## byrnnryb

Hello,

I currently have working code to solve a single first order differential equation using a predictor-corrector. I need to modify this to solve a system of first order differential equations.

Here is the system I need to solve:
y'' = -y' + 6y; y(0)=1; y'(0)=-2, on [0,4]

And here is my code to solve a single first order diff. equation (using dev c++)

``````#include <stdio.h>
#include <vector>
#include <math.h>
#include <cstdlib>

#define MAXDATA  100
using namespace std;

double h, tx[MAXDATA],ty[MAXDATA], x,xi,xf,yi;
int fi,i,n;

double f(double x, double y) {

return (4*x/y - x*y);

}

//classical Runge-Kutta method of order 4
void runge_kutta(double *y) {
double a,b,c,d;
a=h*f(x,*y);
b=h*f(x+h/2,*y+a/2);
c=h*f(x+h/2,*y+b/2);
x+=h;
d=h*f(x,*y+c);
*y =*y + (a+b+b+c+c+d)/6;
}

void equadiff_pc(double *tx,double *ty,double xi,double xf,
double yi,int m,int fi) {
double z=yi,y,w,p[4];
int i,j,k; long ni;
if ((m>MAXDATA) || (fi<1)) return;
h = (xf-xi)/fi/m;
p[3]=f(xi,yi);
tx[0]=xi; ty[0]=yi;
for (k=0, i=1; i<=m; i++) {
ni=(long) (i-1)*fi-1;
for (j=1; j<=fi; j++) {
x=xi+h*(ni+j);
if (++k<4) {
runge_kutta(&z);
p[3-k]=f(x,z);
}
else {
x+=h;
w=z+h/24*(55*p[0]-59*p[1]+37*p[2]-9*p[3]);
do {
y=w;
w=z+h/24*(9*f(x,y)+19*p[0]-5*p[1]+p[2]);
} while (fabs(y-w) > 1e-10);
z=w; p[3]=p[2]; p[2]=p[1];
p[1]=p[0]; p[0]=f(x,z);
}
} // j loop
tx[i]=x; ty[i]=z;
} // i loop
}

int main(void) {

printf("\n Input x begin: "); scanf("%lf",&xi);
printf("\n Input x end  : "); scanf("%lf",&xf);
printf("\n Input y begin: "); scanf("%lf",&yi);
printf("\n Input num of points: "); scanf("%d",&n);
printf("\n Input finesse: "); scanf("%d",&fi);

//print results
printf("\n");
printf("        X               Y     \n");
printf(" -----------------------------\n");
for (i=1; i<=n; i++)
printf("%12.6f     %12.6f\n",tx[i],ty[i]);
printf(" -----------------------------\n");

system("PAUSE");
return EXIT_SUCCESS;
}``````

No love?

## Salem 5,138

> I currently have working code to solve a single first order differential equation using a predictor-corrector.
Did YOU write it?

I mean, if you did, then you should be able to at least take a swing at trying to write the new one yourself as well.

Otherwise it looks like another helpless "Hey, I need an orange - if it's of any use to you, I found this lemon".

## byrnnryb

>
Did YOU write it?

I mean, if you did, then you should be able to at least take a swing at trying to write the new one yourself as well.

Otherwise it looks like another helpless "Hey, I need an orange - if it's of any use to you, I found this lemon".

I wrote part of it (The runge-kutta function.)
Oh trust me, I've already spent countless hours working on this with no luck. I've been told that it's a simple matter of changing some of the doubles to arrays, and wrapping up many of the statements in for loops, but I've just been staring at it so long that I'm too frustrated to do any good, wanted to see if a fresh set of eyes could lend some insight.