Program.cs

using System;
using System.Collections.Generic;

namespace StudentGradeManagement
{
    class Program
    {
        // ──────────────────────────────────────────────
        //  Data stores (shared across all methods)
        // ──────────────────────────────────────────────

        // Maps student ID → student name
        static Dictionary<int, string> students = new Dictionary<int, string>();

        // Maps student ID → list of (subject, grade) pairs
        static Dictionary<int, List<(string Subject, double Grade)>> grades =
            new Dictionary<int, List<(string Subject, double Grade)>>();

        // ──────────────────────────────────────────────
        //  Entry point
        // ──────────────────────────────────────────────
        static void Main(string[] args)
        {
            bool running = true;

            // while loop — keeps the program alive until the user exits
            while (running)
            {
                ShowMenu();
                string choice = Console.ReadLine();

                // switch — dispatches user choice to the right action
                switch (choice)
                {
                    case "1":
                        AddStudent();
                        break;
                    case "2":
                        AddGrade();
                        break;
                    case "3":
                        DisplayStudents();
                        break;
                    case "4":
                        CalculateAverage();
                        break;
                    case "5":
                        running = false;
                        Console.WriteLine("\nGoodbye!");
                        break;
                    default:
                        Console.WriteLine("\n[Error] Invalid option. Please enter a number from 1 to 5.\n");
                        break;
                }
            }
        }

        // ──────────────────────────────────────────────
        //  Method 1 — ShowMenu
        //  Prints the main menu to the console.
        // ──────────────────────────────────────────────
        static void ShowMenu()
        {
            Console.WriteLine("==========================================");
            Console.WriteLine("   STUDENT GRADE MANAGEMENT SYSTEM");
            Console.WriteLine("==========================================");
            Console.WriteLine("1. Add new student");
            Console.WriteLine("2. Add grade to a student");
            Console.WriteLine("3. Display all students and grades");
            Console.WriteLine("4. Calculate and display a student's average");
            Console.WriteLine("5. Exit");
            Console.Write("Choose an option: ");
        }

        // ──────────────────────────────────────────────
        //  Method 2 — AddStudent
        //  Registers a new student with a unique ID.
        // ──────────────────────────────────────────────
        static void AddStudent()
        {
            Console.Write("\nEnter student ID (integer): ");
            string idInput = Console.ReadLine();

            // Validate that the input is a valid integer
            if (!int.TryParse(idInput, out int id))
            {
                Console.WriteLine("[Error] Invalid ID. Please enter a whole number.\n");
                return;
            }

            // if-else — check for duplicate ID
            if (students.ContainsKey(id))
            {
                Console.WriteLine($"[Error] A student with ID {id} already exists.\n");
            }
            else
            {
                Console.Write("Enter student name: ");
                string name = Console.ReadLine();

                // Validate non-empty name
                if (string.IsNullOrWhiteSpace(name))
                {
                    Console.WriteLine("[Error] Name cannot be empty.\n");
                    return;
                }

                students[id] = name;
                grades[id] = new List<(string, double)>();
                Console.WriteLine($"Student \"{name}\" (ID: {id}) added successfully.\n");
            }
        }

        // ──────────────────────────────────────────────
        //  Method 3 — AddGrade
        //  Assigns a grade for a subject to an existing student.
        // ──────────────────────────────────────────────
        static void AddGrade()
        {
            Console.Write("\nEnter student ID: ");
            string idInput = Console.ReadLine();

            if (!int.TryParse(idInput, out int id))
            {
                Console.WriteLine("[Error] Invalid ID. Please enter a whole number.\n");
                return;
            }

            // Check that the student exists
            if (!students.ContainsKey(id))
            {
                Console.WriteLine($"[Error] No student found with ID {id}.\n");
                return;
            }

            Console.Write("Enter subject name: ");
            string subject = Console.ReadLine();

            if (string.IsNullOrWhiteSpace(subject))
            {
                Console.WriteLine("[Error] Subject name cannot be empty.\n");
                return;
            }

            Console.Write("Enter grade (0 - 100): ");
            string gradeInput = Console.ReadLine();

            if (!double.TryParse(gradeInput, out double grade))
            {
                Console.WriteLine("[Error] Invalid grade. Please enter a number.\n");
                return;
            }

            // if-else — validate grade range
            if (grade < 0 || grade > 100)
            {
                Console.WriteLine("[Error] Grade must be between 0 and 100.\n");
            }
            else
            {
                grades[id].Add((subject, grade));
                Console.WriteLine($"Grade {grade} in \"{subject}\" added for {students[id]}.\n");
            }
        }

        // ──────────────────────────────────────────────
        //  Method 4 — DisplayStudents
        //  Shows every student with their recorded grades.
        // ──────────────────────────────────────────────
        static void DisplayStudents()
        {
            Console.WriteLine();

            // Handle empty list
            if (students.Count == 0)
            {
                Console.WriteLine("No students registered yet.\n");
                return;
            }

            // foreach loop — iterate over all students
            foreach (KeyValuePair<int, string> student in students)
            {
                Console.WriteLine("------------------------------------------");
                Console.WriteLine($"  ID: {student.Key}  |  Name: {student.Value}");
                Console.WriteLine("------------------------------------------");

                List<(string Subject, double Grade)> studentGrades = grades[student.Key];

                if (studentGrades.Count == 0)
                {
                    Console.WriteLine("  (no grades recorded)");
                }
                else
                {
                    // for loop — print each grade entry
                    for (int i = 0; i < studentGrades.Count; i++)
                    {
                        Console.WriteLine($"  {i + 1}. {studentGrades[i].Subject}: {studentGrades[i].Grade}");
                    }
                }

                Console.WriteLine();
            }
        }

        // ──────────────────────────────────────────────
        //  Method 5 — CalculateAverage
        //  Computes and displays the average grade for a student.
        // ──────────────────────────────────────────────
        static void CalculateAverage()
        {
            Console.Write("\nEnter student ID: ");
            string idInput = Console.ReadLine();

            if (!int.TryParse(idInput, out int id))
            {
                Console.WriteLine("[Error] Invalid ID. Please enter a whole number.\n");
                return;
            }

            // if-else — check student exists and has grades
            if (!students.ContainsKey(id))
            {
                Console.WriteLine($"[Error] No student found with ID {id}.\n");
            }
            else if (grades[id].Count == 0)
            {
                Console.WriteLine($"{students[id]} has no grades to average.\n");
            }
            else
            {
                double sum = 0;
                List<(string Subject, double Grade)> studentGrades = grades[id];

                // for loop — accumulate the sum of all grades
                for (int i = 0; i < studentGrades.Count; i++)
                {
                    sum += studentGrades[i].Grade;
                }

                double average = sum / studentGrades.Count;
                Console.WriteLine($"Average grade for {students[id]}: {average:F2}\n");
            }
        }
    }
}