lage.c

c拉格朗日插值函数

#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>
double QIP(int n,double t,double *x,double *y)
{
    int i,k;
    double u,v,w,x0,x1,x2,f;
    for(i=0;i<=n-4;i++)
    {
        if(t<=x[i+1])
        {
            k=i;
            if((i!=1)&&(t-x[i]<x[i+1]-t))
                k-=1;
        }
        else
            k=n-3;
    }
    x0=x[k];
    x1=x[k+1];
    x2=x[k+2];
    u=(t-x1)*(t-x2)/((x0-x1)*(x0-x2));
    v=(t-x0)*(t-x2)/((x1-x0)*(x1-x2));
    w=(t-x0)*(t-x1)/((x2-x0)*(x2-x1));
    f=u*y[k]+v*y[k+1]+w*y[k+2];
    return (f);
}
double QIP(int ,double,double *,double *);
main()
{
    int i,n;
    double *x,*y,t,f;
    n=6;
    t=75.5;
    x=(double *)calloc(n,sizeof(double));
    if(x==NULL)
        exit(1);
    y=(double *)calloc(n,sizeof(double));
    if(y==NULL)
        exit(1);
    x[0]=75;
    x[1]=76;
    x[2]=77;
    x[3]=78;
    x[4]=79;
    x[5]=80;
    y[0]=2.768;
    y[1]=2.833;
    y[2]=2.903;
    y[3]=2.979;
    y[4]=3.062;
    y[5]=3.153;
    printf("使用拉哥朗日算法:\n");
    printf("当x=75.5时,结果为 %f \n",QIP(n,t,x,y));
    t=78.3;
    printf("当x=78.3时,结果为 %f \n",QIP(n,t,x,y));
    free(x);
    free(y);
}