Indices of matrix elements
The row and column indices that are returned by the BCL functions XPRBgetrownum and XPRBgetcolnum correspond to the position of variables and constraints in the unpresolved matrix with empty rows or columns removed. The position of matrix elements may be modified by the presolve/preprocessing algorithms. That means, if these algorithms are not switched off (control parameters XPRS_PRESOLVE and XPRS_MIPPRESOLVE), the indices for variables and constraints held by BCL should not be used with any Optimizer library functions. The same rule applies to any other variable or constraint-specific information, such as solution and dual values. This problem does not occur within BCL (that is, if only BCL functions are used) since the solution information is accessible only after the optimization run has finished and the postsolve has been performed by the Optimizer.
An exception from the rule stated above are the Optimizer library functions XPRSgetlpsol / XPRSgetmipsol: XPRSgetlpsol may be used, for instance, in Optimizer library callback functions during the global search to access the current solution values, and in combination with the indices for variables and constraints held by BCL. This is possible because XPRSgetlpsol / XPRSgetmipsol return the postsolved solution.
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