Question

How to sort a vector of structs by multiple columns in C++

Answer 1

#include <iostream>
#include <string>
#include <vector>
#include <algorithm> // sort

struct Item {
    int a;
    int b;
    std::string label;
};

// Create or load the dataset
std::vector<Item> make_data() {
    return {
        {7, 2, "python"},
        {8, 3, "c"},
        {3, 5, "c++"},
        {4, 1, "c#"},
        {3, 2, "java"},
        {7, 1, "go"},
        {1, 2, "rust"},
    };
}

// Sort by column 0, then column 1
void sort_data(std::vector<Item>& data) {
    std::sort(data.begin(), data.end(),
        [](const Item& x, const Item& y) {
            if (x.a != y.a)
                return x.a < y.a;
            return x.b < y.b;
        }
    );
}

// Print the dataset
void print_data(const std::vector<Item>& data) {
    for (const auto& item : data) {
        std::cout << "(" << item.a << ", "
                  << item.b << ", "
                  << item.label << ")\n";
    }
}

int main() {
    auto data = make_data();
    
    sort_data(data);
    print_data(data);
}



/*
run:

(1, 2, rust)
(3, 2, java)
(3, 5, c++)
(4, 1, c#)
(7, 1, go)
(7, 2, python)
(8, 3, c)

*/

 

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Answer 2

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

struct Item {
    int a;
    int b;
    std::string label;
};

// Create the dataset
std::vector<Item> make_data() {
    return {
        {7, 2, "python"},
        {8, 3, "c"},
        {3, 5, "c++"},
        {4, 1, "c#"},
        {3, 2, "java"},
        {7, 1, "go"},
        {1, 2, "rust"},
    };
}

// Sort by column 0, then column 1
void sort_by_two_columns(std::vector<Item>& data) {
    std::sort(data.begin(), data.end(),
        [](const Item& x, const Item& y) {
            if (x.a != y.a)
                return x.a < y.a;
            return x.b < y.b;
        }
    );
}

// Print a single item
void print_item(const Item& item) {
    std::cout << "(" << item.a << ", "
              << item.b << ", "
              << item.label << ")\n";
}

// Print all items
void print_data(const std::vector<Item>& data) {
    for (const auto& item : data) {
        print_item(item);
    }
}

// Find the first item with a matching label
std::optional<Item> find_by_label(const std::vector<Item>& data,
                                  const std::string& label) {
    for (const auto& item : data) {
        if (item.label == label)
            return item;
    }
    return std::nullopt;
}

// Return all items with a matching first column
std::vector<Item> filter_by_first_column(const std::vector<Item>& data, int value) {
    std::vector<Item> result;
    for (const auto& item : data) {
        if (item.a == value)
            result.push_back(item);
    }
    return result;
}

int main() {
    auto data = make_data();

    sort_by_two_columns(data);
    print_data(data);

    std::cout << "\nSearching for 'java':\n";
    if (auto found = find_by_label(data, "java")) {
        print_item(*found);
    }

    std::cout << "\nFiltering items where a == 7:\n";
    auto filtered = filter_by_first_column(data, 7);
    print_data(filtered);
}



/*
run:

(1, 2, rust)
(3, 2, java)
(3, 5, c++)
(4, 1, c#)
(7, 1, go)
(7, 2, python)
(8, 3, c)

Searching for 'java':
(3, 2, java)

Filtering items where a == 7:
(7, 1, go)
(7, 2, python)

*/

 

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