C++使用toupper后输入校验循环仍不接受小写字符问题

错误原因 #

你目前的toupper使用逻辑完全错误：

• 现有代码中toupper('A')是对常量字符'A'做大写转换，转换结果还是'A'本身，完全没有处理用户输入的custPlanType，因此小写输入不可能通过校验
• 你之前尝试的toupper(custPlanType == 'A')是先执行custPlanType == 'A'的布尔判断，再对布尔结果做大写转换，逻辑完全不符合需求

修改步骤 #

1. 新增引入<cctype>头文件，toupper函数定义在该头文件中，缺失头文件属于非标准写法，可能在部分编译器下出现未定义行为
2. 在校验前先对用户输入的custPlanType做大写转换，再和大写的A/B/C/U做比较即可

修改后的完整代码 #

#include <iostream>
// 新增toupper对应的头文件
#include <cctype>

using namespace std;

int main()
{
    bool custGBTypeValid = false;
    bool custPlnTypeValid = false;
    char custPlanType = toupper('Z');
    int custUsedData = 1;

    cout << "Hello, welcome to AT&T wireless. We're here to help you decide if your current plan is what's right for you." << endl;
    cout << "Here are our plans:" << endl;
    cout << "Plan A: For $25 per month 0GB are provided. Data is $10 per GB." << endl;
    cout << "Plan B: For $45 per month 2GB are provided." << endl;
    cout << "Plan C: For $80 per month 6GB are provided." << endl;
    cout << "Plan Unlimited: Unlimited data for $100 per month." << endl;

    while (custPlnTypeValid == false)
    {
        cout << "What type of plan are you on? (Please answer with A, B, C, or U): ";
        cin >> custPlanType;
        // 仅修改判断条件：对用户输入的字符转大写后再比较
        if (toupper(custPlanType) == 'A' 
            || toupper(custPlanType) == 'B' 
            || toupper(custPlanType) == 'C' 
            || toupper(custPlanType) == 'U')
            custPlnTypeValid = true;
        else
            cout << "ERROR: Incorrect data type entered." << endl;
    }
}

补充说明：如果需要更严谨的写法，可以把toupper(custPlanType)替换为toupper(static_cast<unsigned char>(custPlanType))，避免char为有符号类型时，负值传入toupper出现未定义行为，初学者直接写简化版本也可以正常运行。

内容的提问来源于stack exchange，提问作者Samantha Maxam