C++ | An array, switch statement? Need some guidance - c++

Question

My objective here is to take (3) students, give each of them (3) grades, then the average of those grades for each student along with the letter grade. This part works well. The final part I need is taking those (3) average grades, adding them, then taking another average which will be the class grade average.
How should I go about getting the class average? Array? Switch statement? The reason I ask, is because I really have no idea. I am legitimately clueless as how I can accomplish this.
#include <iostream>
#include <ostream>
#include "stdafx.h"
using namespace std;
// start the program
int main() {
// LOCAL VARIABLES
// int student1, student2, student3; //the (3) students -- NOT EVEN SURE
// I NEED THIS
int all_students; // number of students
double grade1, grade2, grade3; // the (3) grade variable declarations
double grade_total; // holds the sum of all grades
double grade_avg; // holds the average of the sum of grades
cout << "Enter the number of students in the class: "; // number of students
// request
cin >> all_students;
all_students = 1; // initialize counter for the loop
// need to create a loop for all three students and input for 3
// grades/student
while (all_students <= 3) // start the loop for the students
{
cout << "\n\nPlease enter (3) numeric grades for student "
<< all_students << ":"; // enters the 3 grades for student
cin >> grade1 >> grade2 >>
grade3; // the grades are stored in these 3 variables
grade_total = grade1 + grade2 +
grade3; // sum is the 3 sets of graders added together
grade_avg = grade_total / 3; // avg. is the sum divided by 3
// displays the output for the student grades, grade average, and letter
// grade
cout << "Student " << all_students << " grades are \n" << grade1 << "\n"
<< grade2 << "\n" << grade3 << "\n"
<< "with an average of: " << grade_avg
<< ", which is letter grade: "; // need this line to also read the
// letter grade for the average
// grade
if (grade_avg >= 90) // 90+ gets an A
cout << "A";
else if (grade_avg >= 80) // 80-89 gets a B
cout << "B";
else if (grade_avg >= 70) // 70-79 gets a C
cout << "C";
else if (grade_avg >= 60) // 60-69 gets a D
cout << "D";
else // anything less than a 60% is an F
cout << "F";
// increases counter! Incrementer.
all_students++; // Yes, I want to increment the count after the loop.
//****************************************************************//
// I need to figure out how to create an array to hold //
// each student average in order to calculate the overall //
// class average. Or use a switch statement? Advice? //
//****************************************************************//
//****************************************************************//
//****************************************************************//
}
}

Answer 1

you need to create an array outside the while with maximum number of students, then inside the while loop (at the end of it) assign each calculated average to one entry in that array.
then outside the while loop, write a for loop to calculate the averge of the elements in the array, which are the averages of each student

Answer 2

The simplest way is to have a running average
float classAvg = 0;
const unit NUM_STUDENTS = 3;
while (all_students <= NUM_STUDENTS) //start the loop for the students
{
...
classAvg += grade_avg;
}
classAvg /= NUM_STUDENTS
cout<<"Class average is " << classAvg;
I also recommend making the part that converts numeric grade to letter grade into a function. That will make your code more modular and cleaner. Something along the lines of:
void displayLetterGrade(float grade)
{
if (grade >= 90) // 90+ gets an A
cout << "A";
else if (grade >= 80) // 80-89 gets a B
cout << "B";
else if (grade >= 70) // 70-79 gets a C
cout << "C";
else if (grade >= 60) // 60-69 gets a D
cout << "D";
else // anything less than a 60% is an F
cout << "F";
}