Is there something wrong in DataFitting interpolate with user defined extrapolate call back routine?

Hi all,

I meet a strange problem in DataFitting Module while call df?InterpolateEx1D function. If I present the call back function for extrapolation , the result was wrong. But if set them to NULL, result is right.

My problem is:

x[] = {1, 1.5, 2.5, 3.5, 4.5, 5}, and y[] = {100, 120, 150, 170, 200, 200}, Interpolate method is stepwise constant interpolant. And I need found y value at x=4.

The following is the sample code. I hope that was something wrong in my code. Can you help me to explain it? thanks.

#include <stdio.h>
#include "mkl_df.h"


// fixed value extrapolate function parameters
struct fixvalue_extrap_params
{
    float value;
};

// fixed value extrapolate function
int fixvalue_extrap( MKL_INT64* n, MKL_INT64 cell[], float site[], float r[],
                     void *params )
{
    fixvalue_extrap_params *p = (fixvalue_extrap_params*)params;
    r[0] = p->value;
    return 0;
}


int main(int argc, char* argv[])
{
    float x[] = { 1., 1.5, 2.5, 3.5, 4.5, 5 };
    float y[] = { 100., 120., 150., 170., 200., 200. };
    int nx = 6;
    int ny = 1;
    const int nsite = 1;
    float site[nsite] = { 4 };
    float r[nsite];
    int cell[nsite];

    DFTaskPtr task;                     // Data Fitting task descriptor

    MKL_INT ndorder = 1;                    // size of array describing derivative
    MKL_INT dorder[] = { 1 };           // only value to calculate

    int errcode = 0;

    /***** Create Data Fitting task *****/
    errcode = dfsNewTask1D(&task, nx, x, DF_NON_UNIFORM_PARTITION, ny, y, DF_NO_HINT);

    /***** Edit task parameters for look up interpolant *****/
    errcode = dfsEditPPSpline1D(task, DF_PP_STD, DF_CL_STEPWISE_CONST_INTERPOLANT, 0, 0, 0, 0, 0, 0);

    /***** Interpolate using lookup method without extrapolate *****/
    errcode = dfsInterpolateEx1D(task, (DF_CELL | DF_INTERP), DF_METHOD_PP,
                                 nsite, site, 0, ndorder,
                                 dorder, 0, r, 0, cell,
                                 0, 0, 0, 0,
                                 0, 0, 0, 0);
    // this answer is right
    printf("interpolate reuslt for [%f] = [%f] in section %d\n", site[0], r[0], cell[0]);

    dfsInterpCallBack le_cb, re_cb;       // interpolation call backs
    void * le_params, *re_params;    // interpolation call backs parameters
    fixvalue_extrap_params le_fixparams, re_fixparams;

    le_cb = fixvalue_extrap;
    re_cb = fixvalue_extrap;
    le_fixparams.value = y[0];
    re_fixparams.value = y[nx-1];
    le_params = &le_fixparams;
    re_params = &re_fixparams;

    /***** Interpolate using lookup method *****/
    errcode = dfsInterpolateEx1D(task, (DF_CELL | DF_INTERP), DF_METHOD_PP,
                                 nsite, site, 0, ndorder,
                                 dorder, 0, r, 0, cell,
                                 le_cb, le_params, re_cb, re_params,
                                 0, 0, 0, 0);

    // this answer is wrong
    printf("interpolate reuslt with extrapolate for [%f] = [%f] in section %d\n", site[0], r[0], cell[0]);

    /***** Delete Data Fitting task *****/
    errcode = dfDeleteTask(&task);

    return 0;
}

 My output is :

interpolate reuslt for [4.000000] = [170.000000] in section 4
interpolate reuslt with extrapolate for [4.000000] = [150.000000] in section 4