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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 :     # object. If the context has more than one variable, the first one
17 :     # 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 :     # ANS(DifferenceQuotient("-a/t(t+dt)","dt")->cmp);
32 :     #
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 :     my $formula = shift;
44 :     my $dx = shift || 'd'.($$Value::context->variables->names)[0];
45 :     #
46 :     # Save the original context, and make a copy to which we
47 :     # add a variable for 'dx'
48 :     #
49 :     my $current = $$Value::context;
50 :     my $context = main::Context($current->copy);
51 :     $context->{_variables}->{pattern} = $context->{_variables}->{namePattern} =
52 :     $dx . '|' . $context->{_variables}->{pattern};
53 :     $context->update;
54 :     $context->variables->add($dx=>'Real');
55 :     $dx = bless $self->SUPER::new($formula), $class;
56 :     $dx->{isValue} = 1; $dx->{isFormula} = 1;
57 :     main::Context($current); # put back the original context;
58 :     return $dx;
59 :     }
60 :    
61 :     sub cmp_class {'a Difference Quotient'}
62 :    
63 :     sub cmp_defaults{(
64 :     shift->SUPER::cmp_defaults,
65 :     ignoreInfinity => 0,
66 :     )}
67 :    
68 :     sub cmp_postprocess {
69 :     my $self = shift; my $ans = shift;
70 :     return if $ans->{score} == 0 || $ans->{isPreview};
71 :     $main::__student_value__ = $ans->{student_value};
72 :     my ($value,$err) = main::PG_restricted_eval('$__student_value__->reduce(dx=>0)');
73 :     $self->cmp_Error($ans,"It looks like you didn't finish simplifying your answer")
74 :     if $err && $err =~ m/division by zero/i;
75 :     }
76 :    

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