当变量之一稍后更新时，计算结果不会更改

在此程序中，所有语句均被顺序执行，每个语句仅执行一次。您必须重写或循环语句重复。

int numberPistols = 5;
int numberKnives = 18;
int numberWeapons = numberPistols + numberKnives;

让我们在您的程序上进行干式运行：

• 首先，您已声明了一个整数类型变量numberPistols，并用值'5'初始化了它。
• 然后，您已声明了一个整数类型变量numberKnives，并以值'18'初始化了它。
• 在下一个语句中，您已声明了一个变量numberWeapons，并以从numberPistols和numberKnives的总和获得的值初始化了它。
• 打印了所有变量后，您将numberPistols的值数递减1。

错误：

您已更新了numberPistols，但您尚未再次更新numberWeapons。更改numberPistols或numberKnives的值不会影响numberWeapons的值，因为它已经存储了，您必须使用新值更新存储的值才能使更改发生。

解决方案：

添加此行numberWeapons = numberPistols + numberKnives;

numberPistols = numberPistols + 1;

现在，您的最终代码将是：

#include <iostream>
using namespace std;
int main() {
    int numberPistols = 5;
    int numberKnives = 18;
    int numberWeapons = numberPistols + numberKnives;
    cout << "Number of Pistols: " << numberPistols << endl;
    cout << "Number of Knives: " << numberKnives << endl;
    cout << "Total of Weapons: " << numberPistols + numberKnives << endl;
    cout << "There's a new Pistol available!" << endl;
    numberPistols = numberPistols + 1;
    numberWeapons = numberPistols + numberKnives;         //update the numberWeapons here
    cout << "New amount of Pistols " << numberPistols << endl;
    cout << "Updated total of Weapons " << numberWeapons << endl;
    return 0;
}

carcigenate已经评论，一种通常的方法是写一个函数

#include <iostream>
using namespace std;
int getNumberWeapons(int numberPistols, int numberKnives)
{
    return numberPistols + numberKnives;
}
int main() {
    int numberPistols = 5;
    int numberKnives = 18;
    cout << "Number of Pistols: " << numberPistols << endl;
    cout << "Number of Knives: " << numberKnives << endl;
    cout << "Total of Weapons: " << getNumberWeapons(numberPistols, numberKnives) << endl;
    cout << "There's a new Pistol available!" << endl;
    numberPistols = numberPistols + 1;
    cout << "New amount of Pistols " << numberPistols << endl;
    cout << "Updated total of Weapons " << getNumberWeapons(numberPistols, numberKnives) << endl;
    return 0;
}

更好的解决方案是上课。这可能会被夸大，但是如果您想扩展"武器卡宾"，那将是合适的。它添加了几个成员功能来操纵课程并访问它们。

#include <iostream>
using namespace std;
class WeaponCabinet
{
public:
    void setNumberPistols(int pistols)
    {
        numberPistols = pistols;
    }
    void setNumberKnives(int knives)
    {
        numberKnives = knives;
    }
    void addPistol()
    {
        ++numberPistols;
    }
    void addKnive()
    {
        ++numberKnives;
    }
    void removePistol()
    {
        if (numberPistols == 0) return;
        --numberPistols;
    }
    void removeKnive()
    {
        if (numberKnives == 0) return;
        --numberKnives;
    }
    int getNumberKnives()
    {
        return numberKnives;
    }
    int getNumberPistols()
    {
        return numberPistols;
    }
    int getNumberWeapons()
    {
        return numberKnives + numberPistols;
    }
private:
    int numberPistols;
    int numberKnives;
};
int main() {
    WeaponCabinet weaponCabinet;
    weaponCabinet.setPistols(5);
    weaponCabinet.setKnives(18);
    cout << "Number of Pistols: " << weaponCabinet.getNumberPistols() << endl;
    cout << "Number of Knives: " << weaponCabinet.getNumberKnives() << endl;
    cout << "Total of Weapons: " << weaponCabinet.getNumberWeapons() << endl;
    cout << "There's a new Pistol available!" << endl;
    weaponCabinet.addPistol();
    cout << "New amount of Pistols " <<  weaponCabinet.getNumberPistols() << endl;
    cout << "Updated total of Weapons " << weaponCabinet.getNumberWeapons() << endl;
    return 0;
}

通过这种方式，您在刀和手枪之间有直接的连接，并且不必每次将它们传递到功能getNumberWeapons。

如果您不需要整个班级处理武器，那么最现代的技术就是使用lambda（使用功能更接近解决方案）

#include <iostream>
using namespace std;
int main() {
    int numberPistols = 5;
    int numberKnives = 18;
    auto numberWeapons = [&numberPistols, &numberKnives]()
        {
            return numberPistols + numberKnives;
        };
    cout << "Number of Pistols: " << numberPistols << endl;
    cout << "Number of Knives: " << numberKnives << endl;
    cout << "Total of Weapons: " << numberWeapons() << endl;
    cout << "There's a new Pistol available!" << endl;
    numberPistols = numberPistols + 1;
    cout << "New amount of Pistols " << numberPistols << endl;
    cout << "Updated total of Weapons " << numberWeapons() << endl;
    return 0;
}

写

int numberWeapons = numberPistols + numberKnives;

这不会创建一个"方程式"，该方程在每次更改后自动更新。该行在开始时一次执行一次，并且由于您没有循环，因此再也不会执行。

您需要在更改后手动更新总计：

#include <iostream>
using namespace std;
int main() {
    int numberPistols = 5;
    int numberKnives = 18;
    int numberWeapons = numberPistols + numberKnives;
    cout << "Number of Pistols: " << numberPistols << endl;
    cout << "Number of Knives: " << numberKnives << endl;
    cout << "Total of Weapons: " << numberPistols + numberKnives << endl;
    cout << "There's a new Pistol available!" << endl;
    numberPistols = numberPistols + 1;
    //Manually update the variable. 
    numberWeapons = numberPistols + numberKnives;
    cout << "New amount of Pistols " << numberPistols << endl;
    cout << "Updated total of Weapons " << numberWeapons << endl;
    return 0;
}

注意，它看起来就像是第一次设置，除了int，因为我不是声明 numberWeapons just repainsing it。

在"真实"程序中，您将使用一个处理更新的函数，但我认为这只会掩盖逻辑听到的声音，因为您只是将2个数字添加在一起。

变量是彼此独立的。为了向您解释，我们可以从苹果开始：

我给露西给了两个苹果。我给你三个苹果，说："我给你一个苹果，加上给露西的苹果数量。"然后，一段时间后，我将另一个苹果给露西。你有几个苹果？

答案很简单：这是三个。就像C 或任何其他编程语言中的变量一样，您和Lucie的苹果数量彼此独立。变量彼此独立。

突变变量的值不会影响另一个变量的值，即使您使用变量计算另一个值的值。

如果您想始终具有更新的号码怎么办？

然后，您必须每次计算值。程序员通常会使用功能来执行此操作。在您的示例中，我们可以将武器总数定义为返回numberPistols加上numberKnives的函数。这就是我的方式：

#include <iostream>
using namespace std;
int main() {
    int numberPistols = 5;
    int numberKnives = 18;
    auto computeNumberWeapons = [&]{ return numberPistols + numberKnives; };
    cout << "Number of Pistols: " << numberPistols << endl;
    cout << "Number of Knives: " << numberKnives << endl;
    cout << "Total of Weapons: " << numberPistols + numberKnives << endl;
    cout << "There's a new Pistol available!" << endl;
    numberPistols = numberPistols + 1;
    cout << "New amount of Pistols " << numberPistols << endl;
    cout << "Updated total of Weapons " << computeNumberWeapons() << endl;
    return 0;
}

现在， computeNumberWeapons不是类型int的变量，而是计算两个变量的总和的函数。您必须使用()操作员调用该功能。

这也许不是您在第一个C 课程中要学习的东西，但是如果您'RE使用C 11，您可以用lambda函数替换int numberWeapons，该功能在调用它时累加numberPistols和numberKnives的当前值：

  auto numberWeapons = [&numberPistols, &numberKnives]() {
    return numberPistols + numberKnives;
  };

numberWeapons现在是一个函数，从语句的[&numberPistols, &numberKnives]部分中知道numberPistol和numberKnives变量。它不需要参数-()-它返回两个变量的总和。

（实际上，()可以在此处省略，因为它们是空的，没有关于此功能的论点，但是我将它们留在其中，因为它使其更具识别为lambda功能，通常看起来像[…](…){…}。

#include <iostream>
using namespace std;
int main() {
  int numberPistols = 5;
  int numberKnives = 18;
  auto numberWeapons = [&numberPistols, &numberKnives]() {
    return numberPistols + numberKnives;
  };
  cout << "Number of Pistols: " << numberPistols << endl;
  cout << "Number of Knives: " << numberKnives << endl;
  cout << "Total of Weapons: " << numberWeapons() << endl; // outputs 23
  //                         note the brackets ^^
  cout << "There's a new Pistol available!" << endl;
  numberPistols = numberPistols + 1;
  cout << "New amount of Pistols " << numberPistols << endl;
  cout << "Updated total of Weapons " << numberWeapons() << endl; // outputs 24
  //                                                  ^^
  return 0;
}

我理解，还尝试了您在这里发布的其他代码，其中一个在我尝试编译和运行时给我错误（下面是代码）。我正在使用Dev-C 5.11。

int main() {
int numberPistols = 5;
int numberKnives = 18;
auto computeNumberWeapons = [&]{ return numberPistols + numberKnives; };
cout << "Number of Pistols: " << numberPistols << endl;
cout << "Number of Knives: " << numberKnives << endl;   
cout << "Total of Weapons: " << numberPistols + numberKnives << endl;   
cout << "There's a new Pistol available!" << endl;
numberPistols = numberPistols + 1;
cout << "New amount of Pistols " << numberPistols << endl;
cout << "Updated total of Weapons " << computeNumberWeapons()) << endl;
return 0;
}

有时我使用的这个IDE 编译器会让我认为是否是错误的代码，或者在安装并在其中启动项目时做错了什么...

错误图像

好的，可以在下面的函数下添加此功能，然后在int main（）

//function for calculating numberWeapons 
int totalWeapons(int numberPistols, int numberKnives){ 
     return numberPistols+numberKnives; 
}

并在您需要更新的NumberWeapons中添加下面给出的行：

numberWeapons = totalWeapons(numberPistols, numberKnives);

即。您的代码就像：

#include <iostream>
using namespace std;
//function for calculating numberWeapons 
int totalWeapons(int numberPistols, int numberKnives){ 
     return numberPistols+numberKnives; 
}
int main() {
    int numberPistols = 5;
    int numberKnives = 18;
    int numberWeapons = totalWeapons(numberPistols, numberKnives); //call function here
    cout << "Number of Pistols: " << numberPistols << endl;
    cout << "Number of Knives: " << numberKnives << endl;
    cout << "Total of Weapons: " << numberWeapons << endl;
    cout << "There's a new Pistol available!" << endl;
    numberPistols = numberPistols + 1;
    //call function here to get updated numberWeapons
    numberWeapons = totalWeapons(numberPistols, numberKnives); 
    cout << "New amount of Pistols " << numberPistols << endl;
    cout << "Updated total of Weapons " << numberWeapons << endl;
    return 0;
}