c++ - C++中双变量的逼近

Question

我正在尝试运行这个程序，在其中插入两个 double 值，然后返回两个数字中哪个更大，如果两个 double 的值相同，则返回一条消息“数字相等” , 如果两个数字之间的差异小于 1/10.000.000，则显示一条消息“数字几乎相等”。

我写的代码是这样的:

    #include "stdafx.h"
    #include <iostream>
    #include <string>
    #include <vector>
    #include <algorithm>
    #include <cmath>
    using namespace std;
    inline void keep_window_open() { char ch; cin >> ch; }

    double square(double x)
    {
        return x * x;
    }

    int main() {
        double number;
        double number2;
        while (cin >> number, cin >> number2) {
            if (number == '|' || number2 == '|') break;
            else {
                cout << number << " " << number2 << endl;
                if (number > number2) cout << number << " is bigger than " << number2 << endl;
                if (number < number2) cout << number2 << " is bigger than " << number << endl;
                double difference = number - number2;
                if (difference > - 1/10000000 && difference < 0) cout << "the numbers are almost equal" << endl;
                if (difference < 1 / 10000000 && difference > 0) cout << "the numbers are almost equal" << endl;
                if (number == number2) cout << "the numbers are equal" << endl;
            }

        }
    }

当我运行这个程序并插入两个差值小于 1/10.000.000 的数字时，程序应该返回“数字几乎相等”，但事实并非如此。

如果我插入“1”和“1.0000000000001”，这就是它返回的内容(当我运行程序并插入两个数字时):

    "1 1.0000000000001

    1 1

    1 is bigger than 1"

我不明白为什么程序将我插入的两个数字视为 1，并返回“1”和“1”作为值。

它也应该给出“数字几乎相等”的返回，因为我写道:

     if (difference > - 1/10000000 && difference < 0) cout << "the numbers are almost equal" << endl;
                        if (difference < 1 / 10000000 && difference > 0) cout << "the numbers are almost equal" << endl;

但事实并非如此。

当我插入两个相差小于 1/10.000.000 的 double 值(例如“1”和“1.0000000000001”)时，如何让程序返回“数字几乎相等”？

Answer 1

当您尝试打印这些 double 值时，cout 会对其进行四舍五入。参见 here

至于您的代码，整数除法 (1/10000000) 将始终返回一个整数，因此您需要执行 double 或十进制值 (1.0/10000000.0) 的除法

这是一个工作代码，对逻辑做了一些小改动

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <cmath>
using namespace std;
inline void keep_window_open() { char ch; cin >> ch; }

double square(double x)
{
    return x * x;
}

int main() {
    double number;
    double number2;
    while (cin >> number, cin >> number2) {
        if (number == '|' || number2 == '|') break;
        else {
            cout.precision(15);
            cout << number << " " << number2 << endl;
            double difference = number - number2;
            if (difference > - 1/10000000.0 && difference < 0) cout << "the numbers are almost equal" << endl;
            else if (difference < 1/10000000.0 && difference > 0) cout << "the numbers are almost equal" << endl;
            else if (number == number2) cout << "the numbers are equal" << endl;
            else if (number > number2) cout << number << " is bigger than " << number2 << endl;
            else if (number < number2) cout << number2 << " is bigger than " << number << endl;
        }

    }
}