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Re: Using overload to parse arithmetic terms

by japhy (Canon)
on Feb 07, 2006 at 12:52 UTC ( #528498=note: print w/ replies, xml ) Need Help??


in reply to Using overload to parse arithmetic terms

If you want to output a properly parenthesized mathematical expression, you'll have to do so on the fly, examining the tree that I assume you have in your Arithmetic object. For example, if you have $x = ($a + $b) * $c;, then you probably have a tree like:

Arithmetic::Multiply( Arithmetic::Add( Variable('a'), Variable('b') ), Variable('c') )
When you're walking that tree, when you get to a Multiply or Divide node, you have to check to see if its children are Add or Subtract nodes; if they are, those child nodes need to be surrounded by parentheses when output.

Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart


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Re^2: Using overload to parse arithmetic terms
by blokhead (Monsignor) on Feb 07, 2006 at 14:41 UTC
    To expand (and generalize) on this a bit, let each type of operation have a precedence. Then as you walk the expression tree, you need to parenthesize operations that have a lower precedence than the expression they appear within.

    A good way to do this is via OO, representing your operation types as subclasses:

    package Op; sub new { my $pkg = shift; bless [@_], $pkg; } sub children { @{ $_[0] } } sub display { my ($self, $precedence) = @_; $precedence = 0 if not defined $precedence; my $s = join $self->op, map { $_->display($self->precedence) } $self->children; $s = "($s)" if $precedence > $self->precedence; return $s; } package Op::Addition; @ISA = qw[Op]; sub precedence { 10 } sub op { " + " } package Op::Multiplication; @ISA = qw[Op]; sub precedence { 20 } sub op { " * " } # ... package Op::Term; @ISA = qw[Op]; sub precedence { 100 } sub display { $_[0][0] }
    Then you just call the display method on an Op tree. For example,
    Op::Multiplication->new( Op::Term->new(5), Op::Addition->new( Op::Term->new(6), Op::Term->new(7) ) )->display; # 5 * (6 + 7) Op::Addition->new( Op::Term->new(5), Op::Multiplication->new( Op::Term->new(6), Op::Term->new(7) ) )->display; # 5 + 6 * 7
    And you can see how easy it would be to add new types of operations, due to the beauty of subclassing.

    blokhead

      That is a beautiful approach and technique. ++ well deserved.

      Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
      How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart
      perfect ... thanks.

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