** Using WeBWorK **

The code below, which defines an answer evaluator, doesn't work. The problem is the + signs. If I replace

    $ans =~ s/\s+//g

with

    $ans =~ s/ //g

and remove the + sign in the if test, everything is fine (except, of course, that then the answer I want from the students doesn't match what's in the if test). I fixed this particular problem by replacing the m/// by a substring test, but how do you program around it in general? And why does it happen? (I also tried $ans =~ s/\+/X/ which doesn't do anything either. None of these things results in an error message; the code just fails silently.)

Thanks again,

Steve

$anseval = sub {

     my $student_ans = shift;
     my $correct = 0;
     my $ans = $student_ans;

     $ans =~ s/\s+//g; # FAILS
     $ans =~ tr/[a-z]/[A-Z]/;
     if ($ans =~ /^P\+PRT$/ || $ans =~ /^P\(1\+RT\)$/) {$correct = 1;} # FAILS

     my $anshash = new AnswerHash();
     $anshash->{'score'} = $correct;
     $anshash->{'correct_ans'} = "P+Prt or P(1+rt)";
     $anshash->{'student_ans'} = $student_ans;
     $anshash->{'type'} = "SPC";

     return $anshash;

};

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Bob

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My guess is that there might be simpler ways of coding this answer evaluator. For the most part, this seems to be a string checker ignoring case and whitespace. I think some version of str_cmp can handle that (or str_cmp with the right flags set). You could then use the pc evaluator to put two of them together as an "or".

Alternatively, you could code this with fun_cmp with variables P, r, and t. Then, "(1+rt)P", "P+rtP", and similar variations, would also be marked correct automatically. If you want to allow anything algebraically equivalent to the right answer, then fun_cmp would be the way to go.

John

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