C++金额转纸币硬币程序求助：小数金额与模运算处理问题

问题描述 #

我有一项作业任务，要编写程序实现用户输入金额后，输出所需纸币和硬币的数量，但在存储找零及获取所需找零金额时遇到了麻烦。我知道不能用整数存储硬币金额，但用double类型的话，又会出现%模运算不能用于double的错误。现在想请教，处理完纸币部分后，怎么用double类型提取剩余金额？当前程序输入456.56时，能正确输出4张百元、2张20元、1张10元等纸币，但没法处理0.56的小数部分。以下是我当前的C++代码：

/*This program will convert the amount of money entered by the user into the amount of bills and change*/

#include<iostream>
#include<iomanip>

using namespace std;

int main()
{
    const int HUNDRED = 100;
    const int TWENTY = 20;
    const int TEN = 10;
    const int FIVE = 5;
    const int DOLLAR = 1;
    const int QUARTER = 25;
    const int DIME = 10;
    const int NICKEL = 05;
    const int PENNY = 01;
    int changeAmount; 

    cout << "Enter amount of money to convert: $";
    cin >> changeAmount;

    cout << "\n";
    
    cout << "Numbe of 100 dollar bills: " << (int)changeAmount / HUNDRED << endl;
    changeAmount = changeAmount % HUNDRED;

    cout << "Numbe of 20 dollar bills: " << (int)changeAmount / TWENTY << endl;
    changeAmount = changeAmount % TWENTY;

    cout << "Numbe of 10 dollar bills: " << (int)changeAmount / TEN << endl;
    changeAmount = changeAmount % TEN;

    cout << "Numbe of 5 dollar bills: " << (int)changeAmount / FIVE << endl;
    changeAmount = changeAmount % FIVE;

    cout << "Numbe of 1 dollar bills: " << (int)changeAmount / DOLLAR << endl;
    changeAmount = changeAmount % DOLLAR;

    cout << "Numbe of Quarters: " << (int)changeAmount / QUARTER << endl;
    changeAmount = changeAmount % QUARTER;

    cout << "Numbe of Dimes: " << (int)changeAmount / DIME << endl;
    changeAmount = changeAmount % DIME;

    cout << "Numbe of Nickles: " << (int)changeAmount / NICKEL << endl;
    changeAmount = changeAmount % NICKEL;

    cout << "Numbe of Pennies: " << (int)changeAmount / PENNY << endl;
    changeAmount = changeAmount % PENNY;

    return 0;
}

解决思路 #

处理货币金额最稳妥的方式是把所有金额转换成整数（以分为单位），完全避免浮点数精度问题。因为浮点数（比如double）存储十进制小数时会有精度丢失（比如0.1无法精确表示），导致计算错误。具体步骤：

• 将用户输入的金额乘以100，转换为整数分，这样所有计算都用整数完成，模运算%也能正常使用。
• 纸币部分按“元”对应的分来计算（比如100元=10000分），硬币部分直接按分的面值计算。
• 修正原代码中的拼写错误（Numbe→Number）和不规范的八进制数写法（05→5，01→1）。

修改后的完整代码 #

/*This program will convert the amount of money entered by the user into the amount of bills and change*/

#include<iostream>
#include<iomanip>

using namespace std;

int main()
{
    // 所有面值以分为单位
    const int HUNDRED = 10000;   // 100元 = 10000分
    const int TWENTY = 2000;     // 20元 = 2000分
    const int TEN = 1000;        // 10元 = 1000分
    const int FIVE = 500;        // 5元 = 500分
    const int DOLLAR = 100;      // 1元 = 100分
    const int QUARTER = 25;      // 25分
    const int DIME = 10;         // 10分
    const int NICKEL = 5;        // 5分
    const int PENNY = 1;         // 1分

    double inputAmount;
    cout << "Enter amount of money to convert: $";
    cin >> inputAmount;

    // 转换为整数分，加0.5避免浮点数精度误差（比如456.56*100可能得到45655.99999999999）
    int totalCents = static_cast<int>(inputAmount * 100 + 0.5);

    cout << "\n";
    
    cout << "Number of 100 dollar bills: " << totalCents / HUNDRED << endl;
    totalCents = totalCents % HUNDRED;

    cout << "Number of 20 dollar bills: " << totalCents / TWENTY << endl;
    totalCents = totalCents % TWENTY;

    cout << "Number of 10 dollar bills: " << totalCents / TEN << endl;
    totalCents = totalCents % TEN;

    cout << "Number of 5 dollar bills: " << totalCents / FIVE << endl;
    totalCents = totalCents % FIVE;

    cout << "Number of 1 dollar bills: " << totalCents / DOLLAR << endl;
    totalCents = totalCents % DOLLAR;

    cout << "Number of Quarters: " << totalCents / QUARTER << endl;
    totalCents = totalCents % QUARTER;

    cout << "Number of Dimes: " << totalCents / DIME << endl;
    totalCents = totalCents % DIME;

    cout << "Number of Nickels: " << totalCents / NICKEL << endl;
    totalCents = totalCents % NICKEL;

    cout << "Number of Pennies: " << totalCents / PENNY << endl;
    totalCents = totalCents % PENNY;

    return 0;
}

关键说明 #

• 用double接收用户输入后，立即转换为整数分，通过+0.5解决浮点数精度问题（比如0.56*100可能计算为55.99999999999999，加0.5后转为整数就是56）。
• 所有面值统一用分作为单位，全程用整数运算，彻底避免模运算不能用于浮点数的问题，同时解决了小数部分无法处理的问题。

内容的提问来源于stack exchange，提问作者needusername