linear_interpolation_not_optimized.cpp

double interpolate(const QList<double>& xs, const QList<double>& ys, double x, bool* ok = 0) {

    if (ys.size()<xs.size()) {
        qDebug() << "double interpolate(xs,ys,x,ok) warning: ys.size()<xs.size()";
    } else if (ys.size()!=xs.size()){
        qDebug() << "double interpolate(xs,ys,x,ok) warning: ys.size()!=xs.size()";
    }

    for (int i=0;i<xs.size()-1;i++) {
        if (x >= xs.at(i) && x <= xs.at(i+1)) {
            if (ok)
                *ok = true;
            double dx = xs[i+1]-xs[i];
            if (dx > 0.0) {
                return (x-xs[i])/dx*(ys[i+1]-ys[i])+ys[i];
            } else {
                if (ok)
                    *ok = false;
                return ys[i];
            }
        }
    }
    if (ok)
        *ok = false;
    if (x < xs.at(0))
        return ys.at(0);
    if (x > xs.at(xs.size()-1))
        return ys.at(ys.size()-1);
    return 0.0;
}