// sample program of forming invariant mass of opposit charge states,  2008.5.26 T. Kondo
#include <vector>
#include <iostream>
#include <math.h>
double rand01();

int main()
{
        std::vector<double> m_p[4];   // 4 momentum vector
        std::vector<int> m_ch;  //stuck for charge
        double p[4],mass,invmass;
        int i,j,k,charge,max;

        rand01();   // dummy call to avoid 0.0        max = 2+rand()%8;  //rand()%8 dives integer 0-8
        for(i = 0; i < max; ++i )
        {
           p[0]=10.*rand01();
           p[1]=10.*rand01();
           p[2]=10.*rand01();
           mass=0.938;
           p[3]=sqrt(p[1]*p[1]+p[2]*p[2]+p[0]*p[0]+mass*mass);
           if(rand01() < 0.5)
               charge=-1;
           else
               charge=1;
           std::cout<<i<<"="<<p[0]<<","<<p[1]<<","<<p[2]<<
           ", E="<<p[3]<<", Charge="<<charge<<std::endl;

           for(j=0;j<4;++j)
           {m_p[j].push_back(p[j]);   // store to STL dynamic array (vector)
           }
           m_ch.push_back(charge);
        }

// invariant mass between opposit charge states
        for(i = 0; i < max; ++i )
        {
           for(j=i+1; j<max ; ++j)
           {
              if(m_ch[i] == m_ch[j]) continue;
               for(k=0; k<4; ++k)
               {
                  p[k]=m_p[k][i]+m_p[k][j]; // retrieve from dynamic array
               }
               invmass= sqrt(p[3]*p[3]-p[0]*p[0]-p[1]*p[1]-p[2]*p[2]);
               std::cout<<"("<<i<<","<<j<<")="<<invmass<<std::endl;
            }
        }
                return 0;
}

./a.exe
0=5.05418,5.91491,5.54785, E=9.60153, Charge=-1
1=2.57732,2.07382,6.26262, E=7.14448, Charge=-1
2=8.43852,0.68778,4.09907, E=9.45325, Charge=1
3=3.1948,9.80568,0.850048, E=10.3904, Charge=1
4=1.02509,9.21978,5.07551, E=10.6158, Charge=1
5=3.3326,6.92409,5.56946, E=9.53667, Charge=-1
6=0.319938,8.58511,0.989845, E=8.69863, Charge=1
(0,2)=6.6613
(0,3)=6.60014
(0,4)=5.46529
(0,6)=7.28166
(1,2)=6.25657
(1,3)=9.07977
(1,4)=6.80826
(1,6)=8.74064
(2,5)=8.40455
(3,5)=5.77805
(4,5)=3.63229
(5,6)=5.96867