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# XSLPsetcbiterstart, XPRSsetcbslpiterstart

Purpose
Set a user callback to be called at the start of each SLP iteration
Topic areas
Callback, SLP
Synopsis
`int XPRS_CC XSLPsetcbiterstart(XSLPprob prob, int (XPRS_CC *iterstart) (XSLPprob cbprob, void *cbdata), void *data);`
Arguments

prob | The current SLP problem.
iterstart | The function to be called at the start of each SLP iteration. iterstart returns an integer value. If the return value is nonzero, the SLP iterations will stop.
cbprob | The problem passed to the callback function.
cbdata | The user-defined object passed as data to XSLPsetcbiterstart.
data | A user-defined object, which can be used for any purpose by the function. data is passed to iterstart as cbdata.

Example
The following example sets up a callback to be executed at the start of the optimization to save to save the values of the variables from the previous iteration:

    double *cSol;
    XSLPsetcbiterstart(prob, CBIterStart, &cSol);

A suitable callback function might resemble this:

    int XPRS_CC CBIterStart(XSLPprob MyProb, void *Obj) {
      XPRSprob xprob;
      double *cSol;
      int nIter;
      cSol = * (double **) Obj;
      XSLPgetintattrib(MyProb, XSLP_ITER, &nIter);
      if (nIter == 0) return 0; /* no previous solution */
      XSLPgetptrattrib(MyProb, XSLP_XPRSPROBLEM, &xprob);
      XPRSgetlpsol(xprob, cSol, NULL, NULL, NULL);
      return 0;
    }

The data argument is used here to hold the address of the array cSol which we populate with the solution values.
Further information
This callback can be used at the start of each SLP iteration before the optimization begins.
Related topics
XSLPsetcbiterend, XSLPsetcbitervar
Parent Topic Library functions and the programming interface

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