Thread: Lagrange Polynomial Program

Code:

#include <stdio.h>#include <stdlib.h>
#include <math.h>
#define pi 4. * atan(1.)
int LaGrange(int x0[],int x[],int n,int m);
main(){
    double x[10]={0,0.1,0.3,0.45,0.66,0.8,1.1,1.4,1.5,1.9};
    double approx[6]={-0.5,0.2,0.9,1.6,2.0,3.0};
    double y[10],yapprox[6],lagrange[6],temp,total;
    int i,n,m;
    
    for(i=0;i<10;i++)
    y[i]=x[i]*sin(x[i]);
    
    for(i=0;i<7;i++)
    yapprox[i]=approx[i]*sin(approx[i]);
    
    for (m=0;m<=6;m++){
        for (n=0;n<=9;n++){
            if (n<=7){
                temp=(((approx[n]-x[n+1])*(approx[n]-x[n+2]))/((x[n]-x[n+1])*(x[n]-x[n+2])))*y[n];
                total += temp;
            }
            else{
                temp=(((approx[n]-x[n-1])*(approx[n]-x[n]))/((x[n-2]-x[n-1])*(x[n-2]-x[n])))*y[n];
                total += temp;
            }
        }
        lagrange[m]=total;
    }
    printf("\nThe LaGrange Values for each value:\n");
    printf("x=-.5:\t%e\nx=0.2:\t%e\nx=0.9:\t%e\nx=1.6:\t%e\nx=2.0:\t%e\nx=3.0:\t%e\n",lagrange[0],lagrange[1],lagrange[2],lagrange[3],lagrange[4],lagrange[5]);
    printf("\nThe actual value for each value is:\n");
    printf("x=-.5:\t%e\nx=0.2:\t%e\nx=0.9:\t%e\nx=1.6:\t%e\nx=2.0:\t%e\nx=3.0:\t%e\n",yapprox[0],yapprox[1],yapprox[2],yapprox[3],yapprox[4],yapprox[5]);
    getchar();
}

So I am suppose estimate the values for xsinx by using Lagrange polynomial and noticed while doing this that my code is almost correct but my equation is not 9th order. How can i make my code shorter instead of writing out a 9th order polynomial. The equation i made was a 2nd order because i was not really understanding the LaGrange Polynomial.


I basically have learning arrays, pointers, functions and thats it for right now. I can't use structures for this (I don't know what they are either.

This is what a LaGrange Polynomial is.
lagrange polynomial - Wolfram|Alpha

Thank you for all the help

Code:

intLaGrange(intx0[],intx[],intn,intm);

I just noticed that i am not even using this

I've not much time but have you already looked at the examples on Wikipedia?

Bye, Andreas

Okay so looking at the examples on google I came up with this code

Code:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define pi 4*atan(1.)
main(){
    double x[10]={0,0.1,0.3,0.45,0.66,0.8,1.1,1.4,1.5,1.9};
    double approx[6]={-0.5,0.2,0.9,1.6,2.0,3.0};
    double y[10],yapprox[6],lagrange[6],temp,total=0,part=1,grandtotal=0;
    int i,n,m,k;
     
    for(i=0;i<10;i++)
    y[i]=x[i]*sin(x[i]);
     
    for(i=0;i<7;i++)
    yapprox[i]=approx[i]*sin(approx[i]);
     
    for (k=0;k<=5;k++){
		for (m=0;m<=9;m++){
			for (n=0;n<=9;n++){
				if (m!=n && k!=n){
					temp = (approx[k]-x[n])/(x[m]-x[n]);
					part = part * temp;
				}
			total = part * y[m];
			}
		grandtotal= total + grandtotal;
		}
	lagrange[k]=grandtotal;
	}
    printf("\nThe LaGrange Values for each value:\n");
    printf("x=-.5:\t%e\nx=0.2:\t%e\nx=0.9:\t%e\nx=1.6:\t%e\nx=2.0:\t%e\nx=3.0:\t%e\n",lagrange[0],lagrange[1],lagrange[2],lagrange[3],lagrange[4],lagrange[5]);
    printf("\nThe actual value for each value is:\n");
    printf("x=-.5:\t%e\nx=0.2:\t%e\nx=0.9:\t%e\nx=1.6:\t%e\nx=2.0:\t%e\nx=3.0:\t%e\n",yapprox[0],yapprox[1],yapprox[2],yapprox[3],yapprox[4],yapprox[5]);
    getchar();
}

But this is still not giving me what values I want and i can't figure out what my mistake is. I have been looking at it for a while and can't figure it out.

Lagrange polynomial - Wikipedia, the free encyclopedia

I followed example one on the foundation of my code. Basically taking the ratio of each part and multiplying it all together through my first loop, then multiplying that by y[m] when all my values for ratios are multiplied together. After that they are all added up to give me some value and that is assigned to each lagrange variable. But i am not getting the results i need.

What are "approx" and "yapprox" supposed to be?

Bye, Andreas