trapzoid_integration.c

#include<stdio.h>
double func(double x){
    return x*x;
}
double trapzoid(double (*f)(double),double a,double h,double b){
    int i,n;
    double sum = 0;
    n = (b-a)/h;
    sum += f(a)/2;
    for(i = 1;i < n;i++){
        sum += f(a+i*h);
    }
    sum += f(b) / 2;
    return h*sum;
}
int main(){
    double result = trapzoid(func,0.5,0.01,1);
    double exact  = (1.0/3) - (0.125/3);
    printf("Integrate x^2 from 0.5 to 1\nTrapzoid = %.6f\nExact = %.6f",result,exact);
    return 0;
}