无论我输入什么字符，我的程序都会循环

Question

我一直在用 C++ 编写一个非常基本的计算程序。它计算一个数字的平方根，如果用户愿意，它也可以平方。这是我到目前为止所拥有的（我知道这可能是垃圾代码，但我是一个初学者，只是在尝试看看它是如何工作的。任何建议都非常感谢）：

#include <iostream>
#include <stdlib.h>
#include <math.h>

using namespace std;

int number;         // Global variables to be used in void functions as well as main.
int squaredNumber;
double sqrtResult;
char input;
char useAgain;

void squareNum();       // Prototypes for the void functions
void sqrtNum();
void useAgainQuery();

int main()
{
retry:                      // Establishing somewhere to send user if their input is invalid.
    system("cls");
    cout << "Square Calcualtions" << endl;
    cout << "******************" << endl;
    cout << endl;
    cout << "Do you want to square a number or find the square root of a number?" << endl;
    cout << "Please select 1 or 2 respectively." << endl;
    cout << endl;
    cin >> input;
    if (input == '1')
    {
        cout << "Please press ENTER to continue." << endl;
        cin.ignore().get();
        squareNum();                // If the input is 1, run the void to square a number.
    }
    else if (input == '2')
    {
        cout << "Please press ENTER to continue." << endl;
        cin.ignore().get();
        sqrtNum();                  // If the input is 2, run the void to sqrt a number.
    }
    else if (input != '1' || '2')
    {
        system("cls");
        cout << "Square Calcualtions" << endl;
        cout << "******************" << endl;
        cout << endl;
        cout << "Your selection was invalid, please enter 1 or 2." << endl;
        cin.ignore().get();
        goto retry;             // If the input isn't either 1 or 2, send back to the start of program.
    }
    return 0;
}

void squareNum()  // function to square the inputted number.
{
    system("cls");
    cout << "Square Calcualtions" << endl;
    cout << "******************" << endl;
    cout << endl;
    cout << "Enter the number you want to square." << endl;
    cin >> number;
    cout << "You have chosen: " << number << endl;
    cout << "Press ENTER to calculate." << endl;
    cin.ignore().get();
    system("cls");
    squaredNumber = number * number;        // Simple maths to find the square number
    cout << "You have squared " << number << "." << endl;
    cout << "The result was " << squaredNumber << "." << endl;
    cout << "Press ENTER to continue." << endl;
    cin.get();
    useAgainQuery();
    return;
}

void sqrtNum()
{
    system("cls");
    cout << "Square Calcualtions" << endl;
    cout << "******************" << endl;
    cout << endl;
    cout << "Enter the number you would like the square root of." << endl;
    cin >> number;
    cout << "You have chosen: " << number << "." << endl;
    cout << "Press ENTER to calculate." << endl;
    cin.ignore().get();
    system("cls");
    sqrtResult = sqrt(number);
    cout << "You have found the square root of " << number << "." << endl;
    cout << "The result was: " << sqrtResult << "." << endl;
    cout << "Press ENTER to continue." << endl;
    cin.get();
    useAgainQuery();
    return;
}

void useAgainQuery()
{
    system("cls");
    cout << "Square Calcualtions" << endl;
    cout << "******************" << endl;
    cout << endl;
    cout << "Would you like to make another calculation?" << endl;
    cout << "Y for Yes and N for No." << endl;
    cout << endl;
    cin >> useAgain;
    if (useAgain == 'Y' || 'y')
    {
    retry2:                     // Establishing somewhere to send user if their input is invalid.
        system("cls");
        cout << "Square Calcualtions" << endl;
        cout << "******************" << endl;
        cout << endl;
        cout << "Do you want to square a number or find the square root of a number?" << endl;
        cout << "Please select 1 or 2 respectively." << endl;
        cout << endl;
        cin >> input;
        if (input == '1')
        {
            cout << "Please press ENTER to continue." << endl;
            cin.ignore().get();
            squareNum();                // If the input is 1, run the void to square a number.
        }
        else if (input == '2')
        {
            cout << "Please press ENTER to continue." << endl;
            cin.ignore().get();
            sqrtNum();                  // If the input is 2, run the void to sqrt a number.
        }
        else if (input != '1' || '2')
        {
            system("cls");
            cout << "Square Calcualtions" << endl;
            cout << "******************" << endl;
            cout << endl;
            cout << "Your selection was invalid, please enter 1 or 2." << endl;
            cin.ignore().get();
            goto retry2;
        }
    }
    else if (useAgain != 'Y' || 'y')
        return;
    return;
}  

所以是的，当我通过它并询问“你想再玩一次吗”时，它会一遍又一遍地通过它。我按什么键并不重要，但它会循环。任何帮助将不胜感激！

Answer 1

在此处更改您的条件：

    if (useAgain == 'Y' || 'y')

到

    if (useAgain == 'Y' || useAgain=='y')

另外，改变这个：

 else if (useAgain != 'Y' || 'y')
{
    return;
}

到这里：

else if (useAgain != 'Y' && useAgain!='y')
{
    return;
}

Answer 2

也许尝试创建一个控制整个主循环的布尔变量，如下所示：

#include <InsertLibrariesHere>
int main(){
    bool running=true;
    while(running){
        //calculations here
        //continue(Y/N)?
        if (input == N || input == n){running = false;}
    }
}

Answer 3

1. 不要使用 goto，而是使用 while(1)。

2. 你的最后一个 if 语句是错误的，它应该是

   if (input != '1' && input != '2')

Answer 4

您可以简化与字母的比较：

if (std::toupper(useAgain) == 'Y')

或

if (std::tolower(useAgain) == 'y')

输入后也可以转换大小写：

cin >> useAgain;
useAgain = std::toupper(useAgain);
if (useAgain == 'Y')

还有std::transform 是您需要将std::string 转换为全部小写或全部大写。