如何从循环中写入多个输出

我用这个程序来计算对应字母等级的等级，并让它循环用户想要输出的次数。但是，由于某些原因，它只将最后一个已知的输出写入其文本文件。有人能告诉我我在这里做错了什么吗？

#include <fstream>
#include <iostream>
#include <iomanip>
#include <string>
#include <cstdlib>
int weighted1 = 0;
int weighted2 = 0;
int weighted3 = 0;
int weighted4 = 0;
int weighted_average = 0;
const int MAX = 20;
int flag = 0;
int choice;
double sum = 0;
double average = 0;
char name [10];
char letter;
char file [MAX];
int num = 0;
int count = 0;
int main( )
{
//Initiate input/output stream
std::ifstream in_stream;
std::ofstream out_stream;
in_stream.open("grade.txt");
out_stream.open("finalgrade.dat");
double first, second, third, fourth;
in_stream >> first >> second >> third >> fourth >> name;
//std::cout >> " 1: " >> first >> " 2: " >> second >> 
double grade = 0.0;
grade = (first + second + third + fourth)/4;
//Gives user the choice of reading student records from keyboard or file
bool menu = true;
while (menu != false)
{
      std::cout << "Would you like to open as keyboard or file?" << 'n';
std::cout << "1. keyboard" << 'n';
std::cout << "2. file" << 'n';
std::cin >> choice;
switch (choice)
{
//Enter the number students the grades will enter
case 1:
std::cout << "How many students? ";
std::cin >> num;
for(count =0; count < num; count++) 
{
    {
    std::cout << "Student's Name: ";
    std::cin >> name;
    }
do
{
flag = 0;
std::cout << "Please input your first exam grade and press enter: n";
    std::cin >> first;
    if ((first < 0) || (first > 100))
    {
        std::cout << "You've entered invalid data!" << 'n';
        flag = 1;
    }
}while (flag == 1);
do
{
flag = 0;
std::cout << "Please input your second exam grade and press enter: n";
    std::cin >> second;
    if ((second < 0) || (second > 100))
    {
        std::cout << "You've entered invalid data!" << 'n';
        flag = 1;
    }
}while (flag == 1);
do
{
flag = 0;
std::cout << "Please input your third exam grade and press enter: n";
    std::cin >> third;
    if ((third < 0) || (third > 100))
    {
        std::cout << "You've entered invalid data!" << 'n';
        flag = 1;
    }
}while (flag == 1);
do
{
flag = 0;
std::cout << "Please input your final exam grade and press enter: n";
    std::cin >> fourth;
    if ((fourth < 0) || (fourth > 100))
    {
        std::cout << "You've entered invalid data!" << 'n';
        flag = 1;
    }
}while (flag == 1);
//Formulas that calculate student average
grade = (first + second + third + fourth)/4;
sum = first + second + third + fourth;
average = sum/4;
//Letter grade and it's weighted averages
 letter = 'A';
 letter = 'B';
 letter = 'C';
 letter = 'D';
 letter = 'F';
 if(grade >= 90)
 { 
 letter = ('A');
 std::cout<<letter<<'n';
 }
 else if(grade >= 80)
 {
 letter = ('B');
 std::cout<<letter<<'n';
 }
 else if(grade >= 70)
 {
 letter = ('C');
 std::cout<<letter<<'n';
 }
 else if(grade >= 60)
 {
 letter = ('D');
 std::cout<<letter<<'n';
 }
 else if (grade < 60)
 {
 letter = ('F');
 std::cout<<letter<<'n';
 }
 weighted1 = (first  * .20);
 weighted2 = (second * .20);
 weighted3 = (third  * .20);
 weighted4 = (fourth * .40);
 weighted_average = (weighted1 + weighted2 + weighted3 + weighted4);
 //Output
 std::cout << "Exam Grades: " <<  first << "," << second << "," << third << "," << fourth << 'n';
 std::cout << "This is the average for " << name << ": " << weighted_average << 'n';
 std::cout << "This is the letter grade: " << letter << 'n';
  }
  {
//Writing the grade into grades.txt
  for(count =0; count < num; count++)   
  {
    std::ofstream myfile;
    myfile.open ("grades.txt");
    myfile << "Writing this to a file: ";
    myfile << name << ' ';
    myfile << weighted_average << ' ';
    myfile << letter << 'n';
    myfile << "****";
    myfile.close();
  }
  break;
  }
//Here we open "grade.txt" to output grade to screen
case 2:
in_stream.open("grade.txt");
out_stream.open("finalgrade.dat");
 letter = 'A';
 letter = 'B';
 letter = 'C';
 letter = 'D';
 letter = 'F';
 if(grade >= 90)
 letter = ('A');
 else if(grade >= 80)
 letter = ('B');
 else if(grade >= 70)
 letter = ('C');
 else if(grade >= 60)
 letter = ('D');
 else if (grade < 60)
 letter = ('F');
 weighted1 = (first  * .20);
 weighted2 = (second * .20);
 weighted3 = (third  * .20);
 weighted4 = (fourth * .40);
 weighted_average = (weighted1 + weighted2 + weighted3 + weighted4);
std::cout << "Enter file name: ";
     std::cin >> file;
if(file != "grade.txt")
{
     std::cout << std::fixed << "The average grade for: " << name << 'n';
     std::cout << "average in grade.txt is: "<< weighted_average << std::setprecision(2) << 'n';
     std::cout << "and the letter grade is: " << letter << 'n'; 
}
else
{
    return 0;
}
in_stream.close();
out_stream.close();
}
return 0;
}
}