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Hey,

When looking through Coin-Or, I found they were referring to a constraint type called "free constraint". For instance here.

Does any one know what a free constraint is?

Thanks

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1 Answer

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Basically, it's a linear expression whose value as a function of the variable values should be tracked, but is not bounded. Another way to think about it is that a constraint in a general form is b_u <= ax <= b_u, where the constraint is equality if b_l = b_u, less-or-equal if b_u is finite and b_l is (negative) infinite, and greater-or-equal if b_l is finite and b_u is infinite. Then a free constraint is one where both b_l and b_u are infinite.

(BTW, "constraint programming" isn't a very good tag for this question.)

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Hey, Thanks for the response. So if I understand this correctly, a free constraint us useful to keep track of since there is a chance it might become an actual constraint in the course of the tree search? PS - What tag would you recommend? – Sid Jan 22 at 20:16
It's more general than that. It's really more of a placeholder for a constraint that you could make active later on for a variety of reasons. It might also might be used as an alternate objective function. (I think that was its main role in the original MPS input file format was.) I think just "optimization" and/or "modeling" would be more suitable as a tag. "Constraint programming" is a different sort of modeling process and search algorithm, with some connections to mathematical programming. See the "CP and Integer Programming" post. – Matthew Saltzman Jan 22 at 20:27
So on what basis is a free constraint generated. For instance, I assume it does not just consist of randomly generated coefficients for the variables. Any reference would be appreciated. – Sid Jan 22 at 20:53
I don't really know what would auto-generate one of these as part of a tree search. I've always thought of it in the context of inputting a model. – Matthew Saltzman Jan 22 at 21:12

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