输入验证和退出循环，创建函数

如何解决输入验证和退出循环，创建函数

我正在尝试创建一个用于输入验证的循环，如果用户要退出，则尝试创建另一个循环。还可以创建四个功能，一个用于菜单，一个用于三个区域计算中的每一个。

我应该在哪里实现更新的代码？我应该使用新变量还是利用旧变量。谢谢你

到目前为止，这是我的代码：

#include <iostream>
#include <cmath>    
using namespace std;
int main()
{
    const float PI = 3.14159;
    int user_choice;
    // main screen prompting for choice
    cout << "\nGeometry Calculator\n"
        << "   1. Calculate the Area of a Circle\n"
        << "   2. Calculate the Area of a Rectangle\n"
        << "   3. Calculate the Area of a Triangle\n"
        << "   4. Quit\n"
        << "\nEnter you choice (1-4): ";
    cin >> user_choice;
    cout << endl;

    switch (user_choice)
    {
        float area;
        // if user chooses 1
    case 1:
        int radius;
        // prompt user for radius
        cout << "What is the radius: ";
        cin >> radius;
        // if user enters radius as negative number
        if (radius < 0)
        {
            cout << "\nThe radius must be a positive number.\n"
                << "Try again.\n"
                << endl;
        }
        else
            // calculate area
        {
            area = PI * pow(radius,2);

            cout << "The area of the circle is  "
                << area << endl;
        }
        break;
        // if user chooses 2
    case 2:
        float width,length;
        // prompt user for height and width
        cout << "What is the length? " << endl;
        cin >> length;

        if (length > 0)
        {
            cout << "What is the width? " << endl;
            cin >> width;
            // calculate and display area
            if (width > 0) {
                area = length * width;
                cout << "The area of rectangle is "
                    << area
                    << endl;
            }
            // if user enters a height or width less than 0
            else {
                cout << "\nWidth must be a positive number"
                    << endl;
                cout << "Try again." << endl;
            }

        }
        else
        {
            cout << "\nLength must be a positive number" << endl;
            cout << "Try again." << endl;
        }
        break;
        // if user chooses 3
    case 3:
        float height,base;
        // prompt user for base and height
        cout << "What is the base? ";
        cin >> base;

        if (base > 0)
        {
            cout << "What is the height? ";
            cin >> height;
            // calculate and display area
            if (height > 0)
            {
                area = (base * height) * .5;
                cout << "Area of triangle is "
                    << area
                    << endl;
            }
            // if user inputs a height or base less than 0
            else
            {
                cout << "\nHeight must be a positive number\n"
                    << "Try again."
                    << endl;
            }

        }
        else
        {
            cout << "\nBase length must be a positive number\n"
                << "Try again." << endl;
        }
        break;
        // if user chooses 4
    case 4:
        cout << "You have chose to quit the program. Good-bye." << endl;
        break;
        // if user enters input other than 1-4
    default:
        cout << "\nYour choice must be between 1 and 4.\n"
            << "Try again."
            << endl;
        break;
    }

解决方法

如果我了解您想要的内容，则应创建一个while(true)循环，其中包括用户的主要选择（主菜单）。我会做这样的事情：

void calculateCircleArea() {
    // Circle area calculation
}

void calculateRectangleArea() {
    // Rectangle area calculation
}

void calculateTriangleArea() {
    // Triangle area calculation
}

int main()
{
    // other main stuff

    int user_choice=0;
    while (user_choice!=4)
    {
        cout << "\nGeometry Calculator\n"
        << "   1. Calculate the Area of a Circle\n"
        << "   2. Calculate the Area of a Rectangle\n"
        << "   3. Calculate the Area of a Triangle\n"
        << "   4. Quit\n"
        << "\nEnter you choice (1-4): ";
        cin >> user_choice;

        if (user_choice<1 || user_choice>4)    // example to inform the user of a bad input
        {
            cout << "Wrong input\n";
            cin.clear();
            cin.ignore(INTMAX_MAX,'\n');
            // Ignoring and clearing all the input until the delimiter character ('\n')
            continue; // it is unnecessary,but this keyword will skip the cycle in which it is to its next iteration
        }
        
        switch(user_choice)
        {
            case 1:
            calculateCircleArea();
            break;
            case 2:
            calculateRectangleArea();
            break;
            case 3:
            calculateTriangleArea();
            break;
            case 4:
            // nothing,because the value of the variable user_choice is 4,so the loop will end
            break;
            default:
            // default stuff
            break;
        }    // switch
    }    // while

}    // main

如您所见，我包含了三个函数，每个计算一个。我建议您尝试更多地使用函数，这将使您的代码更具可读性和简洁性，而它们又由于您将要学习的其他原因而有用。

编辑：按照建议，我没有想到要输入字符，因此只要输入错误，输入缓冲区就会被清除，并且它将忽略最后插入的行，以防止意外行为。