c++ - 为什么我的输入检查 if else 语句不起作用？

Question

我是一名 C++ 初学者，目前正在开发一个简单的计算器项目。我希望输入只是数字，所以我做了一个 if else 语句来检查输入是否只是数字，如果是，则进行计算并返回到“输入运算符”bla bla。请忽略情况 2 3 4，因为我想让输入检查器正常工作。我做错了什么以及如何解决这个问题？先感谢您 !但由于某种原因，它就是行不通。

# include <iostream>

using namespace std;

int main() {
    bool whileRunning = true;
    while(whileRunning){
        char op;
        float num1, num2;
        cout << "\n Enter operator either 1 (+) or 2 (-) or 3 (*) or 4 (/) or 5 to exit: ";
        cin >> op;
       
        switch (op)
        {
        case '1':     
            cout << "Enter two operands: ";
            cin >> num1 >> num2;
            if (isdigit(num1) == 0 && isdigit(num2) == 0) {
                cout << num1 + num2;
            }
            else
                cout << " Please only input numbers";
                break;
            break;
        case '2':
            cout << "Enter two operands: ";
            cin >> num1 >> num2;
            if (isdigit(num1) && isdigit(num2)) {
                cout << num1 - num2;
            }
            else
                break;

            break;

        case '3':
            cout << "Enter two operands: ";
            cin >> num1 >> num2;
            if (isdigit(num1) && isdigit(num2)) {
                cout << num1 * num2;
            }
            else
                break;
            break;

        case '4':
            cout << "Enter two operands: ";
            cin >> num1 >> num2;
            if (isdigit(num1) && isdigit(num2)) {
                if (num2 == 0) {
                    cout << "You tried to divide by 0, Try again!";
                    break;
                }
                else
                    cout << num1 / num2;
            }
            else
                break;
            

            break;
        case '5':
            whileRunning = false;
            break;

        default:
            // If the operator is other than +, -, * or /, error message is shown
            cout << "Error! Your input must be >= 1 and  <= 5. Please try again";
            break;

        }
    }
 

    return 0;
};

Answer 1

您在 isDigit 中进行了错误的检查和isDigit 不能与 float 一起使用。

正如C++引用 所说，返回值为:

A value different from zero (i.e., true) if indeed c is a decimaldigit. Zero (i.e., false) otherwise.

所以你需要检查 isDigit 是否函数返回一个不同于 0 的值。正确的代码是这样的:

# include <iostream>

using namespace std;

int main() {
    bool whileRunning = true;
    while(whileRunning){
        char op;
        int num1, num2;
        cout << "\n Enter operator either 1 (+) or 2 (-) or 3 (*) or 4 (/) or 5 to exit: ";
        cin >> op;
       
        switch (op)
        {
        case '1':     
            cout << "Enter two operands: ";
            cin >> num1 >> num2;
            if (isdigit(num1) != 0 && isdigit(num2) != 0) {
                cout << num1 + num2;
            }
            else
                cout << " Please only input numbers";
                break;
        
        ....
         
        default:
            // If the operator is other than 1, 2, 3, 4, 5 error message is shown
            cout << "Error! Your input must be >= 1 and  <= 5. Please try again";
            break;

        }
    }
 

    return 0;
};

在您的代码中，您测试相反的代码。

您可以在 isDigit 上找到更多阅读内容在此功能 site .

对于 float ，您可以使用其他方法:

if(scanf("%f", &userNum) == 1) {
    // Handle item float here
}

所以你的代码看起来像:

# include <iostream>

using namespace std;

int main() {
    bool whileRunning = true;
    while(whileRunning){
        char op;
        float num1, num2;
        cout << "\n Enter operator either 1 (+) or 2 (-) or 3 (*) or 4 (/) or 5 to exit: ";
        cin >> op;
       
        switch (op)
        {
        case '1':     
            cout << "Enter two operands: ";
            if(scanf("%f %f", &num1, &num2) == 2){ {
                cout << num1 + num2;
            }
            else
                cout << " Please only input numbers";
                break;
        
        ....
         
        default:
            // If the operator is other than 1, 2, 3, 4, 5 error message is shown
            cout << "Error! Your input must be >= 1 and  <= 5. Please try again";
            break;

        }
    }
 

    return 0;
};

您可以找到scanf引用here .