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# XPRSaddcbslpconstruct

Purpose
Add a user callback to be called during the Xpress-SLP augmentation process
Topic areas
Callback, SLP
Synopsis
`int XPRS_CC XPRSaddcbslpconstruct(XPRSprob prob, int (XPRS_CC *slpconstruct) (XPRSprob cbprob, void *cbdata), void *data, int priority);`
Arguments

prob | The current problem.
slpconstruct | The function to be called during problem augmentation. slpconstruct returns an integer value. See below for an explanation of the values.
cbprob | The problem passed to the callback function.
cbdata | The user-defined object passed as data to XPRSaddcbslpconstruct.
data | Address of a user-defined object, which can be used for any purpose by the function. data is passed to slpconstruct as cbdata.
priority | An integer that determines the order in which callbacks of this type will be invoked. The callback added with a higher priority will be called before a callback with a lower priority. Set to 0 if not required.

Example
The following example sets up a callback to be executed during the Xpress-SLP problem augmentation:

    double *cValue;
    cValue = NULL;
    XPRSaddcbslpconstruct(prob, CBConstruct, &cValue, 0);

The following sample callback prints information about the number of rows and columns in the augmented problem.

    int XPRS_CC CBConstruct(XPRSprob MyProb, void *Obj) {
      int ncol;
      int nrow;
      XPRSgetintattrib(MyProb, XPRS_COLS, &ncol);
      XPRSgetintattrib(MyProb, XPRS_ROWS, &nrow);
      printf("The augmented problem has %i rows and %i columns\n", nrow, ncol);
      return 0;
    }

Further information
1. This callback can be used during the problem augmentation, generally (although not exclusively) to change the initial values for the variables.
2. The following return codes are accepted:

0
  Normal return: augmentation continues
-1
  Return to recalculate matrix values
-2
  Return to recalculate row weights and matrix entries
other
  Error return: augmentation terminates, XPRSslpconstruct terminates with a nonzero error code.

3. The return values -1 and -2 will cause the callback to be called a second time after the matrix has been recalculated. It is the responsibility of the callback to ensure that it does ultimately exit with a return value of zero.
Related topics
XPRSslpconstruct XPRSremovecbslpconstruct
Parent Topic Library functions and the programming interface

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