How to find the lowest average score in a structure file

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C++

I need to find the lowest average score among all the students in the file.
I was looking for a grade point average, but this is a search for the last student whose was entered at file, how do I look for a grade point average among all students?

What I have tried:

My code:

#include <iostream>
#include <iomanip>
#include <fstream>
#include <string>
#include <Windows.h>

using namespace std;

enum Specialty { KOMP_NAYK, INFORMATUKA, MATEM_EKONOM, FIZ_MATEM, TRUD_NAVCH };

string specialtyStr[] = { "KOMP_NAYK", "INFORMATUKA", "MATEM_EKONOM", "FIZ_MATEM", "TRUD_NAVCH " };

struct Student
{
	char surname[64];
	unsigned course;
	Specialty specialty;
	unsigned physics;
	unsigned maths;
	union
	{
		unsigned programming;
		unsigned ch_methods;
		unsigned pedagogy;
	};
};

void CreateBIN(const char* filename);
void PrintBIN(const char* filename);
double LowMark(const char* filename);

int main()
{                             

	char filename[100] = "";

	int menuItem;
	do {
		cout << endl;
		cout << " Choose an action: " << endl << endl;
		cout << " [1] - data entry from the keyboard " << endl;
		cout << " [2] - output to the screen " << endl;
		cout << " [0] - exit and end of the program " << endl << endl;
		cout << " Enter a value: "; 
		cin >> menuItem;
		cout << endl;

		switch (menuItem)
		{
		case 1:
			if (strcmp(filename, "") == 0) {
				cout << "File name: ";
				cin >> filename;
			}
			CreateBIN(filename);
			break;
		case 2:
			PrintBIN(filename);
			cout << " The lowest average score: " << LowMark(filename) << endl;
			break;
		case 0:
			break;
		default:
			cout << " You entered an incorrect value! "
				" You must enter the number - the number of the selected menu item " << endl;
		}
	} while (menuItem != 0);

	return 0;
}

void CreateBIN(const char* filename)
{
	ofstream f(filename, ios::binary); 
	char ch; 
	int n = 1;
	int specialty;
	Student student;

	do {
		cout << " Student № " << n << ":" << endl;
		cin.get();
		cin.sync();
		cout << " Surname: ";
		cin >> student.surname;
		cout << " Course: ";
		cin >> student.course;
		cout << " Specialty (0 - KOMP_NAYK, 1 - INFORMATUKA, 2 - MATEM_EKONOM, 3 - FIZ_MATEM, 4 - TRUD_NAVCH): ";
		cin >> specialty;
		student.specialty = (Specialty)specialty;
		cout << " mark physics: ";
		cin >> student.physics;
		cout << " mark mathematics: ";
		cin >> student.maths;

		switch (student.specialty)
		{
		case KOMP_NAYK:
			cout << " mark programming: ";
			cin >> student.programming;
			break;
		case INFORMATUKA:
			cout << " mark numerical method: ";
			cin >> student.ch_methods;
			break;
		case MATEM_EKONOM:
		case FIZ_MATEM:
		case TRUD_NAVCH:
			cout << " mark pedagogy: ";
			cin >> student.pedagogy;
			break;
		}
		cout << endl;
		f.write((char*)&student, sizeof(Student));
		n++;
		cout << "Add more? (y/n): ";
		cin >> ch;
		cout << endl;
	}

	while (ch == 'y' || ch == 'Y');
	cout << endl;
	f.close();
}

void PrintBIN(const char* filename)
{
	ifstream f(filename, ios::binary);

	cout << "=================================================================================================================================" << endl;
	cout << " |  №  |    Surname    | Course |      Specialty      | Physics | Maths | Programming | Num_Methods | Pedagogy |" << endl;
	cout << "---------------------------------------------------------------------------------------------------------------------------------" << endl;

	Student student;
	int i = 1;

	while (f.read((char*)&student, sizeof(Student))) {
		cout << " | " << setw(2) << right << i << " ";
		cout << " | " << setw(14) << left << student.surname;
		cout << " |  " << setw(2) << right << student.course << " ";
		cout << " | " << setw(23) << left << specialtyStr[student.specialty];
		cout << " | " << setw(4) << right << student.physics << "  ";
		cout << " | " << setw(6) << right << student.maths << "    ";

		switch (student.specialty)
		{
		case KOMP_NAYK:
			cout << " | " << setw(7) << right << student.programming << "     " << "  | " << setw(18) << right << " | " << setw(12) << right << "|" << endl;
			break;
		case INFORMATUKA:
			cout << " | " << setw(16) << right << "| " << setw(8) << right << student.ch_methods << "      " << "  | " << setw(12) << right << "|" << endl;
			break;
		case MATEM_EKONOM:
		case FIZ_MATEM:
		case TRUD_NAVCH:
			cout << " | " << setw(16) << right << "| " << setw(18) << right << "  | " << setw(6) << right << student.pedagogy << "     " << "|" << endl;
			break;
		}
		i++;
	}
	cout << "=================================================================================================================================" << endl;
	cout << endl;
}

double LowMark(const char* filename)
{
	ifstream f(filename, ios::binary);

	Student student;
	double low;
	double low_mark;

while (f.read((char*)&student, sizeof(Student)))
	{
		switch (student.specialty)
		{
		case KOMP_NAYK:
			low = (student.physics + student.maths + student.programming) / 3.0;
			break;
		case INFORMATUKA:
			low = (student.physics + student.maths + student.ch_methods) / 3.0;
			break;
		case MATEM_EKONOM:
		case FIZ_MATEM:
		case TRUD_NAVCH:
			low = (student.physics + student.maths + student.pedagogy) / 3.0;
			break;
		}
	}
	while (f.read((char*)&student, sizeof(Student)))
	{
		switch (student.specialty)
		{
		case KOMP_NAYK:
			low_mark = (student.physics + student.maths + student.programming) / 3.0;
			break;
		case INFORMATUKA:
			low_mark = (student.physics + student.maths + student.ch_methods) / 3.0;
			break;
		case MATEM_EKONOM:
		case FIZ_MATEM:
		case TRUD_NAVCH:
			low_mark = (student.physics + student.maths + student.pedagogy) / 3.0;
			break;
		}
		if (low_mark < low)
		{
			low = low_mark;
		}
	}

	f.close();

	return low;
}

UPDATE:

double LowMark(const char* filename)
{
	ifstream f(filename, ios::binary);

	Student student;
	double low = 5;
	double low_mark = 5;

	while (f.read((char*)&student, sizeof(Student)))
	{
		switch (student.specialty)
		{
		case KOMP_NAYK:
			low_mark = (student.physics + student.maths + student.programming) / 3.0;
			break;
		case INFORMATUKA:
			low_mark = (student.physics + student.maths + student.ch_methods) / 3.0;
			break;
		case MATEM_EKONOM:
		case FIZ_MATEM:
		case TRUD_NAVCH:
			low_mark = (student.physics + student.maths + student.pedagogy) / 3.0;
			break;
		}
		if (low_mark < low)
		{
			low = low_mark;
		}
	}
		
	f.close();
	return low;
}