Linear interpolation C++

InterpolateLin.h

// Linear interpolation procedure
// c++ code by [email protected]
// SEE
// http://www.cplusplus.com/forum/general/216928/

template<typename T>
inline T InterpolateLin(const T* data_x, const T* data_y, size_t data_length, T x, size_t& i)
{
    if (x >= data_x[data_length - 2]) i = data_length - 2;
    else while (x > data_x[i + 1]) i++;
    const T dydx = (data_y[i + 1] - data_y[i]) / (data_x[i + 1] - data_x[i]);
    return data_y[i] + dydx * (x - data_x[i]);
}

template<typename T>
inline void InterpolateLin(const T* data_x, const T* data_y, size_t data_length, const T* sampled_x, T* sampled_y, size_t threshold)
{
    size_t j = 0;
    for (size_t i = 0; i < threshold; i++) {
        sampled_y[i] = InterpolateLin(data_x, data_y, data_length, sampled_x[i], j);
    }
}