## WeBWorK Problems

### a weak alternative to a polymorphic custom checker

by Dick Lane -
Number of replies: 2
After lots of naive tries at an answer checker to handle student responses of different types, I realized that this task might be better handled before sending a student response to a single ANS command.

Problem setup:  3 points given
Task: enter a normal to plane they determine or NONE if they are collinear
student response to process: vector or string

current solution: (with $normal equal to cross product of suitable vectors and$abNorm being its norm)
if  ($abNorm == 0) { ANS( List('NONE') -> cmp() ;$msg = 'comment about collinear points' ;
}  else  {
ANS( $normal -> cmp( parallel => 1 ) ) ;$msg = 'scalar multiples are also correct' ;
}
Note: both of those ANS modes seem relatively quiet about chiding a student for entering the wrong type of info.
Late insight: this provides a method for having a relevant $msg within my SOLUTION. failed attempts at a custom checker (with$ab being cross product of PQ and PR vectors)
[the first presumes a "lazy evaluation" of compound clauses]
ANS( $ab -> cmp( checker => sub { my ($correct , $student ,$ansHash ) = @_ ;
return ( ( (norm($correct) == 0) && ($student == "NONE") )
||      ( $correct -> isParallel($student ) ) ) ;
} ,
showCoordinateHints => 0 )
) ;

ANS( $ab -> cmp( parallel => 1 , checker => sub { my ($correct , $student ,$ansHash ) = @_ ;
return $student == "none" if (norm($correct) == 0) ;
return  ($correct ==$student) ;
} ,
showCoordinateHints => 0 )
) ;

### Re: a weak alternative to a polymorphic custom checker

by Gavin LaRose -
Hi Dick,

I may be misinterpreting your intent here. I would be inclined to have a single answer blank for the vector, or the string "none" if the three points are collinear. Something like

  Context("Vector");

for ( my $i=0;$i<3; $i++ ) {$x[$i] = random(1,10,1);$y[$i] = random(1,10,1);$z[$i] = random(1,10,1); }$pq = Vector( ( $x[1]-$x[0], $y[1]-$y[0], $z[1]-$z[0] ) );
$pr = Vector( ($x[2]-$x[0],$y[2]-$y[0],$z[2]-$z[0] ) );$ab = $pq x$pr;
$abnorm = norm($ab);

BEGIN_TEXT
enter the vector, or none: \{ ans_rule(25) \}
END_TEXT

ANS( $ab->cmp( checker=>sub { my ($c, $s,$ans ) = @_;
return ( $abnorm &&$c == $s ) || ( !$abnorm && String("none") == $s ); } ) );  (This is mostly just a skeleton of a problem, and isn't tested at all.) Gavin In reply to Gavin LaRose ### Re: a weak alternative to a polymorphic custom checker by Dick Lane - Thanks, Gavin, for your improvement to my stab at a suitable custom checker. I didn't respond right after I read and tried it because I wanted to explore the ways in which your code was better. I suspected the use of String was crucial and confirmed that. Then I thought to make it more self-contained by not having the norm be obtained via reference to a global, so inserted a norm computation, e.g., ANS($ab->cmp( checker=>sub {
my ( $c,$s, $ans ) = @_; my$nc = norm($c) ; return ($nc && $c ==$s ) ||
( ! $nc && String("none") ==$s );
} ) );

Perhaps here, or after a further bit of tinkering, things went awry.  Part of my testing involved inserting an assignment so that the points would be collinear --- that condition had been recognized before but not after some recent change.  Even if I reverted to your code (pasted-in), what worked before did not now.

Hence, I am reverting to the version I had when writing my note: the problem template identifies, via the  if ... else  block, which of two states exist (the points determine a plane or are collinear) and then produces a single answer box which is associated with an appropriate checker (and causes the CORRECT item to be suitable --- I was not paying attention to that detail when your version worked initially, but suspect it showed <0,0,0> for the collinear case).

I find this a bit strange.  Although I did not keep written notes during my test of your version and my initial changes to it, I know collinearity was identified before it wasn't.  [I will keep one of the current strange versions to examine further when I am less rushed.]