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(At least) ExactSumConstraint doesn't handle negative numbers properly #33

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ltskinol opened this issue Aug 26, 2017 · 3 comments
Open

(At least) ExactSumConstraint doesn't handle negative numbers properly #33

ltskinol opened this issue Aug 26, 2017 · 3 comments
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@ltskinol
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ltskinol commented Aug 26, 2017 •
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myth1 python-constraint-master # python
Python 2.7.12 (default, Dec 17 2016, 17:32:09)
[GCC 4.9.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from constraint import *
>>> p = Problem()
>>> p.addVariable('a', [-1,0,1])
>>> p.addVariable('b', [-1,0,1])
>>> p.addConstraint(ExactSumConstraint(0), ['a', 'b'])
>>> print p.getSolutions()
[{'a': 0, 'b': 0}]
>>>

Shouldn't a==-1, b==1 and a==1, b==-1 also be solutions?

Thanks.
@s-celles
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What is your opinion @niemeyer about this?

@agriffis
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ExactSumConstraint implicitly assumes the inputs are non-negative and optimizes internally for that assumption. I didn't write it, but my guess is that finding exact sums from sets of non-negative numbers fits a certain problem domain, and this was intended for that.

It might be better to update the documentation for ExactSumConstraint than to change its behavior, and @ltskinol could write a separate constraint class for their problem domain.

@fjwillemsen
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This seems like the ExactSumConstraint doesn't behave as can be expected from the name. I don't have the time to look into it, if someone else could that would be great.

@fjwillemsen fjwillemsen added the bug label Feb 1, 2025
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@agriffis @s-celles @fjwillemsen @ltskinol