simpson_integration.c

#include<stdio.h>
double func(double x){
    return x*x;
}
double trapzoid(double (*f)(double),double a,int n,double b){
    int k;
    double sum_odd = 0, sum_even = 0,h;
    n = n * 2; //simpson จะแบ่งช่วงย่อยเป็นช่วงคู่และช่วงคี่
    h = (b-a)/n;
    for(k = 0;k<n - k;k++){
        sum_odd+= f(a+h+2*k*h);
    }
    for(k = 1;k<n - k;k++){
        sum_even+= f(a+2*k*h);
    }
    return (h/3)*(f(a)+4*sum_odd+2*sum_even+f(b));
}
int main(){
    double result = trapzoid(func,0.5,100,1);
    double exact  = (1.0/3) - (0.125/3);
    printf("Integrate x^2 from 0.5 to 1\nSimpson = %.6f\nExact = %.6f",result,exact);
    return 0;
}