C++ (Qt)
int GetNext( const std::vector<double> & src, const double & value, const double & tolerance)
{
    auto it = std::lower_bound(src.begin(), src.end(), value, [&](const double & x, const double & val)->bool { return ((x - val) < tolerance); });
 
    return (it != src.end()) ? it - src.begin() : -1;
}
 

C++ (Qt)
int GetNext( const std::vector<double> & src, double value, double tolerance )
{
    auto it = std::lower_bound(src.begin(), src.end(), value + tolerance);
    return (it != src.end()) ? std::distance(src.begin(), it) : -1;
}
 
int GetPrev( const std::vector<double> & src, double value, double tolerance )
{
    auto it = std::upper_bound(src.begin(), src.end(), value - tolerance);
    return std::distance(src.begin(), it) - 1;
}