How to find a qVector <double> element from a qVector <double> index

[pedromenezes]

Hi guys,
I have two double vectors (x, y) and i need to compare the X vector element with a doubleSpin value and if they are equal I need do get the Y value at the same position.

I tried this, but it doesn't work properly:

QVector<double> x(512), y(512);
double rate = prom->doubleSpinBox->value();
double chk_rate = 0;

for (int i=0; i<x.size(); ++i)
       {
       x[i] = i * 0.1;
       y[i] = 10 * qAbs(qSin(2.0 * M_PI * i ));
if (rate == x[i]) chk_rate = y[i];
}

There is a warning : comparing floating point with == or != is unsafe!
And besides this in sometimes the result is correct but most of times it shows y[i] = 0;

I was trying to use this way, but i don't know how to finish:

QVector<double> x(512), y(512);
double rate = prom->doubleSpinBox->value();
double chk_rate = 0;

auto it = std::find(x.begin(), x.end(), rate);
auto index = std::distance(x.begin(), it);
chk_rate = y[index]; 

There is other warning: implicit conversion loses integer precision: 'long' to 'int'

Could someone help me please?

Cheers

[Kent-Dorfman]

you can safely compare equivalence of floating point numbers but you have to compare the difference between them to a tolerance value. you can try something liek

const float tolerance(0.0001f);
if (fabs(x - y) < tolerance) {
    cout << "equivalent";
} else {
    cout < "not equivalent";
}

[VRonin]

@pedromenezes said in How to find a qVector <double> element from a qVector <double> index:

There is a warning : comparing floating point with == or != is unsafe!

Replace if (rate == x[i]) with if(qFuzzyIsNull(rate - x[i]))

but most of times it shows y[i] = 0;

When i is 0 then 10 * qAbs(qSin(2.0 * M_PI * i )); is 0 so it's a perfectly acceptable answer

[beecksche]

@pedromenezes

Due to periodicity

10 * qAbs(qSin(2.0 * M_PI * i ))

is always zero.

[pedromenezes]

Thank you guys. I separated the process and it worked perfectly.

First it generate the vector:

void promediation::rateEffect(){

    int num = 512;
    int freq = 60;
  
    QString label_t = prom->label_2->text();
    double amplitude = (100-label_t.toDouble())/2;// interference adjust
    double sine = freq*2.0*M_PI;
    double r = 0;
    QSharedPointer<QCPGraphDataContainer > graphData = prom->customPlot_3->graph(0)->data();
    const double time = std::max_element(graphData->begin(),graphData->end(),
                                          [](const QCPGraphData& a, const QCPGraphData& b)->bool{return a.key<b.key;})->key;

    double timewindow = time/num;


    QVector<double> x(num), y(num), y_final(num);

        for (int i=0; i<x.size(); ++i)
        {
        r =  QRandomGenerator::global()->bounded(-100,100); //create a noise
        x[i] = i*timewindow; // 0.0208 to show 512 points in 10.66 msec
        double time = x[i]/1000;//time in (msec)
        y[i] = amplitude*qSin(sine*(time)+r/1000);// Y = Amplitude*Sin(Frequency*2*Pi*time + phase)*ajust
        }

        for (int i=0; i<x.size(); ++i)
        {
            y_final[i] = y[i]+plot3Data->at(i)->value;

        }
}

And the doubleSpinBox set the values to be plotted

void promediation::StimRate(){


    int num = 700;// max rate 70 Hz

    auto plot6 = prom->customPlot_6;
    auto plotkey6 = plot6->graph(0)->data();
    double taxa = round(prom->doubleSpinBox->value()*1000.0)/1000.0;
    QString label_t = prom->label_2->text();
    double taxa_v = label_t.toDouble();

    QVector<double> x(num), y(num);

        for (int i=0; i<x.size(); ++i)
        {
        x[i] = taxa;
        y[i] = i*0.00142;
        }
    const int PEN_WIDTH = 2;
    plot6->addGraph();
    plot6->graph(1)->setData(x,y);
    QPen iPen;
    iPen.setWidth(100);
    plot6->graph(1)->setPen(QPen(iPen));


        if (taxa_v >= 90){
            plot6->graph(1)->setPen(QPen(QColor(0,128,0), PEN_WIDTH));
        }
        if (taxa_v >= 80 and taxa_v < 90){
            plot6->graph(1)->setPen(QPen(QColor(87,122,15), PEN_WIDTH));
        }

        if (taxa_v >= 70 and taxa_v < 80){
            plot6->graph(1)->setPen(QPen(QColor(30,80,180), PEN_WIDTH));
        }

        if (taxa_v >= 50 and taxa_v < 70){
            plot6->graph(1)->setPen(QPen(QColor(10,10,10), PEN_WIDTH));
        }
        if (taxa_v >= 20 and taxa_v < 50){
            plot6->graph(1)->setPen(QPen(QColor(200,120,0), PEN_WIDTH));
        }

        if (taxa_v < 20){
            plot6->graph(1)->setPen(QPen(QColor(255,0,0), PEN_WIDTH));
        }

    plot6->rescaleAxes();
    plot6->replot();
}