f0610.cpp

it is a usefull thing

//==================================
// f0610.cpp
// 矩形积分
//==================================
#include<iostream>
#include<fstream>
#include<cmath>
using namespace std;
//----------------------------------
double g(double x){
  return 1/x;
}//---------------------------------
double rectangle(double a, double b, double(*f)(double)){
  double w=b-a, sumNew=w*(f(a)+f(b))/2, sumOld=0;
  for(int n=1; abs(sumNew-sumOld)>=1e-4; n*=2){
    sumOld=sumNew;
    sumNew=0;
    for(int i=0; i<n; ++i)
      sumNew += f(a + w*(i+0.5)/n);
    sumNew *= w/n;
  }
  return sumNew;
}//---------------------------------
int main(){
  ifstream in("integral.txt");
  cout<<fixed; cout.precision(3);
  for(double b; in>>b; )
    cout<<rectangle(1,b,g)<<"\n";
}//=================================