Sorting by Functions in C++

Sometimes we’ll want to sort a collection by something other than its natural order. For example, suppose we wanted to sort strings by their length instead of alphabetically. Here’s an example of custom sorts in C++.

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

int main() {
    std::vector<std::string> fruits = {"peach", "banana", "kiwi"};

    // We implement a comparison function for string lengths.
    auto lenCmp = [](const std::string& a, const std::string& b) {
        return a.length() < b.length();
    };

    // Now we can call std::sort with this custom comparison function
    // to sort fruits by name length.
    std::sort(fruits.begin(), fruits.end(), lenCmp);
    
    for (const auto& fruit : fruits) {
        std::cout << fruit << " ";
    }
    std::cout << std::endl;

    // We can use the same technique to sort a vector of
    // values that aren't built-in types.
    struct Person {
        std::string name;
        int age;
    };

    std::vector<Person> people = {
        {"Jax", 37},
        {"TJ", 25},
        {"Alex", 72}
    };

    // Sort people by age using std::sort.
    std::sort(people.begin(), people.end(),
        [](const Person& a, const Person& b) {
            return a.age < b.age;
        });

    for (const auto& person : people) {
        std::cout << "{" << person.name << " " << person.age << "} ";
    }
    std::cout << std::endl;

    return 0;
}

In this C++ version, we use std::sort from the <algorithm> header instead of slices.SortFunc. The comparison functions are implemented as lambda expressions.

For sorting the fruits vector, we define a lenCmp lambda that compares string lengths. We then pass this to std::sort along with the iterators for the beginning and end of the vector.

For the Person struct, we sort directly within the std::sort call using an inline lambda expression that compares the age field.

Note that in C++, we need to explicitly print each element of the sorted containers. We use range-based for loops for this purpose.

To compile and run this program:

$ g++ -std=c++11 sorting_by_functions.cpp -o sorting_by_functions
$ ./sorting_by_functions
kiwi peach banana 
{TJ 25} {Jax 37} {Alex 72}

This example demonstrates how to use custom sorting functions in C++ to sort both built-in types (strings) and user-defined types (Person structs) based on specific criteria.

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Sorting by Functions in C++