c语言代码解决问题

解题思路:

注意事项:

参考代码:

#include<stdio.h>

#include<string.h>

#include<ctype.h>

int main()

{

    char str[1001];

    gets(str);

    str[strlen(str)] = '\0';

    int i,flag=1;

    if (isdigit(str[0]) == 0)

    {

        for (i = 0; i < strlen(str); i++)

        {

            if (!((str[i] >= 'A' && str[i] <= 'Z') || (str[i] >= 'a' && str[i] <= 'z') || (str[i] == '_') || (str[i] >= '0' && str[i] <= '9')))

            {

                flag = 0;

                break;

            }

        }

    }

    else

        flag = 0;

    if (flag)

    {

        printf("yes");

    }

    else

        printf("no");

    return 0;

}