The STL and professional C++ · Lesson 21 of 21

Project: a grade book

Classes, vectors, maps, algorithms and exceptions in one program.

15 minutes3 games

We'll build a small grade book for a SeaCode course that brings together everything we've learned. Read and run the code first, then complete the three tasks below, each building on the last.

C++

#include <algorithm>
#include <iostream>
#include <map>
#include <stdexcept>
#include <string>
#include <vector>

class GradeBook {
public:
    void add(const std::string& student, int score) {
        if (score < 0 || score > 100) throw std::out_of_range("score must be 0-100");
        scores_[student].push_back(score);
    }

    double average(const std::string& student) const {
        auto it = scores_.find(student);
        if (it == scores_.end()) throw std::invalid_argument("unknown student " + student);
        const std::vector<int>& s = it->second;
        int sum = 0;
        for (int x : s) sum += x;
        return static_cast<double>(sum) / s.size();
    }

    std::vector<std::string> students() const {
        std::vector<std::string> names;
        for (const auto& entry : scores_) names.push_back(entry.first);
        return names;
    }

private:
    std::map<std::string, std::vector<int>> scores_;
};

int main() {
    GradeBook book;
    book.add("Laila", 90);
    book.add("Laila", 80);
    book.add("Omar", 70);
    book.add("Nour", 100);
    book.add("Nour", 95);

    for (const std::string& name : book.students()) {
        std::cout << name << ": " << book.average(name) << "\n";
    }
    try {
        book.add("Omar", 120);
    } catch (const std::exception& e) {
        std::cout << "rejected: " << e.what() << "\n";
    }
    return 0;
}

GradeBook
- scores_: map<string, vector<int>>
  - "Laila" → [90, 80]
  - "Nour" → [100, 95]
  - "Omar" → [70]
- add() → validates, then push_back
- average() const → throws if unknown

Task 1: ranking students

Exercise

Add a ranking() method (const) returning student names sorted by average, highest first. Print them on one line separated by spaces.

Expected output: Nour Laila Omar

C++

#include <algorithm>
#include <iostream>
#include <map>
#include <string>
#include <vector>

class GradeBook {
public:
    void add(const std::string& student, int score) { scores_[student].push_back(score); }
    double average(const std::string& student) const {
        const std::vector<int>& s = scores_.at(student);
        int sum = 0;
        for (int x : s) sum += x;
        return static_cast<double>(sum) / s.size();
    }
    std::vector<std::string> ranking() const {
        std::vector<std::string> names;
        // fill and sort names here
        return names;
    }
private:
    std::map<std::string, std::vector<int>> scores_;
};

int main() {
    GradeBook book;
    book.add("Laila", 90); book.add("Laila", 80);
    book.add("Omar", 70);
    book.add("Nour", 100); book.add("Nour", 95);
    for (const std::string& n : book.ranking()) std::cout << n << " ";
    std::cout << "\n";
    return 0;
}

Task 2: letter grades

Exercise

Write char letter(double avg): 90 and up is A, 80 is B, 70 is C, anything lower F. For each student (alphabetically) print name letter.

Expected output:

Ali C
Laila B
Nour A
Omar F

C++

#include <iostream>
#include <map>
#include <string>
#include <vector>

char letter(double avg) {
    return '?';
}

int main() {
    std::map<std::string, std::vector<int>> scores = {
        {"Laila", {90, 80}}, {"Omar", {70, 60}}, {"Nour", {100, 95}}, {"Ali", {72, 78}}
    };
    for (const auto& entry : scores) {
        int sum = 0;
        for (int x : entry.second) sum += x;
        double avg = static_cast<double>(sum) / entry.second.size();
        std::cout << entry.first << " " << letter(avg) << "\n";
    }
    return 0;
}

Task 3: course statistics

Exercise

From all scores {90, 80, 70, 100, 95, 55}: print the highest, the lowest, and how many are 60 or more, on one line as max 100 min 55 passed 5. Use the algorithms.

Expected output: max 100 min 55 passed 5

C++

#include <algorithm>
#include <iostream>
#include <vector>

int main() {
    std::vector<int> all = {90, 80, 70, 100, 95, 55};
    
    return 0;
}

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#include <algorithm> #include <iostream> #include <map> #include <stdexcept> #include <string> #include <vector> class GradeBook { public: void add(const std::string& student, int score) { if (score < 0 || score > 100) throw std::out_of_range("score must be 0-100"); scores_[student].push_back(score); } double average(const std::string& student) const { auto it = scores_.find(student); if (it == scores_.end()) throw std::invalid_argument("unknown student " + student); const std::vector<int>& s = it->second; int sum = 0; for (int x : s) sum += x; return static_cast<double>(sum) / s.size(); } std::vector<std::string> students() const { std::vector<std::string> names; for (const auto& entry : scores_) names.push_back(entry.first); return names; } private: std::map<std::string, std::vector<int>> scores_; }; int main() { GradeBook book; book.add("Laila", 90); book.add("Laila", 80); book.add("Omar", 70); book.add("Nour", 100); book.add("Nour", 95); for (const std::string& name : book.students()) { std::cout << name << ": " << book.average(name) << "\n"; } try { book.add("Omar", 120); } catch (const std::exception& e) { std::cout << "rejected: " << e.what() << "\n"; } return 0; }

Task 1: ranking students

Exercise

Add a ranking() method (const) returning student names sorted by average, highest first. Print them on one line separated by spaces.

Expected output: Nour Laila Omar

C++

#include <algorithm>
#include <iostream>
#include <map>
#include <string>
#include <vector>

class GradeBook {
public:
    void add(const std::string& student, int score) { scores_[student].push_back(score); }
    double average(const std::string& student) const {
        const std::vector<int>& s = scores_.at(student);
        int sum = 0;
        for (int x : s) sum += x;
        return static_cast<double>(sum) / s.size();
    }
    std::vector<std::string> ranking() const {
        std::vector<std::string> names;
        // fill and sort names here
        return names;
    }
private:
    std::map<std::string, std::vector<int>> scores_;
};

int main() {
    GradeBook book;
    book.add("Laila", 90); book.add("Laila", 80);
    book.add("Omar", 70);
    book.add("Nour", 100); book.add("Nour", 95);
    for (const std::string& n : book.ranking()) std::cout << n << " ";
    std::cout << "\n";
    return 0;
}

Task 2: letter grades

Exercise

Write char letter(double avg): 90 and up is A, 80 is B, 70 is C, anything lower F. For each student (alphabetically) print name letter.

Expected output:

Ali C
Laila B
Nour A
Omar F

C++

#include <iostream>
#include <map>
#include <string>
#include <vector>

char letter(double avg) {
    return '?';
}

int main() {
    std::map<std::string, std::vector<int>> scores = {
        {"Laila", {90, 80}}, {"Omar", {70, 60}}, {"Nour", {100, 95}}, {"Ali", {72, 78}}
    };
    for (const auto& entry : scores) {
        int sum = 0;
        for (int x : entry.second) sum += x;
        double avg = static_cast<double>(sum) / entry.second.size();
        std::cout << entry.first << " " << letter(avg) << "\n";
    }
    return 0;
}

Task 3: course statistics

Exercise

From all scores {90, 80, 70, 100, 95, 55}: print the highest, the lowest, and how many are 60 or more, on one line as max 100 min 55 passed 5. Use the algorithms.

Expected output: max 100 min 55 passed 5

C++

#include <algorithm>
#include <iostream>
#include <vector>

int main() {
    std::vector<int> all = {90, 80, 70, 100, 95, 55};
    
    return 0;
}

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