C++中一个简单的if语句实践中的逻辑错误答案

【问题标题】：Logic Error in a Simple If Statement Practice in C++C++中一个简单的if语句实践中的逻辑错误
【发布时间】：2017-03-16 03:14:42
【问题描述】：

我不确定为什么 CHARGE_1 被评估为 .80，无论我输入的数量是 1 还是 19。其他 if 语句也是如此。我不断得到奇怪的数字，比如 .80 和 .64 作为输出。请帮我弄清楚这有什么问题。

#include <iostream>
#include <iomanip>

using namespace std;

int main()
{
    const int MONTHLY_FEE = 10;
    const double CHARGE_1 = .10;
    const double CHARGE_2 = .08;
    const double CHARGE_3 = .06;
    const double CHARGE_4 = .04;
            int numChecks;
          double charge1Total = numChecks * CHARGE_1;
          double charge2Total = numChecks * CHARGE_2;
          double charge3Total = numChecks * CHARGE_3;
          double charge4Total = numChecks * CHARGE_4;

    cout << "Please enter how many checks you have used.\n";
    cout << "and I will tell you how much your charge is. ";
    cin >> numChecks;

    if((numChecks > 0) && (numChecks < 20))
    {
        cout << "Your total is $" << fixed << setprecision(2) <<    
        (charge1Total + MONTHLY_FEE) << ".";
    }
    else if ((numChecks >= 20) && (numChecks <= 39))
    {
        cout << "Your total is $" << fixed << setprecision(2) <<    
       (charge2Total + MONTHLY_FEE) << ".";
    }
    else if ((numChecks >= 40) && (numChecks <= 59))
    {
        cout << "Your total is $" << fixed << setprecision(2) <<    
        (charge3Total + MONTHLY_FEE) << ".";
    }
    else if (numChecks >=60)
    {
        cout << "Your total is $" << fixed << setprecision(2) << 
        (charge4Total + MONTHLY_FEE) << ".";
    }
}

【问题讨论】：

您在获得支票号码之前计算您的总数。放在 cin >> numChecks; 行之后。

标签： c++ if-statement logic conditional

【解决方案1】：

使用前需要初始化 numChecks 的值。有四个地方使用了 uninitialized 变量，这意味着该值可以是任何值。

在行中

  double charge1Total = numChecks * CHARGE_1;
  double charge2Total = numChecks * CHARGE_2;
  double charge3Total = numChecks * CHARGE_3;
  double charge4Total = numChecks * CHARGE_4;

numChecks 可以是任何东西，所以把这些行放在你所在的语句下面

cin >> numChecks;

【讨论】：

【解决方案2】：

chargeXTotal 变量在编译时进行评估，因为您在变量声明中将它们的公式定义为：

double charge1Total = numChecks * CHARGE_1;

因为numChecks 是一个未初始化的变量：

int numChecks;

因此，chargeXTotal 每个都已经设置了一个数字，而不是您对cin 的期望，因此正确的方法是在cin 之后保留chargeXTotal 的声明：

cin >> numChecks;
double charge1Total = numChecks * CHARGE_1;

这将使编译器在用户输入后计算charge1Total 的正确值，而不是在编译时使用未初始化的变量。

【讨论】：

【解决方案3】：

您的代码中有一个简单的逻辑错误：您甚至在初始化之前就使用numChecks 执行了数学运算。计算完成后接受numChecks 的输入：

int numChecks; // numChecks is uninitialized here; it is only defined as an int variable

// in the following 4 calculations, you multiply CHARGE_N with an uninitialized variable
double charge1Total = numChecks * CHARGE_1;
double charge2Total = numChecks * CHARGE_2;
double charge3Total = numChecks * CHARGE_3;
double charge4Total = numChecks * CHARGE_4;

// here you accept the input of numChecks after you have oerformed the calculations with it
cout << "Please enter how many checks you have used.\n";
cout << "and I will tell you how much your charge is. ";
cin >> numChecks;

由于您在执行数学运算之前没有初始化numChecks，因此每次运行程序时在运算中使用的值都是未知的，因为编译器会将变量设置为某个默认值。一些编译器将变量初始化为零；但是，其他编译器会将其初始化为某个“unknown”值。

在编译器执行数学运算并将 不正确 值存储在 chargeXtotal 之后，您初始化变量 numChecks，因此，您可以将奇怪的值作为输出。

您应该更改您的代码，以便在完成chargeXtotal 的数学运算之前输入numChecks：

int numChecks;

cout << "Please enter how many checks you have used.\n";
cout << "and I will tell you how much your charge is. ";
cin >> numChecks;

double charge1Total = numChecks * CHARGE_1;
double charge2Total = numChecks * CHARGE_2;
double charge3Total = numChecks * CHARGE_3;
double charge4Total = numChecks * CHARGE_4;

希望这会有所帮助！

【讨论】：