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PORunge Kutta in C for Lorenz equation
 

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  1. PORunge Kutta in C for Lorenz equation
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    <p>i'm trying to compute the Lorenz system using Runge Kutta method, but i can't find where my code have an error. When I run it, the system goes to a static point and i should obtain a butterfly (the Lorenz attractor). I think it's something on the 'for'loops in the Runge Kutta method section. Here's my code</p> <pre><code>#include&lt;stdio.h&gt; #include&lt;stdlib.h&gt; #include&lt;math.h&gt; double f(int,double,double []); double s,r,b; FILE *output; int main() { output=fopen("lorenzdata.dat","w"); int j,N=3; double x[2],dt,y[2],K1[2],K2[2],K3[2],K4[2],ti,t,i; printf("\n Introduce parameters s, r and b sepparated by a space:"); scanf("%lf %lf %lf",&amp;s,&amp;r,&amp;b); printf("\n Introduce initial conditions t0,x0,y0 and z0:"); scanf("%lf %lf %lf %lf",&amp;ti,&amp;x[0],&amp;x[1],&amp;x[2]); printf("\n Introduce time step:"); scanf("%lf",&amp;dt); printf("\n Specify time to compute in the equations:"); scanf("%lf",&amp;t); /* Loop para Runge Kutta 4 */ do { /* printf("%9.4f %9.4f %9.4f %9.4f \n",ti,x[0],x[1],x[2]); */ i++; for(j=0;j&lt;N;j++) { K1[j] = f(j,ti,x); } for(j=0;j&lt;N;j++) { y[j] = x[j]+(K1[j]/2)*dt; } for(j=0;j&lt;N;j++) { K2[j] = f(j,ti+dt/2,y); } for(j=0;j&lt;N;j++) { y[j] = x[j]+(K2[j]/2)*dt; } for(j=0;j&lt;N;j++) { K3[j] = f(j,ti+dt/2,y); } for(j=0;j&lt;N;j++) { y[j] = x[j]+(K3[j])*dt; } for(j=0;j&lt;N;j++) { K4[j] = f(j,ti+dt,y); } for(j=0;j&lt;N;j++) { x[j] += dt*((K1[j]/6)+(K2[j]/3)+(K3[j]/3)+(K4[j]/6)); } ti +=dt; fprintf(output,"%9.4f %9.4f %9.4f \n",x[0],x[1],x[2]); }while(i*dt &lt;= t); fclose(output); return 0; } /* Definimos la funcion */ double f(int m,double h,double x[]) { if(m==0) { return s*(x[1]-x[0]); } else if(m==1) { return x[0]*(r-x[2])-x[1]; } else if(m==2) { return x[0]*x[1]-b*x[2]; } } </code></pre> <p>Thanks in advance</p> <p><strong>EDIT</strong></p> <p>I wrote the code again (in a more simplified way) on linux and it runs ok, it seems my editor on Windows had something (maybe the encoding) that when i compiled the code, it throwed a lot of infinites values. Don't know why and it took me a lot of time to notice that. Thanks for your help</p>
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    1. USDavid Winchester
    1. USDavid Winchester
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      1. PORunge Kutta in C for Lorenz equation
      1. USRenan
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    1. COWhat is the point of double h as input into the function f? It doesn't appear to get used. Is it necessary?
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    2. COmm nop, i put h there just to make a more generalizaed code, in case that the diff equations depend explicitly on t
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