C Program for Lagrange's Interpolation

#include<stdio.h>

#include<conio.h>

#define MAX 10

void main(){

clrscr();

float x[MAX+1], fx[MAX+1], num, den, val, y=0;

int i, j, n;

printf("\t\tLAGRANGE'S INTERPOLATION");

printf("\n\nEnter the degree of interpolation(n): ");

scanf("%d", &n);

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

printf("\n x%d: ",i);

scanf("%f", &x[i]);

printf("\n fx%d: ",i);

scanf("%f", &fx[i]);

}

printf("\nEnter x to estimate fx: ");

scanf("%f", &val);

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

num=1;

den=1;

for (j=0; j<=n; j++)

if(j!=i){

num *= val-x[j];

den *= x[i]-x[j];

}

y+=(num/den)*fx[i];

}

printf("\nApproximate value at %.3f: %.3f",val,y);

getch();

}

Sample Output:

LAGRANGE’S INTERPOLATION
Enter the degree of interpolation(n): 2
x0: 1
fx0: 1
x1: 2
fx1: 1.4142
x2: 3
fx2: 1.7321

Enter x to estimate fx: 2.5
Approximate value at 2.500: 1.585

©Dixit Bhatta 2013