// In this code, show using 'structs' how to group related variables together

#include <iostream>
using namespace std;

int main() 
{

 //Suppose we are writing a program involving particles, and that for each particle, 
 //we will are interested in its mass and momentum

  // we could create a variable for each of these quantities, like this:
  // double mass_1;
  // double mass_2;
  // double mass_3;
  // double momentum_1;
  // double momentum_2;
  // double momentum_3;

  // But this can get quite tediuos/complex
  
  //--------------------------------------------------------------------------------------
    
  //Solution (in C++) -- struct -- a data structure that can link together related variables

  //Define a struct which contains two variables: mass and momentum

  struct particle {
    double mass;
    double p;
  }; // note the semicolon after the closing brace of the struct here
  // this is important! -- if omitted, it can cause compilation issues

  // now that we have defined this 'particle' struct, we can use it just like the basic data types
  // you learned about earlier like (int, double)! 
 
  // create an object which is an instance of this struct type:
    
  particle p1;

  // Can assign the values of the variables for this object, using the syntax:
  // object.variable = value;
    
  p1.mass = 0.938; //GeV (proton mass)
  p1.p = 0.0;

  // anytime we want to access the variables within the struct, 
  // we do it by writing object.variable

  cout << "Particle 1: mass = " << p1.mass;
  cout << "; momentum = " << p1.p <<endl;

  // and we can make a second 'particle' using the same defined struct

  particle p2;
  p2.mass = 0.9396; //GeV (proton mass);
  p2.p = 25.0;

  cout << "Particle 2: mass = " << p2.mass;
  cout << "; momentum = " << p2.p <<endl;

  // This is much nicer than making a bunch of variables
  // Also now with the structure,  we know that p1.mass and p1.momentum are both properties of particle p1 

  return 0;
}