c++ - 数组中的元素相乘在 C++ 中不起作用？

Question

我正在尝试用 C++ 编写一个简单的程序，该程序获取购买商品的价格和数量，将其存储在数组中，然后以表格格式输出每件商品的总计。然而，当我将代码中的数字相乘时，我得到了完全奇怪的答案!有人可以告诉我发生了什么事吗？

代码:

#include <iostream>

using namespace std;

int main(int argc, _TCHAR* argv[]) {
    float price[4], tot[4];
    int amt[4];
    cout << "Please enter the price and amount of 4 items:\n";
    for (int i = 0; i<4; i++) {
        cout << "Price of item " << i + 1 << ": ";
        cin >> price[i];
        cout << "Amount of item " << i + 1 << ": ";
        cin >> amt[i];
        if (price[i] <= 0 || amt[i] <= 0) {
            cout << "Invalid Input Entry!\n";
            break;
        }
        tot[i] = price[i] * amt[i]; // I can't really see how I could have messed this up...
    }
    cout << "Total\t\tPrice\t\tAmount\n";
    cout << "-----\t\t-----\t\t------\n";
    for (int i = 0; i < 4; i++) {
        cout << "$" << fixed << cout.precision(2) << tot[i] << "\t\t$" << price[i] << "\t\t" << amt[i] << endl;
    }
    system("pause");
    return 0;
}

输出:

Answer 1

问题是你输出的是 cout.precision(2) 的返回值(它返回之前的精度，在本例中，最初是 6，然后是 2)在每个总价之前。

您需要:

• 不传递cout.precision()的返回值至 operator<< :

  cout << "$" << fixed;
  cout.precision(2);
  cout << tot[i] << ...
  

  或者，调用 precision()在进入循环之前执行一次:

  cout.precision(2);
  for (int i = 0; i < 4; i++) {
      cout << "$" << fixed << tot[i] << "\t\t$" << price[i] << "\t\t" << amt[i] << endl;
  }
  

• 使用 std::setprecision() 流操纵器而不是调用 cout.precision()直接:

  #include <iomanip>
  
  for (int i = 0; i < 4; i++) {
      cout << "$" << fixed << setprecision(2) << tot[i] << "\t\t$" << price[i] << "\t\t" << amt[i] << endl;
  }
  

  或

  #include <iomanip>
  
  cout << setprecision(2);
  for (int i = 0; i < 4; i++) {
      cout << "$" << fixed << tot[i] << "\t\t$" << price[i] << "\t\t" << amt[i] << endl;
  }
  

---

附带说明一下，您不应该使用 \t字符来控制表格的格式。使用像 std::setw() 这样的流操纵器, std::left , 等等:

#include <iostream>
#include <sstream>
#include <iomanip>
#include <cstdlib>    
using namespace std;

const int maxItems = 4;

string moneyStr(float amount)
{
    ostringstream oss;

    // in C++11 and later, you can replace this with std::put_money() instead:
    // http://en.cppreference.com/w/cpp/io/manip/put_money
    //
    // oss << showbase << put_money(amount);
    oss << "$" << fixed << setprecision(2) << amount;

    return oss.str();
}

int main(int argc, _TCHAR* argv[])
{
    float price[maxItems], tot[maxItems];
    int amt[maxItems];
    int cnt = 0;

    cout << "Please enter the price and amount of " << maxItems << " items:" << endl;
    for (int i = 0; i < maxItems; ++i)
    {
        cout << "Price of item " << i + 1 << ": ";
        cin >> price[i];
        cout << "Amount of item " << i + 1 << ": ";
        cin >> amt[i];
        if (price[i] <= 0 || amt[i] <= 0) {
            cout << "Invalid Input Entry!" << endl;
            break;
        }
        tot[i] = price[i] * amt[i];
        ++cnt;
    }

    cout << left << setfill(' ');
    cout << setw(16) << "Total" << setw(16) << "Price" << setw(16) << "Amount" << endl;
    cout << setw(16) << "-----" << setw(16) << "-----" << setw(16) << "------" << endl;
    for (int i = 0; i < cnt; i++) {
        cout << setw(16) << moneyStr(tot[i]) << setw(16) << moneyStr(price[i]) << setw(16) << amt[i] << endl;
    }

    system("pause");
    return 0;
}

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