How to Sort a Vector Based on Lengths in C++

C++ Sort a Vector Based on Lengths

You can use the std::sort function from the algorithm library with a custom comparison function to sorting a vector based on the lengths of its elements.

Syntax of std::sort

The syntax to import the required libraries and use the std::sort function with the custom comparison function is as shown in the following.

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

// Sort a vector based on lengths using a custom comparison function
std::sort(vector.begin(), vector.end(), [](const auto& a, const auto& b) {
    return a.size() < b.size();
});

Examples

Example 1: Sort a Vector of Strings by Length in Ascending Order of Lengths

This example demonstrates sorting a vector of strings in ascending order of their lengths using a lambda expression as the comparison function.

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

int main() {
    std::vector<std::string> words = {"apple", "banana", "kiwi", "cherry", "grape"};

    // Sort the vector based on lengths in ascending order
    std::sort(words.begin(), words.end(), [](const std::string& a, const std::string& b) {
        return a.size() < b.size();
    });

    std::cout << "Strings sorted by length (ascending):" << std::endl;
    for (const auto& word : words)
        std::cout << word << " ";
    std::cout << std::endl;

    return 0;
}

Output:

Strings sorted by length (ascending):
kiwi apple grape banana cherry

Explanation:

1. Initialize the vector: A std::vector named words is created and initialized with strings: {"apple", "banana", "kiwi", "cherry", "grape"}. This vector contains words of varying lengths to demonstrate sorting by their lengths.
2. Sort the vector by string length: The std::sort function is called with three arguments:
   • words.begin(): Specifies the starting point of the range to be sorted.
   • words.end(): Specifies the end of the range to be sorted.
   • A lambda expression: Defines the custom sorting logic that compares the lengths of two strings, a and b. It returns true if the length of a is less than the length of b, ensuring ascending order by length.
   After sorting, the shorter strings appear before the longer ones.
3. Print the sorted vector: A range-based for loop iterates over the sorted vector, printing each word to the console using std::cout. The words are displayed separated by spaces, preceded by the message "Strings sorted by length (ascending):".

Example 2: Sort a Vector of Strings by Length in Descending Order of Lengths

This example demonstrates sorting a vector of strings in descending order of their lengths using a lambda expression.

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

int main() {
    std::vector<std::string> words = {"apple", "banana", "kiwi", "cherry", "grape"};

    // Sort the vector based on lengths in descending order
    std::sort(words.begin(), words.end(), [](const std::string& a, const std::string& b) {
        return a.size() > b.size();
    });

    std::cout << "Strings sorted by length (descending):" << std::endl;
    for (const auto& word : words)
        std::cout << word << " ";
    std::cout << std::endl;

    return 0;
}

Output:

Strings sorted by length (descending):
banana cherry apple grape kiwi

Example 3: Stable Sorting a Vector of Strings by Length

You can use std::stable_sort to sort a vector of strings by their lengths while preserving the relative order of strings with equal lengths.

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

int main() {
    std::vector<std::string> words = {"kiwi", "apple", "pear", "banana", "grape"};

    // Stable sort the vector based on lengths
    std::stable_sort(words.begin(), words.end(), [](const std::string& a, const std::string& b) {
        return a.size() < b.size();
    });

    std::cout << "Strings stably sorted by length:" << std::endl;
    for (const auto& word : words)
        std::cout << word << " ";
    std::cout << std::endl;

    return 0;
}

Output:

Strings stably sorted by length:
kiwi pear apple grape banana

Example 4: Sorting a Vector of Numbers Based on Length

This example demonstrates sorting a vector of numbers, where each number is represented as a string, based on the length of the strings. The shorter strings appear first in ascending order of their lengths.

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

int main() {
    // Initialize a vector of numbers represented as strings
    std::vector<std::string> numbers = {"123", "45", "7", "8901", "56", "789", "2"};

    // Sort the vector based on the length of the strings
    std::sort(numbers.begin(), numbers.end(), [](const std::string& a, const std::string& b) {
        return a.size() < b.size(); // Ascending order of string lengths
    });

    std::cout << "Numbers sorted by length:" << std::endl;
    for (const auto& number : numbers)
        std::cout << number << " ";
    std::cout << std::endl;

    return 0;
}

Output:

Numbers sorted by length:
7 2 45 56 123 789 8901

Explanation:

Sort the vector: The std::sort function is used with a custom lambda expression to sort the strings by their lengths. The lambda function compares the lengths of two strings, an and b, and returns true if the length of a is less than the length of b. This results in sorting the vector in ascending order of string lengths.