C++ Mini Projects for Students

#include <iostream>
#include <fstream>
#include <iomanip>
#include <vector>
#include <string>
#include <algorithm>
#include <limits>

const int SUBJECTS = 5;
const std::string DATA_FILE = "students.txt";

class Student {
public:
    int roll = 0;
    std::string name;
    int marks[SUBJECTS] = {0};

    int total() const {
        int t = 0;
        for (int m : marks) t += m;
        return t;
    }
    double percentage() const { return total() * 100.0 / (SUBJECTS * 100); }
    char grade() const {
        double p = percentage();
        if (p >= 90) return 'A';
        if (p >= 75) return 'B';
        if (p >= 60) return 'C';
        if (p >= 33) return 'D';
        return 'F';
    }
};

int readInt(const std::string& prompt, int lo, int hi) {
    int x;
    while (true) {
        std::cout << prompt;
        if (std::cin >> x && x >= lo && x <= hi) return x;
        std::cout << "  Please enter a number from " << lo << " to " << hi << ".\n";
        std::cin.clear();
        std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
    }
}

void save(const std::vector<Student>& list) {
    std::ofstream out(DATA_FILE);
    for (const auto& s : list) {
        out << s.roll << ' ' << s.name;
        for (int m : s.marks) out << ' ' << m;
        out << '\n';
    }
}

std::vector<Student> load() {
    std::vector<Student> list;
    std::ifstream in(DATA_FILE);
    Student s;
    while (in >> s.roll >> s.name) {
        for (int& m : s.marks) in >> m;
        list.push_back(s);
    }
    return list;
}

void addStudent(std::vector<Student>& list) {
    Student s;
    s.roll = readInt("Roll number: ", 1, 9999);
    for (const auto& x : list)
        if (x.roll == s.roll) { std::cout << "  Roll number already exists.\n"; return; }
    std::cout << "Name (one word): ";
    std::cin >> s.name;
    for (int i = 0; i < SUBJECTS; i++)
        s.marks[i] = readInt("  Marks in subject " + std::to_string(i + 1) + ": ", 0, 100);
    list.push_back(s);
    save(list);
    std::cout << "  Saved.\n";
}

void printTable(const std::vector<Student>& list) {
    if (list.empty()) { std::cout << "  No records yet.\n"; return; }
    std::cout << std::left << std::setw(6) << "Roll" << std::setw(12) << "Name"
              << std::right << std::setw(6) << "Total" << std::setw(8) << "%"
              << std::setw(7) << "Grade" << "\n";
    for (const auto& s : list) {
        std::cout << std::left << std::setw(6) << s.roll << std::setw(12) << s.name
                  << std::right << std::setw(6) << s.total()
                  << std::setw(8) << std::fixed << std::setprecision(1) << s.percentage()
                  << std::setw(7) << s.grade() << "\n";
    }
}

void meritList(std::vector<Student> list) {            // copy: keep original order
    std::sort(list.begin(), list.end(),
              [](const Student& a, const Student& b) { return a.total() > b.total(); });
    printTable(list);
}

void search(const std::vector<Student>& list) {
    int r = readInt("Roll to search: ", 1, 9999);
    auto it = std::find_if(list.begin(), list.end(),
                           [r](const Student& s) { return s.roll == r; });
    if (it == list.end()) { std::cout << "  Not found.\n"; return; }
    printTable({*it});
}

int main() {
    std::vector<Student> students = load();
    int choice;
    do {
        std::cout << "\n=== Report Card System (" << students.size() << " students) ===\n"
                  << "1. Add student\n2. Show all\n3. Merit list\n4. Search by roll\n0. Exit\n";
        choice = readInt("Choice: ", 0, 4);
        switch (choice) {
            case 1: addStudent(students); break;
            case 2: printTable(students); break;
            case 3: meritList(students);  break;
            case 4: search(students);     break;
        }
    } while (choice != 0);
    std::cout << "Goodbye!\n";
    return 0;
}