| Subject: | simplify() wrong on sum of negative multiple |
| Date: | Tue, 28 Dec 2010 08:16:08 +1100 |
| To: | bug-Math-Symbolic [...] rt.cpan.org |
| From: | Kevin Ryde <user42 [...] zip.com.au> |
In Math::Symbolic 0.604 on recent i386 debian perl 5.10.1 the program
foo.pl below "simplifies" an expression
x + (-5)*y
to
x + (-(-5 * y))
which I think has wrongly doubled the negation. I expected maybe
x + (-(5 * y))
It seems when a sum "x + (-5)*y" is simplified the "-5" multiple of y
becomes a U_MINUS but also leaves the negative on the multiplier "-5".
Is it meant to have an abs() if it does that, per below?
I struck this when trying "2*x^2 + (-1)*x" which should be the hexagonal
numbers but gave wrong results unless entered as "2*x^2 - x".
I might have been happy for the simplifier to leave a -5 constant
instead of turning it into an extra U_MINUS operation, at least for
numerical use where a negative -5 is the same price as a +5. Unless
that's an option for that I haven't spotted. I suppose for human
readability a subtract instead of add+uminus would be possible too, like
"x - (5*y)", unless that's meant to be left to the print stage.
use strict;
use warnings;
use Math::Symbolic;
my $tree = Math::Symbolic->parse_from_string('x + (-5)*y');
print "parsed: ",$tree->to_string,"\n";
print " ",$tree->to_string('prefix'),"\n";
my $simp = $tree->simplify;
print "simplified: ",$simp->to_string,"\n";
print " ",$simp->to_string('prefix'),"\n";
--- Operator.pm.orig 2010-12-27 15:24:47.000000000 +1100
+++ Operator.pm 2010-12-27 15:24:47.000000000 +1100
@@ -835,7 +835,7 @@
}
my $mul = Math::Symbolic::Operator->new(
'*',
- Math::Symbolic::Constant->new($num),
+ Math::Symbolic::Constant->new(abs($num)),
Math::Symbolic::Variable->new($_)
);
push @ops, $num < 0