How to call on a variable in c++?

Question

I am very new to C++ and I don't know the syntax or many of the functions. I am working on an assignment and I am very lost. I am trying to access a variable from another page to use in an equation and I'm not sure how to do so. Here is the code I have written (I know it's very wrong):

#include <fstream>
#include <iostream>
#include <string>

#include "Event.h"

int main()
{
    const std::string filename = R"(..\data.txt)";

    // Open the data file
    std::ifstream inputFile(filename);

    // Throw an exception if the file is not where we expected
    if (!inputFile)
    {
        throw std::runtime_error("File " + filename + " not found!");
    }

    // Find the number of events in the file
    int nEvents;
    inputFile >> nEvents;

    // Loop over each event
    int count = 0;
    for (int i = 0; i < nEvents; ++i)
    {
        Event event;
        inputFile >> event;
        
        
        if (Event(3) = 1 or -1) 
        {
            int mass = sqrt(2 * Particle::getpT[1] * Particle::getpT[2] * (cosh(Particle::getPhi[1] - Particle::getPhi[2]) - cos(Particle::getEta[1] - Particle::getEta[2])));

                if (180 < mass < 220)
                {
                    count = count + 1;
                };
        }
        return count;
        // Here is where you would put the code to look at the particles in each event
        // You want to see if the event has both same-sign pairs of invariant masses 
        // falling into the range 180-220
        // Count how many events do this
    }

    int nEvents = count;
    // When you are done, you should print out the answer:
    
        
    std::cout << "Total events in range: " << nEvents << '\n';

    std::cout << "Finished running!\n";
}

And the variables I am trying to get are here:

#pragma once

#include <iostream>

// A class that contains basic information about a particle
class Particle
{
public:
    // Default constructor needed for streaming
    Particle() : pt(0), phi(0), eta(0), charge(0) {}
    Particle(double ipT, double iphi, double ieta, int icharge) : pt(ipT), phi(iphi), eta(ieta), charge(icharge) {}

    // Getters
    double getpT() const { return pt; }
    double getPhi() const { return phi; }
    double getEta() const { return eta; }
    int getCharge() const { return charge; }

    // Setters (needed for streaming)
    void setpT(double newPt) { pt = newPt; }
    void setPhi(double newPhi) { phi = newPhi; }
    void setEta(double newEta) { eta = newEta; }
    void setCharge(int newCharge) { charge = newCharge; }

private:
    double pt, phi, eta;
    int charge;
};

// Streaming operators for I/O
std::ostream& operator<<(std::ostream& stream, const Particle& part);
std::istream& operator>>(std::istream& stream, Particle& part);

There's also this file attached:

#pragma once

#include <iostream>
#include <vector>

#include "Particle.h"

// An event contains a number of Particles
// In our case, there will always be four particles
class Event
{
public:
    // Gets a particle by a given index - zero indexed
    // So 0, 1, 2, and 3 are the possible values
    const Particle& getParticle(int index) const { return particles.at(index); }

    // The total number of particles - always 4 in our case
    int getNParticles() const { return static_cast<int>(particles.size()); }

    // Adds a new particle to the collection
    void addParticle(Particle part) { particles.push_back(part); }

private:
    std::vector<Particle> particles;
};

// Streaming operators for I/O
std::ostream& operator<<(std::ostream& stream, const Event& event);
std::istream& operator>>(std::istream& stream, Event& event);

If you can make any sense of this and what I need to know, I would really appreciate it!

Answer 1

Particle and Event are not variables, they are types.
You are supposed to access the Particles in an Event using its getNParticles and getParticle functions.

Example of a loop over all of them:

Event event;
inputFile >> event;
for (int i = 0; i < event.getNParticles(); i++)
{
    const Particle& p = event.getParticle(i);
    // Use p.getpT() et al in your formula (it's not an equation).
}

It is also a good idea to put the mass calculation in a function so you can write for instance,

if (mass(particle1) == mass(particle2)) 
    ...

Also, 180 < mass < 220 does not mean what you think it means – you need to write 180 < mass && mass < 220.
You should not return count; inside the loop; that will exit main immediately.
And it's unclear what you're trying to accomplish with Event(3) = 1 or -1, but I would guess it's an attempt at "the event has both same-sign pairs of invariant masses".
You're going to need to think harder about that.
And do get yourself a good book. You can't learn C++ by guessing.