter does is to add the string 'experimental' to the 
# answer message of the AnswerHash, just to prove that we've actually done
# something.  You could do much more.

# Final note -- I often use $rh_ans, the $rh_ means it's a pointer (reference) to a 
# hash value, this way I can remember that the scalar variable holds something
# more complicated than a single value.  I also use $r_ for reference, and $ra for 
# a reference to an array.  Unfortunately I'm not consistent about this, but I use
# technique when I'm likely to get confused as to what is a value and what is a pointer.

ANS($new_eval);


ENDDOCUMENT();        
# This should be the last executable line in the problem.

Here is what the problem looks like.

This problem demonstrates how you enter numerical answers into WeBWorK. Evaluate the expression :

# we rely on $opts{'var'} being defined as an array reference
if ( defined( $opts{'var'} ) ) {<br /> $opts{'var'} =
( ref($opts{'var'}) ? $opts{'var'} : [$opts{'var'}] );
} else {<br /> $opts{'var'} = [ 'x' ];
}

my @outEvaluators = ();
foreach my $cAns ( @cAnsList ) {<br /> push(@outEvaluators, diffl_eval($cAns, %opts));<br /> }
return (wantarray) ? @outEvaluators : $outEvaluators[0];
}
sub diffl_eval { <br /> my $cAns = shift();<br /> my %opts = @_;</p><p><br /><br /> my $evaluator = new AnswerEvaluator('correct_ans' => $cAns,<br /> 'type' => 'diffl_fun_cmp',<br /> 'original_correct_ans' =><br /> $cAns);<br /> $evaluator->install_pre_filter( ~~&replace_differentials_filter,<br /> %opts );<br /> $evaluator->install_evaluator( ~~&diffl_fun_eval, %opts );<br /> $evaluator->install_post_filter( ~~&restore_vars_filter, %opts );<br /> return $evaluator;<br />}
sub diffl_fun_eval {<br /> my $rh_ans=shift; <br /> my %opts = @_;</p><p><br /><br /> my $cAns = $rh_ans->{'correct_ans'};

my @vars = ( @{$opts{'var'}}, @{$rh_ans->{'added_vars'}} );
$opts{'var'} = [ @vars ];

my $func_eval = fun_cmp($cAns, %opts);
$rh_ans = $func_eval->evaluate( $rh_ans->{'student_ans'} );

return $rh_ans;
}
sub replace_differentials_filter {<br /> my $rh_ans = shift();<br /> my %opts = @_;</p><p><br /><br /> my $student_input = $rh_ans->input();<br /> my $correct_answer = $rh_ans->{'correct_ans'};

my @subs = ( 'P', 'R', 'S', 'M', 'N', 'O' );
my @addedVars = ();
for ( my $i=0; $i<@{$opts{'var'}}; $i++ ) {<br /> $student_input =~ s/d$opts{'var'}->[$i]/$subs[$i]/g;
$correct_answer =~ s/d$opts{'var'}->[$i]/$subs[$i]/g;
push( @addedVars, $subs[$i] );
}
$rh_ans->input($student_input);
$rh_ans->{'correct_ans'} = $correct_answer;
$rh_ans->{'added_vars'} = [ @addedVars ];

return $rh_ans;
}
sub restore_vars_filter {<br /> my $rh_ans = shift();<br /> my %opts = @_;</p><p><br /><br /> my $student_input = $rh_ans->input();<br /> my $correct_answer = $rh_ans->{'correct_ans'};
my $original_input = $rh_ans->{'original_student_ans'};
my $text_preview = $rh_ans->{'preview_text_string'};
my $latex_preview = $rh_ans->{'preview_latex_string'};

my @subs = ( 'P', 'R', 'S', 'M', 'N', 'O' );
for (my $i=0; $i<@{$opts{'var'}}; $i++) {<br /> $student_input =~ s/$subs[$i]/d$opts{'var'}->[$i]/g;
$correct_answer =~ s/$subs[$i]/d$opts{'var'}->[$i]/g;
$original_input =~ s/$subs[$i]/d$opts{'var'}->[$i]/g;
$text_preview =~ s/$subs[$i]/d$opts{'var'}->[$i]/g;
$latex_preview =~ s/$subs[$i]/d$opts{'var'}->[$i]/g;
}

$rh_ans->input($student_input);
$rh_ans->{'correct_ans'} = $correct_answer;
$rh_ans->{'original_student_ans'} = $original_input;
$rh_ans->{'preview_text_string'} = $text_preview;
$rh_ans->{'preview_latex_string'} = $latex_preview;
delete( $rh_ans->{'added_vars'} );

return $rh_ans;
}

Thanks again, Mike
Gavin

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