用c＋＋编程实现：已知A商品单价为7.3元，购买数量20个及以上时打八折。从键盘输入商品数量，若商品数量不为负数，则计算出商品总金额并输出，再输入付款金额，计算出找零金额并输出。要求所有输出均保留两位小数

以下是用 C++ 实现的代码：

#include <iostream>
#include <iomanip>

using namespace std;

int main()
{
const double unitPrice = 7.3;
const int discountQuantity = 20;
const double discountRate = 0.8;

int quantity;
double totalPrice, payment, change;

cout << "请输入商品数量：";
cin >> quantity;

if (quantity < 0)
{
cout << "商品数量不能为负数！" << endl;
return 1;
}

if (quantity >= discountQuantity)
{
totalPrice = quantity * unitPrice * discountRate;
}
else
{
totalPrice = quantity * unitPrice;
}

cout << "商品总金额为：" << fixed << setprecision(2) << totalPrice << "元" << endl;

cout << "请输入付款金额：";
cin >> payment;

if (payment < totalPrice)
{
cout << "付款金额不足！" << endl;
return 1;
}

change = payment - totalPrice;

cout << "找零金额为：" << fixed << setprecision(2) << change << "元" << endl;

return 0;
}

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