lagrange.c

拉格朗日插值逼近：在离散数据基础上补插除连续函数是计算数学中最基本最常用的手段是函数逼近的重要方法。

#include<stdio.h>
#define N 1000
void main()
{
	int i,j,k,p;
	float x[N],l[N],f[N],m[N];
	float y,L;
	printf("Please put in the number of the sum of the crunodes:");

	scanf("%d",&k);

	printf("Please put in the values of the crunodes and the function values in order:\n");

	for(i=0;i<k;i++)
		scanf("%f%f",&x[i],&f[i]);

	printf("Please put in the value that you want to get:");
	scanf("%f",&y);

	for(i=0;i<k;i++)
	{
		for(j=0,l[i]=1;j<k;j++)
		{
			if(j==i)
				m[j]=1;
			else
				m[j]=(y-x[j])/(x[i]-x[j]);

			l[i]=l[i]*m[j];
		}
	}
	for(i=0,L=0;i<k;i++)
		L=L+l[i]*f[i];

	printf("The function value you want to get is:%f\n",L);
}