Lagrange Function C + +

  
 

   
  

  
#include <iostream>
   
  

  
using namespace std;
   
  

  
//拉格朗日插值求解方程
   
  

  

   
  

  
double * xs; //all input x
   
  

  
double * ys; //all input y
   
  

  
int n; //size
   
  

  

   
  

  
//1.1275 1.1503 1.1735 1.1972
   
  

  
//0.1191 0.13954 0.15932 0.17903
   
  

  
//1.130
   
  

  
void init()
   
  

  
{
   
  

  
 cout << "please input n " << endl;
   
  

  
 cin >> n;
   
  

  
 xs = new double[n];
   
  

  
 ys = new double[n];
   
  

  

   
  

  
//input x
   
  

  
 cout << "please input the x‘s value !" << endl;
   
  

  
 for(int i=0; i<n; i++)
   
  

  
 {
   
  

  
 cin >> xs[i];
   
  

  
 }
   
  

  

   
  

  
//input y
   
  

  
 cout << "please input the y‘s value !" << endl;
   
  

  
 for(int i=0; i<n; i++)
   
  

  
 {
   
  

  
 cin >> ys[i];
   
  

  
 }
   
  

  
}
   
  

  

   
  

  
double everyItem(int num, double x)
   
  

  
{
   
  

  
 double y = ys[num];
   
  

  
 double up = 1;
   
  

  
 double down = 1;
   
  

  
 for(int i=0; i<n; i++)
   
  

  
 {
   
  

  
 if(i != num)
   
  

  
 {
   
  

  
 up *= (x - xs[i]);
   
  

  
 down *= (xs[num] - xs[i]);
   
  

  
 }
   
  

  
 }
   
  

  

   
  

  
 return y*up/down;
   
  

  
}
   
  

  

   
  

  
double lagrange(double x)
   
  

  
{
   
  

  
 double total = 0;
   
  

  
 for(int i=0; i<n; i++)
   
  

  
 {
   
  

  
 total = total + everyItem(i , x);
   
  

  
 }
   
  

  

   
  

  
 return total;
   
  

  
}
   
  

  

   
  

  

   
  

  

   
  

  
int main()
   
  

  
{
   
  

  

   
  

  
 init();
   
  

  
 double input;
   
  

  
 cout << "please input you x !" <<endl;
   
  

  
 cin >> input;
   
  

  

   
  

  
 double result = lagrange(input);
   
  

  
 cout << "the result is " << result << endl;
   
  

  
 return 0;
   
  

  
}
  
 

From for notes (Wiz)

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Lagrange Function C + +