القائمة حسب المادة

student_records = {}

def add_student(name, age, courses):
    if name in student_records:
        print(f"Student '{name}' already exists.")
        return
    student_records[name] = {"age": age, "grades": set(), "courses": set(courses)}
    print(f"Student '{name}' added successfully.")

def add_grade(name, grade):
    if name not in student_records:
        print(f"Student '{name}' not found.")
        return
    student_records[name]["grades"].add(grade)
    print(f"Grade {grade} added for student '{name}'.")

def is_enrolled(name, course):
    if name not in student_records:
        print(f"Student '{name}' not found.")
        return False
    return course in student_records[name]["courses"]

def calculate_average_grade(name):
    if name not in student_records:
        print(f"Student '{name}' not found.")
        return None
    grades = student_records[name]["grades"]
    if not grades:
        return 0
    return sum(grades) / len(grades)


add_student("Alice", 20, ["Math", "Physics"])
add_student("Bob", 22, ["Biology", "Chemistry"])
add_grade("Alice", 90)
add_grade("Alice", 85)
add_grade("Bob", 75)
print(calculate_average_grade("Alice"))  # Should return 87.5
print(calculate_average_grade("Bob"))  # Should return 75.0
print(calculate_average_grade("Charlie"))  # Non-existent student, should print message and return None
print(calculate_average_grade("Alice"))  # Should return 87.5 again