xstoerm.c

Numerical Recipes in C的源代码

/* Driver for routine stoerm */

#include <stdio.h>
#include <math.h>
#define NRANSI
#include "nr.h"
#include "nrutil.h"

#define NVAR 4
#define X1 0.0
#define HTOT 1.570796

float d2y1(float x)
{
	return sin(x)+x;
}

float d2y2(float x)
{
	return cos(x)+x*x-2;
}

void derivs(float x,float y[],float dydx[])
{
	dydx[1]=x-y[1];
	dydx[2]=x*x-y[2];
}

int main(void)
{
	int i;
	float a1,a2,xf,*y,*yout,*d2y;

	y=vector(1,NVAR);
	yout=vector(1,NVAR);
	d2y=vector(1,NVAR);
	y[1]=0.0;
	y[2] = -1.0;
	y[3]=2.0;
	y[4]=0.0;
	derivs(X1,y,d2y);
	xf=X1+HTOT;
	a1=d2y1(xf);
	a2=d2y2(xf);
	printf("Stoermer's Rule:\n");
	for (i=5;i<=45;i+=10) {
		stoerm(y,d2y,NVAR,X1,HTOT,i,yout,derivs);
		printf("\n%s %5.2f %s %5.2f %s %2d %s \n",
			"x=",X1," to ",X1+HTOT," in ",i," steps");
		printf("%14s %9s\n","integration","answers");
		printf("%12.6f %12.6f\n",yout[1],a1);
		printf("%12.6f %12.6f\n",yout[2],a2);
	}
	free_vector(d2y,1,NVAR);
	free_vector(yout,1,NVAR);
	free_vector(y,1,NVAR);
	return 0;
}
#undef NRANSI