An example of * linear interpolation* in

**C++**.

typedef std::vector<double> DoubleVec; int findNearestNeighbourIndex(const double ac_dfValue, DoubleVec x) { double lv_dfDistance = DBL_MAX; int lv_nIndex = -1; for (unsigned int i = 0; i < x.size(); i++) { double newDist = ac_dfValue - x[i]; if (newDist >= 0 && newDist < lv_dfDistance) { lv_dfDistance = newDist; lv_nIndex = i; } } return lv_nIndex; } DoubleVec interpolation(DoubleVec x, DoubleVec y, DoubleVec xx) { double dx, dy; DoubleVec slope, intercept, result; slope.resize(x.size()); intercept.resize(x.size()); result.resize(xx.size()); int indiceEnVector; for (unsigned i = 0; i < x.size(); i++) { if (i < x.size() - 1) { dx = x[i + 1] - x[i]; dy = y[i + 1] - y[i]; slope[i] = dy / dx; intercept[i] = y[i] - x[i] * slope[i]; } else { slope[i] = slope[i - 1]; intercept[i] = intercept[i - 1]; } } for (unsigned i = 0; i < xx.size(); i++) { indiceEnVector = findNearestNeighbourIndex(xx[i], x); if (indiceEnVector != -1) { result[i] = slope[indiceEnVector] * xx[i] + intercept[indiceEnVector]; } else result[i] = DBL_MAX; } return result; }

Returns the interpolated values for the XX grid(std::vector), using as reference the X (std::vector<double, argument variable>) – x axis and Y (std::vector<double, argument variable) – y axis.

How to test it:

int main() { DoubleVec x = { 1, 5, 10 }; DoubleVec y = { 10, 50, 100 }; DoubleVec xx; for (unsigned i = 1; i < 10; ++i) xx.push_back(i); DoubleVec res = interpolation(x, y, xx); return 0; }

The result:

10 ; 20 ; 30 ; 40 ; 50 ; 60 ; 70 ; 80 ; 90 ;

Even at the risk that I misunderstood something, but your function “findNearestNeighbourIndex” does not find the nearest value, but the nearest next smaller value. That’s perfectly fine for this particular case, but the name of the function is somewhat misleading for someone like me who came here via Google for a quick and dirty “findnearestneighbour” – template 🙂

Just sayin