if语句判断字符问题

#include<iostream>
#include<string>
using namespace std;
int main(){
    int i; //控制循环变量
    int reNum[1];
    int AnswerNum;//测验题数 *int
    string Answer;//测验题数  *string
    string strNum[100];//测验题答案
    int isNumber(string str);
    cout << "                 **************测试批改系统**************" <<endl;
    cout << "请输入本次测验总题目数：" <<endl;
    tab1:
    cin >> Answer;
    reNum[1] = isNumber(Answer);
    if (!reNum[1]){
    cout << "请确保正确的有效数字！\n"
         <<"请再次输入本次测试总题目数："<<endl;
    goto tab1;}
    AnswerNum = atoi(Answer.c_str());
    cout << "请输入每一题的答案、按回车键确认。" <<endl;
    for(i = 0;i != AnswerNum;i++){
          cout << "第" <<i+1 <<"题：";
          while (cin >> strNum[i])
          {if(strNum[i][0] == '\n')
              {continue;}
          else
              {break;}
          }
    }
    cout<<"您已经输入完毕，待系统检测完毕...\n\n"<<endl;
    cout<< "                Download...\n\n\n"<<endl;
    for(i = 0;i != AnswerNum;i++){
        cout << "第" <<i+1 <<"题："<< strNum[i] <<"     ";
          }
    cout<<endl;
    cout<< " 系统已经录入您输入的答案，请检测。" << endl;
    cout<<"并且发现第";
    for(i = 0;i != AnswerNum;i++){        
          if(strNum[i].length() > 1){
                 cout<< i+1<<" ";
                 continue;
          }else if(strNum[i][0] == 'A' || strNum[i][0] == 'a'){
                    cout<< i+1<<" ";
                    continue;
           }else if(strNum[i][0] == 'B' || strNum[i][0] == 'b'){
                 cout<< i+1<<" ";
                 continue;
            }else if(strNum[i][0] == 'C' || strNum[i][0] == 'c'){
                  cout<< i+1<<" ";
                  continue;
             }else if(strNum[i][0] == 'D' || strNum[i][0] == 'd'){
                  cout<< i+1<<" ";
                  continue;
                   }
                 
                }
    cout<< "题有明显错误！ "<<endl;
    cout<<endl;
    system("pause");
}
int isNumber(string str){  
    int result[3];
    int i = 0;
    int havePoint = 0;
    int haveReduce = 0;
    int isNum = 1;
    for(i ; i != (str.length()); i++){
        
        
          if(isdigit(str[i])){
                     
          }else{
                isNum = 0;   
          }
}
    result[0] = isNum;   
    return result[0];
}    这个代码简单判断选择题的答案、先输入。再次判断输入的答案是不是等于ABDC时候出错、求怎么解决中间那段判断输入值是不是等于ABCD。