Switch语句问题

我做了一些修改，现在我的switch语句出现了问题。这些函数应该读取随机访问的文件。当我运行程序时，它会显示菜单，当我选择一个选项时，它只输出我选择的第一个选项。如果我选择另一个选项，它不会输出字母。我该怎么解决这个问题？

//Random-Access Files Program
//This program reads a file from beg to end, end to beg, beg to 4th position,
//8th to 15th position, end to 21st position, and 22nd position to the end
#include <iostream>
#include <fstream>
using namespace std;
void begToEnd(fstream &, char);
void endToBeg(fstream &, char);
void begTo4th(fstream &, char);
void eighthTo15th(fstream &, char);
void endTo21st(fstream &, char);
void Twenty2ndToEnd(fstream &, char);
int main()
{
    char letters;   //holds the character
    int choice;
    fstream file("alphabet.txt", ios::in | ios::binary);  //This opens the file
    do
    {
         cout << "Enter 1 to read from beginning to end" << endl
         << "Enter 2 to read from end to beginning" << endl
         << "Enter 3 to read from beginning to 4th position" << endl
         << "Enter 4 to read from 8th to 15th position" << endl
         << "Enter 5 to read from end to 21st position" << endl
         << "Enter 6 to read from 22nd position to the end" << endl;
         cin >> choice;
         switch(choice)
         {
         case 1:
              begToEnd(file, letters);
              break;
         case 2:
              endToBeg(file, letters);
              break;
         case 3:
              begTo4th(file, letters);
              break;
         case 4:
              eighthTo15th(file, letters);
              break;
         case 5:
              endTo21st(file, letters);
              break;
         case 6:
              Twenty2ndToEnd(file, letters);
              break;
         }
         cout << endl;
         system("pause");
         file.close(); //Closes the file
}
while(choice != 'N' && choice != 'n');
return 0;    
}
void begToEnd(fstream &in, char letter)
{
     in.seekg(0L, ios::beg);  //Displays beginning to end
     for(int i = 0; i < 25; i++)
     {
          in.get(letter);
          cout <<"Beginning to end: " << letter << endl;
     }
}
void endToBeg(fstream &in, char letter)
{
     in.seekg(0, ios::end);  //Displays end to beginning
     int size = in.tellg();
     for (int i=1; i <= size; i++)
     {
         in.seekg(-i, ios::end);
         letter=in.get();
         cout << "End to beginning: " << letter << endl;
     }
}
void begTo4th(fstream &in, char letter)
{
     in.seekg(0L, ios::beg);  //Displays beginning to 4th position
     for(int i = 0; i < 4; i++)
     {
         in.get(letter);
         cout << "Beginning to the 4th letter: " << letter << endl;
     }
}
void eighthTo15th(fstream &in, char letter)
{
     in.seekg(7L, ios::beg);  //Displays 8th to 15th position
     for(int i = 0; i < 7; i++)
     {
     in.get(letter);
     cout << "8th to 15th letter: " << letter << endl;
     }
}
void endTo21st(fstream &in, char letter)
{
     in.seekg(0, ios::end);  //Displays end to 21st position
     int size = in.tellg();
     for (int i=1; i <= 5; i++)
     {
         in.seekg(-i, ios::end);
         letter=in.get();
         cout << "End to 21st: " << letter << endl;
     }
}
void Twenty2ndToEnd(fstream &in, char letter)
{    
     in.seekg(21L, ios::beg);  //Displays the 22nd position to the end
     for(int i = 0; i < 5; i++)
     {
         in.get(letter);
         cout << "22nd to end: " << letter << endl;
     }
}