Runge-Kutta 4th order program copying array element

[Leonardo Machado]

Hello, thanks for your interest in may help me, i appreciate it, really.

My question is, I've wrote this code to solve an physical EDO system. But for some reason it copies the element k[1][3] at k[2][3].

After i call the initials elements the code becomes:

   for ( int n=1 ; n<=1; n++ ){
       
        pe= dpe*(n-1);
       
        cout << endl << endl << "pe= " << pe << endl ;
             
        file << endl << "pe= " << pe << "  ";
       
        for ( int l=1 ; l<=4 ; l++){
           
            for ( int j=1 ; j<=i ; j++){
               
                   
               
            if ( l==1){
                   
                px[j][l]= p[j];
                pex[j][l]= pe;
               
                }
           
            if ( l==2){
               
                px[j][l]= p[j]+ dpe*k[j][1]/2;
                pex[j][l]= pe+ dpe/2;
           
                }
               
            if ( l==3){
               
                px[j][l]= p[j]+ dpe*k[j][2]/2;
                pex[j][l]= pe+ dpe/2;
           
                }
           
            if ( l==4){               
                                   
                    px[j][l]= p[j]+ dpe*k[j][3];
                    pex[j][l]= pe+ dpe;
       
                }
               
                cout << "px" << j << "." << l << "= " << px[j][l] << "  " ; 
           
            }
           
       
            for ( int j=1; j<=i; j++){
           
               
                if ( j==1){
                       
                    f[j]=px[3][l];
                       
                    }
                   
                if ( j==2){
                       
                    f[j]= px[4][l];
                       
                    }   
   
                if (j==3){
                       
                        f[j]= (-GM) * px[1][l] / pow ( sqrt (  pow ( px[1][l], 2) + pow ( px[2][l], 2)), 3);
                    }
                   
                if (j==4){
                       
                        f[j]= (-GM) * px[2][l] / pow ( sqrt (  pow ( px[1][l], 2) + pow ( px[2][l], 2)), 3);
                    }   
               
                k[j][l]= f[j];
               
            }
             
            for ( int j=1; j<=i; j++){
               
                cout << "k" << j << "." << l << "= " << k[j][l] << "  " ;
               
            }
           
            cout << endl << endl;
           
        }
  
           
       
        for ( int j=1 ; j<=i ; j++){
          
                   
            file << p[j] << "  ";
            cout << "p." << j << "= " << p[j] << "  ";
            p[j]=p[j]+ dpe*(k[j][1]+k[j][4]+ 2*(k[j][2]+k[j][3]))/6;
           
           
        }
           
       
    }
       
    }

Help me please, i have no more ideas !

 

[Pepper Mint]

You may debug the program and observe how each variable gets changed in every step.

I think this line

k[j][l]= f[j];

may start the problem.
E.g j=1, l=3 the k_1,3=f₁=px_3,3
j=2, l=3 the k_2,3=f₂=px_4,3
Looking up further above, you will see
px_3,3 = px_4,3

I may overlook something though.

 

[chiro]

Hey Leonardo Machado.

Your code has some undefined variables - where have you defined (as an example) the variable i?