C语言中如何判断变量是否等于多个值？CS50初学者求助

原代码的核心错误解析 #

第一版代码问题 #

strcmp是C标准库中用于比较两个字符串的函数，仅接受两个字符串参数，你错误地将多个比较逻辑通过||塞进了strcmp的参数列表里，导致编译器提示参数数量超标。另外，else if块里还写错了判断条件（误将同意选项Yes/Y加入了拒绝判断）。

第二版代码问题 #

1. 没有定义过名为str的标识符，你可能混淆了字符串函数的调用形式；
2. 用单引号'包裹字符串（如'yes'）是错误的——C语言中，单引号用于表示单个字符，字符串必须用双引号"；
3. 变量名拼写错误：usename应为username。

修正后的可运行代码 #

#include <stdio.h>
#include <cs50.h>
#include <string.h>

int main(void)
{
    string username = get_string("What would you like your username to be? ");
    string agreement = get_string("Do you agree to the following terms & conditions? ");
    
    // 对每个同意选项单独调用strcmp，用||连接逻辑判断
    if (!strcmp(agreement, "yes") || !strcmp(agreement, "y") || !strcmp(agreement, "Yes") || !strcmp(agreement, "Y"))
    {
        printf("You can now continue using the service, %s! \n", username);
    }
    // 同理处理拒绝选项，修正之前的条件错误
    else if (!strcmp(agreement, "no") || !strcmp(agreement, "n") || !strcmp(agreement, "No") || !strcmp(agreement, "N"))
    {
        printf("You need to agree to the terms & conditions to use the service\n");
    }
    else
    {
        printf("You need to select an option\n");
    }
}

简化优化方案（可选） #

如果不想重复判断大小写，可以先把输入字符串转成小写（或大写），减少重复判断：

#include <stdio.h>
#include <cs50.h>
#include <string.h>
#include <ctype.h>

// 自定义函数：将字符串转为全小写
void str_to_lower(char *str)
{
    for (int i = 0; str[i] != '\0'; i++)
    {
        str[i] = tolower((unsigned char)str[i]);
    }
}

int main(void)
{
    string username = get_string("What would you like your username to be? ");
    string agreement = get_string("Do you agree to the following terms & conditions? ");
    
    str_to_lower(agreement);
    // 只需判断小写形式即可覆盖所有大小写输入
    if (!strcmp(agreement, "yes") || !strcmp(agreement, "y"))
    {
        printf("You can now continue using the service, %s! \n", username);
    }
    else if (!strcmp(agreement, "no") || !strcmp(agreement, "n"))
    {
        printf("You need to agree to the terms & conditions to use the service\n");
    }
    else
    {
        printf("You need to select an option\n");
    }
}

内容的提问来源于stack exchange，提问作者Korabii