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1 : dpvc 2730 loadMacros('Parser.pl');
2 :    
3 :     sub _parserDifferenceQuotient_init {}; # don't reload this file
4 :    
5 :     ######################################################################
6 :     #
7 :     # This is a Parser class that implements an answer checker for
8 :     # difference quotients as a subclass of the Formula class. The
9 :     # standard ->cmp routine will work for this. The difference quotient
10 :     # is just a special type of formula with a special variable
11 :     # for 'dx'. The checker will give an error message if the
12 :     # student's result contains a dx in the denominator, meaning it
13 :     # is not fully reduced.
14 :     #
15 :     # Use DifferenceQuotient(formula) to create a difference equation
16 : dpvc 3867 # object. If the context has more than one variable, the last one
17 : dpvc 2730 # alphabetically is used to form the dx. Otherwise, you can specify
18 :     # the variable used for dx as the second argument to
19 :     # DifferenceQuotient(). You could use a variable like h instead of
20 :     # dx if you prefer.
21 :     #
22 :     # Usage examples:
23 :     #
24 :     # $df = DifferenceQuotient("2x+dx");
25 :     # ANS($df->cmp);
26 :     #
27 :     # $df = DifferenceQuotient("2x+h","h");
28 :     # ANS($df->cmp);
29 :     #
30 :     # Context()->variables->are(t=>'Real',a=>'Real');
31 : dpvc 3661 # ANS(DifferenceQuotient("-a/[t(t+dt)]","dt")->cmp);
32 : dpvc 2730 #
33 :    
34 :     Context("Numeric");
35 :    
36 :     sub DifferenceQuotient {new DifferenceQuotient(@_)}
37 :    
38 :     package DifferenceQuotient;
39 :     our @ISA = qw(Value::Formula);
40 :    
41 :     sub new {
42 :     my $self = shift; my $class = ref($self) || $self;
43 : dpvc 3867 my $formula = shift; my $current = $$Value::context;
44 :     my $dx = shift || $current->flag('diffQuotientVar') || 'd'.($current->variables->names)[-1];
45 : dpvc 2730 #
46 :     # Save the original context, and make a copy to which we
47 :     # add a variable for 'dx'
48 :     #
49 :     my $context = main::Context($current->copy);
50 : dpvc 3867 $context->variables->add($dx=>'Real') unless ($context->variables->get($dx));
51 : dpvc 3662 $q = bless $self->SUPER::new($formula), $class;
52 : dpvc 3971 $q->{isValue} = 1; $q->{isFormula} = 1; $q->{'dx'} = $dx;
53 : dpvc 2730 main::Context($current); # put back the original context;
54 : dpvc 3662 return $q;
55 : dpvc 2730 }
56 :    
57 :     sub cmp_class {'a Difference Quotient'}
58 :    
59 :     sub cmp_defaults{(
60 :     shift->SUPER::cmp_defaults,
61 :     ignoreInfinity => 0,
62 :     )}
63 :    
64 :     sub cmp_postprocess {
65 : dpvc 3971 my $self = shift; my $ans = shift; my $dx = $self->{'dx'};
66 : dpvc 2730 return if $ans->{score} == 0 || $ans->{isPreview};
67 :     $main::__student_value__ = $ans->{student_value};
68 : dpvc 3971 my ($value,$err) = main::PG_restricted_eval('$__student_value__->substitute(\''.$dx.'\'=>0)->reduce');
69 : dpvc 2730 $self->cmp_Error($ans,"It looks like you didn't finish simplifying your answer")
70 :     if $err && $err =~ m/division by zero/i;
71 :     }
72 :    

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