double* wh = std::lower_bound(srcVectorArgs, srcVectorArgs + srcSize, new_x);

#include <math.h>
#include <iostream>
#include <algorithm>
#include <stdio.h>


void linInterp(double* srcVectorArgs, double* srcVectorFuncs, int srcSize,
               double* newVectorArgs, double* newVectorFuncs, int resSize)
{
    for (int i=0;i<resSize;i++)
    {
       double new_x = newVectorArgs[i];
       int index;
       if (new_x <= srcVectorArgs[0])
           index = 0;
       else if (new_x >=srcVectorArgs[srcSize-1])
           index = srcSize-2;
       else
       { 
           double* wh = std::lower_bound(srcVectorArgs, srcVectorArgs + srcSize, new_x);
           index = wh - srcVectorArgs-1;            
       }
             
       if(new_x == srcVectorArgs[index])
       {
           newVectorFuncs[i] = srcVectorFuncs[index];
       }
       else
       {     
           double xLower = srcVectorArgs[index];
           double xHigher = srcVectorArgs[index+1];
           double yLower = srcVectorFuncs[index];
           double yHigher = srcVectorFuncs[index+1];
           double slope = (yHigher - yLower)/(xHigher - xLower);
           newVectorFuncs[i] = slope * (new_x-xLower) + yLower;

       }
    }   
}




int main()
{
    int srcSize=100;
    int resSize=400;

    double xnew[resSize];
    double ynew[resSize];

    double x[srcSize];
    double y[srcSize];

  
    char data[] = "data.txt";
    char grid_file[] = "grid.txt";
    FILE* file = fopen(data, "r");
    int j=0;
    while(!feof(file)) {
        fscanf(file, "%lf %lf", &x[j], &y[j]);
        j++;
    }
    FILE* fgrid = fopen(grid_file, "r");
    int i=0;
    while(!feof(fgrid)) {
        fscanf(fgrid, "%lf", &xnew[i]);
        i++;
    }
    for(int j=0;j<srcSize;j++){
        printf("%lf %lf\n", x[j], y[j]);
    }

   linInterp(x,y,srcSize,xnew,ynew,resSize);    
   for(int j=0;j<resSize;j++){
       printf("%lf %lf\n", xnew[j], ynew[j]);
   }

    return 0;
}