拉格朗日插值作业.c

极有用的数据处理程序

#include<stdio.h>
main()
{
	double X;
	int n;
	double lgrr(int n,double X);
	printf("input the aim number'X' "); 
	printf("input the number of pairs of numbers 'n':");
	scanf("%lf%d",&X,&n);
	lgrr(n,X);
	return 0;
}
	
double lgrr(int n,double X)
{
    FILE *fp;
	double s=0,t,z;
	double x[81],y[81];
	int i,j,k;
    if((fp=fopen("lgrr.txt","r+"))==NULL)
	printf("can not open the file.");
	printf("input n pairs of numbers:\n");
    for(i=0;i<n;i++)	
		scanf("%lf%lf",&x[i],&y[i]);
	
	for(k=0;k<=20;k++)
	{for(i=0;i<n;i++)
	{
		z=X+.01*k;
		t=y[i];
		for(j=0;j<n;j++)
		{if(j!=i)
		t=t*(z-x[j])/(x[i]-x[j]);}

		s=s+t;
	}
    printf("s=%f\n",s);
    fprintf(fp,"s=%f\n",s);
	}
	return 0;
}