如何在 C++ 中的循环外使用 For 循环中的变量？

Question

所以我的编程技能有点生疏，除了我过去上过的大学课程外，没有任何现实世界的经验。我正在为一个类编写程序，但遇到了障碍；我无法弄清楚如何在循环外的For 循环内使用变量的值。这是我引用的代码：

#include "iostream"

using namespace std;

int want, have, need;
int counter = 0;

char response, cont;

int diff(int want, int have);

int main(){
cout << "Welcome!\n";
cout << "This program will help you reach your money saving goals!\n";
cout << "Would you like to use this program? (y/n)\n";
    cin >> response;
    while (response == 'y'){
        cout << "Please enter all amounts in whole dollars only!\n";
        cout << "Please enter the amount of money you would like to have saved: $";
        cin >> want;
        cout << "\nPlease enter the amount of money you currently have saved: $";
        cin >> have;
        if (have >= want){
            cout << "You have already reached or exceeded your goal, this program will not be able to help you!\n";
            system("Pause");
            return 0;
        }
        cout << "\nYou need to save $" << diff(want, have) << " more money to reach your goal!\n";
        cout << "Would you like me to help you with a savings plan?";
        cin >> cont;
        while (cont == 'y'){
            int menu;
            cout << "Please select from the following options: \n";
            cout << "1 - Daily Saving Plan\n";
            cout << "2 - Weekly Saving Plan\n";
            cout << "3 - Monthly Saving Plan\n";
            cout << "Enter the number associated with your choice: \n";
            cin >> menu;

            switch (menu){
                case 1: {
                    int daily;
                    cout << "You have chosen the Daily Savings Plan\n";
                    cout << "How much money can you save every day? $";
                    cin >> daily;
                    for (int x = daily; x < need; x++){
                        daily = daily + daily;
                        counter++;
                    }
                    cout << "\nIt will take you " << counter << " days to reach your goal!\n";
                    break;
                }
                case 2: {
                    int weekly;
                    cout << "You have chosen the Weekly Savings Plan\n";
                    cout << "How much money can you save every week? $";
                    cin >> weekly;
                    for (int x = weekly; x < need; x++){
                        counter++;
                    }
                    cout << "\nIt will take you " << counter << " weeks to meet your goal!\n";
                    break;
                }
                case 3: {
                    int monthly;
                    cout << "You have chosen the Monthly Savings Plan\n";
                    cout << "How much money can you save every month? $";
                    cin >> monthly;
                    for (int x = monthly; x < need; x++){
                        monthly = monthly + monthly;
                        counter++;
                    }
                    cout << "\nIt will take you " << counter << " months to reach your goal!\n";
                    break;

                }
                default: cout << "You made an invalid selection";

                    cout << "Would you like to look at a different saving plan? (y/n)\n";
                    cin >> cont;
            }
        }
    }
}

int diff(int want, int have){
return want - have;
}

所以，当我运行程序时，一切正常，但在最后的cout 语句中，计数器的值始终显示为“0”。

我想我理解它为什么这样做，这是因为循环外的“int counter = 0”声明，所以我假设循环退出后它会回到那个值。

如果我不启动计数器变量，我会得到一个错误，如果我在循环中声明该值，我会得到一个错误，试图在上面的 cout 语句中使用它。

我什至不确定我的 for 循环结构是否正确...基本上我希望它自己添加每周变量，直到总数为 x = need。我还想捕获它需要多少次迭代，然后将其输出为周数。希望一切都说得通；感谢您提供任何和所有帮助。

Answer 1

似乎您想做的事情可以用ceil(double(need/weekly)) 完成，并且将需求除以每周。

在您完成for 循环后，您在循环外的声明不会影响您cout 的值。

至于您的问题，您似乎从未初始化 need，因此您的 for 循环从未进行过迭代，因为 undefined 不小于或等于 0。

Answer 2

你的程序的 C++11 草图。

char prompt( std::initializer_list<const char*> message, std::initializer_list<const char*> question, std::initializer_list<char> options ) {
  for( auto msg:message )
    std::cout << msg;
  while(true) {
    char response;
    for( auto q:question)
      std::cout << q;
    std::cin >> response;
    for (char option:options) {
      if (response == option)
        return response;
    }
  }
}
int prompt( std::initializer_list<const char*> message, std::initializer_list<const char*> question, int min = 0, int max = std::numeric_limits<int>::max() ) {
  for( auto msg:message )
    std::cout << msg;
  while(true) {
    int response;
    for( auto q:question)
      std::cout << q;
    std::cin >> response;
    if (response >= min && response <= max)
      return response;
    }
  }
}
void saving( const char* plan, const char* unit, const char* units, int target ) {
  int daily = prompt(
    {"You have chosen the ", plan,  "\n"},
    {"How much money can you save every ", unit, "? $"}, 
    1 // min saving of 1$ per unit time to prevent infinite loops
  );
  std::cout  << "\n";
  int counter = 0;
  int amount_saved = 0;
  while (amount_saved < target) {
    ++counter;
    amount_saved += daily;
  }
  if (counter != 1)
    std::cout << "It will take you " << counter << " " << units << " to reach your goal\n";
  else
    std::cout << "It will take you " << counter << " " << unit << " to reach your goal\n";
}

int main() {
  while(
    prompt(
      {"Welcome!\nThis program will help you reach your money saving goals!\n"},
      {"Would you like to use this program? (y/n)\n"},
      {'y', 'n'}
    ) == 'y'
  )
  {
    int want = prompt( {"Please enter all amounts in whole dollars only!\n"},
      {"Please enter the amount of money you would like to have saved?"} );
    int have = prompt( {"\n"}, {"Please enter the amount of money you currently have saved?\n"} );
    std::cout << "\n";
    if (have >= want) {
      std::cout << "You win!\n";
      system("Pause"); // ick
      return 0;
    }
    std::cout << "You need to save $" << (have-want) << " more money to reach your goal!\n";
    while( 'y' == prompt(
      {},
      {"Would you like me to help you with a savings plan? (y/n)"},
      { 'y', 'n' }
    )) {
       char menu = prompt(
         {
           "Please select from the following options: \n",
           "1 - Daily Saving Plan\n",
           "2 - Weekly Saving Plan\n",
           "3 - Monthly Saving Plan\n"
         },
         {"Enter the number associated with your choice: \n"},
         { '1', '2', '3' }
       );
       switch (menu) {
         case '1': {
           saving( "Daily Saving Plan", "day", "days", have-want);
         } break;
         case '2: {
           saving( "Weekly Saving Plan", "week", "weeks", have-want);
         } break;
         case '3': {
           saving( "Monthly Saving Plan", "month", "months", have-want);
         } break;
       }

    }
  }
}