[SCIP] R: Quadratic Constraint Modification

[Stefan Vigerske]

Hi,

On 16/07/2024 15:41, Enrico Calandrini wrote:
> Hi Stefan,
> Thank you very much for your reply! That is exactly what I was looking for!
> I just have a quick follow-up question on what you mentioned. Let's say I have an expression like x + 2xy + 3y². Following your procedure, I should be able to retrieve all the terms of the sum (x, xy, y²) using SCIPexprGetChildren(). However, I'm now wondering how I can directly access the individual expressions to determine which variables are active within each one. Is there a method to retrieve the indices of the active variables in the expression?

You may want to inspect the expression in each child to see whether it 
is x, xy, or y^2. Use functions like SCIPisExprVar(), 
SCIPisExprProduct(), and SCIPisExprPower(). Then use SCIPgetVarExprVar() 
to get the variable in a var-expression or SCIPgetExponentExprPow() to 
get the exponent in a power expression. For a product, use 
SCIPexprGetNChildren() to get the number of children and check that each 
is a var-expression.

> Assuming this is possible, if I retrieve the vector of coefficients with SCIPgetCoefsExprSum() and modify it, is it sufficient to make the change active in SCIP? Do I need to "provide back" the new vector to SCIP in any way?

Modifying the values in SCIPgetCoefsExprSum() should be sufficient, as 
the array returned is a pointer to internal data. It is not so clean to 
do this, but there is currently no elegant way (as far as I remember).

Stefan

> Thank you very much for your time!
> Enrico
> ________________________________
> Da: Stefan Vigerske <svigerske at gams.com>
> Inviato: martedì 16 luglio 2024 15:24
> A: Enrico Calandrini <e.calandrini at studenti.unipi.it>; scip at zib.de <scip at zib.de>
> Oggetto: Re: [SCIP] Quadratic Constraint Modification
> 
> Hi,
> 
> SCIPcreateConsBasicQuadraticNonlinear() is correct. This is a wrapper
> around SCIPcreateConsBasicNonlinear(), which makes it easier to create
> nonlinear constraints that are quadratic.
> 
> There is no SCIPchgLinearCoefQuadratic() in current SCIP versions anymore.
> 
> Since SCIP 8, quadratic constraints are no longer distinguished from
> other nonlinear constraints. The function of a quadratic constraint is
> stored as an expression that is a sum of variables, squares of
> variables, and products of two variables, each term in the sum can have
> a coefficient. If you just want to change a coefficient in the sum, then
> you can get the expression of the constraint (SCIPgetExprNonlinear()),
> check with SCIPisExprSum() that it is indeed a sum, and use
> SCIPgetCoefsExprSum() to get the coefficients vector. I think it should
> be ok to change this vector when SCIP is in problem stage. To get the
> expressions that make up the terms of sum, use SCIPexprGetChildren().
> For the number of terms, use SCIPexprGetNChildren().
> 
> To change the problem after a solve, call SCIPfreeTransform(). That
> should bring SCIP back into problem stage.
> 
> Hope that helps,
> Stefan
> 
> On 15/07/2024 14:59, Enrico Calandrini wrote:
>> Hi everyone,
>> First of all, thank you all for your amazing work!
>> I was wondering if someone could help me understand how to properly modify a quadratic constraint, if this is possible. Specifically, I'm adding a quadratic constraint using the function SCIPcreateConsBasicQuadraticNonlinear. Firstly, is this method the correct one in the current version of SCIP? Additionally, it is possible that after the model has been solved, I may need to modify that constraint.
>> I have noticed that in the constraint handler for quadratic constraints (cons_quadratic.h), there are functions like SCIPchgLinearCoefQuadratic() to modify both linear and quadratic coefficients. However, I'm unsure whether these methods can be used for a constraint created as described above. Moreover, I have seen in the documentation that these changes are allowed only in the problem creation stage, meaning that after a call to SCIPsolve, this constraint can't be modified anymore?
>> I'm a little bit confused, so thank you in advance for your support!
>> Enrico Calandrini
>>
>>
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>