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Friday 16 August 2024
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C++ How to Sort a Vector

Sorting a vector in C++ is a common operation that can be easily accomplished using the std::sort function from the <algorithm> library. The std::sort function is powerful and flexible, allowing you to sort vectors in ascending, descending, or custom order with ease.

Using std::sort for Ascending Order

The simplest way to sort a vector in ascending order is to use the std::sort function, which sorts the elements between two iterators, typically begin() and end().

Example: 

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

int main() {
    std::vector<int> numbers = {4, 2, 7, 1, 9};
    std::sort(numbers.begin(), numbers.end());

    for (int num : numbers) {
        std::cout << num << " ";
    }
    return 0;
}

Output: 

1 2 4 7 9

Sorting in Descending Order

To sort a vector in descending order, you can pass the std::greater<int>() comparator to the std::sort function.

Example: 

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

int main() {
    std::vector<int> numbers = {4, 2, 7, 1, 9};
    std::sort(numbers.begin(), numbers.end(), std::greater<int>());

    for (int num : numbers) {
        std::cout << num << " ";
    }
    return 0;
}

Output: 

9 7 4 2 1

Custom Sorting Criteria

If you need a custom sort order, you can provide a custom comparator function or lambda expression. The comparator should return true if the first element should appear before the second.

Example: 

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

bool customSort(int a, int b) {
    // Sort based on even numbers first, then by ascending order
    if ((a % 2 == 0) && (b % 2 != 0)) return true;
    if ((a % 2 != 0) && (b % 2 == 0)) return false;
    return a < b;
}

int main() {
    std::vector<int> numbers = {4, 2, 7, 1, 9};
    std::sort(numbers.begin(), numbers.end(), customSort);

    for (int num : numbers) {
        std::cout << num << " ";
    }
    return 0;
}

Output: 

2 4 1 7 9

Examples of Sorting a Vector in C++

1. Sorting a vector of strings in alphabetical order: 

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

    int main() {
        std::vector<std::string> words = {"apple", "orange", "banana", "grape"};
        std::sort(words.begin(), words.end());

        for (const auto& word : words) {
            std::cout << word << " ";
        }
        return 0;
    }

Output: 

apple banana grape orange

2. Sorting a vector of pairs by the second element: 

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

    int main() {
        std::vector<std::pair<int, int>> pairs = {{1, 5}, {2, 3}, {4, 2}};
        std::sort(pairs.begin(), pairs.end(), [](auto &left, auto &right) {
            return left.second < right.second;
        });

        for (const auto& p : pairs) {
            std::cout << "{" << p.first << ", " << p.second << "} ";
        }
        return 0;
    }

Output: 

{4, 2} {2, 3} {1, 5}

References

For more information, check out the official C++ documentation:

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Jorge García

Fullstack developer