Hello,

Does anyone know if there is an easy way to determine whether a constraint from a CPLEX problem object is a knapsack constraint using the C API functions of CPLEX? My current thought process would be to use CPXgetrows() along with CPXgetctype() to determine if all variables of a particular constraint are binary variables. If so, then I would consider it a knapsack constraint.

I thought I would ask if there is an easier way before I go ahead and try to implement this strategy. Basically, I am trying to test some cutting planes to the knapsack polyhedra using some problems from MIPLIB (https://miplib.zib.de/). But I need to identify the knapsack constraints of the problem first so I can correctly implement my cuts.

Any suggestions are appreciated. Thank you for your time.

Best,
Fabio

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Fabio Vitor
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#DecisionOptimization

I don't think there is a better way to check.

Depending on how many binary variables you have in your model in relation to other variables, you could pre-filter the rows: Call CPXXgetctype() to find the binary columns. Then use CPXXgetcols() to find the rows that intersect with at least one binary variable.

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Daniel Junglas
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That makes sense too. Thank you Daniel.

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Fabio Vitor
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Original Message:
Sent: Wed May 06, 2020 09:36 AM
From: Daniel Junglas
Subject: Finding Knapsack Constraints using the C API Functions of CPLEX

I don't think there is a better way to check.

Depending on how many binary variables you have in your model in relation to other variables, you could pre-filter the rows: Call CPXXgetctype() to find the binary columns. Then use CPXXgetcols() to find the rows that intersect with at least one binary variable.

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Daniel Junglas
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I think you would also want to check whether the nonzero coefficients all have the same sign, right?

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Paul Rubin
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I think that's right too. Thank you Paul.

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Fabio Vitor
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Original Message:
Sent: Wed May 06, 2020 04:14 PM
From: Paul Rubin
Subject: Finding Knapsack Constraints using the C API Functions of CPLEX

I think you would also want to check whether the nonzero coefficients all have the same sign, right?

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Paul Rubin
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