C++ How To: Sort an Array

Sorting arrays is a fundamental operation in computer programming, and C++ offers several efficient ways to accomplish this task. Whether you're dealing with integers, floating-point numbers, or custom objects, understanding how to sort arrays can significantly improve your coding skills and the performance of your applications.

Table of Contents

The Importance of Sorting

Before we dive into the various sorting methods, let's briefly discuss why sorting is crucial:

🔹 Efficiency: Sorted data allows for faster searching and retrieval.
🔹 Organization: Sorted arrays make data more readable and manageable.
🔹 Preparation: Many algorithms require sorted input for optimal performance.

Now, let's explore different techniques to sort arrays in C++, starting from the simplest to more advanced methods.

1. Bubble Sort: A Simple Starting Point

Bubble Sort is an elementary sorting algorithm that repeatedly steps through the array, compares adjacent elements, and swaps them if they're in the wrong order. While not the most efficient for large datasets, it's an excellent place to start understanding sorting concepts.

Here's a C++ implementation of Bubble Sort:

#include <iostream>
using namespace std;

void bubbleSort(int arr[], int size) {
    for (int i = 0; i < size - 1; i++) {
        for (int j = 0; j < size - i - 1; j++) {
            if (arr[j] > arr[j + 1]) {
                // Swap elements
                int temp = arr[j];
                arr[j] = arr[j + 1];
                arr[j + 1] = temp;
            }
        }
    }
}

int main() {
    int arr[] = {64, 34, 25, 12, 22, 11, 90};
    int size = sizeof(arr) / sizeof(arr[0]);

    cout << "Original array: ";
    for (int i = 0; i < size; i++) {
        cout << arr[i] << " ";
    }
    cout << endl;

    bubbleSort(arr, size);

    cout << "Sorted array: ";
    for (int i = 0; i < size; i++) {
        cout << arr[i] << " ";
    }
    cout << endl;

    return 0;
}

Output:

Original array: 64 34 25 12 22 11 90
Sorted array: 11 12 22 25 34 64 90

💡 Pro Tip: Bubble Sort has a time complexity of O(n²), making it inefficient for large datasets. However, it's easy to understand and implement, making it useful for educational purposes or sorting very small arrays.

2. Selection Sort: Finding the Minimum

Selection Sort is another simple sorting algorithm. It repeatedly selects the smallest (or largest) element from the unsorted portion of the list and moves it to the sorted portion.

Here's how you can implement Selection Sort in C++:

#include <iostream>
using namespace std;

void selectionSort(int arr[], int size) {
    for (int i = 0; i < size - 1; i++) {
        int minIndex = i;
        for (int j = i + 1; j < size; j++) {
            if (arr[j] < arr[minIndex]) {
                minIndex = j;
            }
        }
        if (minIndex != i) {
            swap(arr[i], arr[minIndex]);
        }
    }
}

int main() {
    int arr[] = {64, 25, 12, 22, 11};
    int size = sizeof(arr) / sizeof(arr[0]);

    cout << "Original array: ";
    for (int i = 0; i < size; i++) {
        cout << arr[i] << " ";
    }
    cout << endl;

    selectionSort(arr, size);

    cout << "Sorted array: ";
    for (int i = 0; i < size; i++) {
        cout << arr[i] << " ";
    }
    cout << endl;

    return 0;
}

Output:

Original array: 64 25 12 22 11
Sorted array: 11 12 22 25 64

🔍 Note: Selection Sort also has a time complexity of O(n²), but it generally performs fewer swaps compared to Bubble Sort, which can be advantageous in certain situations.

3. Insertion Sort: Building a Sorted Portion

Insertion Sort builds the final sorted array one item at a time. It's much less efficient on large lists than more advanced algorithms such as QuickSort, HeapSort, or Merge Sort. However, it provides several advantages:

Simple implementation
Efficient for small data sets
More efficient in practice than most other simple quadratic algorithms
Adaptive, i.e., efficient for data sets that are already substantially sorted

Let's implement Insertion Sort in C++:

#include <iostream>
using namespace std;

void insertionSort(int arr[], int size) {
    for (int i = 1; i < size; i++) {
        int key = arr[i];
        int j = i - 1;

        while (j >= 0 && arr[j] > key) {
            arr[j + 1] = arr[j];
            j = j - 1;
        }
        arr[j + 1] = key;
    }
}

int main() {
    int arr[] = {12, 11, 13, 5, 6};
    int size = sizeof(arr) / sizeof(arr[0]);

    cout << "Original array: ";
    for (int i = 0; i < size; i++) {
        cout << arr[i] << " ";
    }
    cout << endl;

    insertionSort(arr, size);

    cout << "Sorted array: ";
    for (int i = 0; i < size; i++) {
        cout << arr[i] << " ";
    }
    cout << endl;

    return 0;
}

Output:

Original array: 12 11 13 5 6
Sorted array: 5 6 11 12 13

⚡ Efficiency Insight: Insertion Sort has a best-case time complexity of O(n) when the array is already sorted, making it efficient for nearly sorted arrays.

4. Quick Sort: Divide and Conquer

Quick Sort is a highly efficient, divide-and-conquer sorting algorithm. It works by selecting a 'pivot' element from the array and partitioning the other elements into two sub-arrays, according to whether they are less than or greater than the pivot.

Here's a C++ implementation of Quick Sort:

#include <iostream>
using namespace std;

int partition(int arr[], int low, int high) {
    int pivot = arr[high];
    int i = (low - 1);

    for (int j = low; j <= high - 1; j++) {
        if (arr[j] < pivot) {
            i++;
            swap(arr[i], arr[j]);
        }
    }
    swap(arr[i + 1], arr[high]);
    return (i + 1);
}

void quickSort(int arr[], int low, int high) {
    if (low < high) {
        int pi = partition(arr, low, high);

        quickSort(arr, low, pi - 1);
        quickSort(arr, pi + 1, high);
    }
}

int main() {
    int arr[] = {10, 7, 8, 9, 1, 5};
    int size = sizeof(arr) / sizeof(arr[0]);

    cout << "Original array: ";
    for (int i = 0; i < size; i++) {
        cout << arr[i] << " ";
    }
    cout << endl;

    quickSort(arr, 0, size - 1);

    cout << "Sorted array: ";
    for (int i = 0; i < size; i++) {
        cout << arr[i] << " ";
    }
    cout << endl;

    return 0;
}

Output:

Original array: 10 7 8 9 1 5
Sorted array: 1 5 7 8 9 10

🚀 Performance Note: Quick Sort has an average time complexity of O(n log n), making it one of the fastest sorting algorithms available. However, its worst-case time complexity is O(n²), which occurs when the pivot is always the smallest or largest element.

5. Merge Sort: Divide, Sort, and Merge

Merge Sort is another divide-and-conquer algorithm that works by dividing the unsorted list into n sublists, each containing one element (a list of one element is considered sorted). Then, it repeatedly merges sublists to produce new sorted sublists until there is only one sublist remaining.

Here's how you can implement Merge Sort in C++:

#include <iostream>
using namespace std;

void merge(int arr[], int left, int middle, int right) {
    int n1 = middle - left + 1;
    int n2 = right - middle;

    int L[n1], R[n2];

    for (int i = 0; i < n1; i++)
        L[i] = arr[left + i];
    for (int j = 0; j < n2; j++)
        R[j] = arr[middle + 1 + j];

    int i = 0, j = 0, k = left;

    while (i < n1 && j < n2) {
        if (L[i] <= R[j]) {
            arr[k] = L[i];
            i++;
        } else {
            arr[k] = R[j];
            j++;
        }
        k++;
    }

    while (i < n1) {
        arr[k] = L[i];
        i++;
        k++;
    }

    while (j < n2) {
        arr[k] = R[j];
        j++;
        k++;
    }
}

void mergeSort(int arr[], int left, int right) {
    if (left < right) {
        int middle = left + (right - left) / 2;

        mergeSort(arr, left, middle);
        mergeSort(arr, middle + 1, right);

        merge(arr, left, middle, right);
    }
}

int main() {
    int arr[] = {12, 11, 13, 5, 6, 7};
    int size = sizeof(arr) / sizeof(arr[0]);

    cout << "Original array: ";
    for (int i = 0; i < size; i++) {
        cout << arr[i] << " ";
    }
    cout << endl;

    mergeSort(arr, 0, size - 1);

    cout << "Sorted array: ";
    for (int i = 0; i < size; i++) {
        cout << arr[i] << " ";
    }
    cout << endl;

    return 0;
}

Output:

Original array: 12 11 13 5 6 7
Sorted array: 5 6 7 11 12 13

📊 Stability: Merge Sort is a stable sort, which means that the relative order of equal elements is preserved in the sorted output. This can be crucial when sorting objects with multiple fields.

6. Using C++ Standard Library: std::sort

While implementing sorting algorithms from scratch is an excellent learning exercise, C++ provides a highly optimized sorting function in its Standard Library: std::sort. This function implements a hybrid sorting algorithm, typically introsort, which combines quicksort, heapsort, and insertion sort to get the best performance in all cases.

Here's how you can use std::sort:

#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;

int main() {
    vector<int> arr = {64, 34, 25, 12, 22, 11, 90};

    cout << "Original array: ";
    for (int num : arr) {
        cout << num << " ";
    }
    cout << endl;

    sort(arr.begin(), arr.end());

    cout << "Sorted array: ";
    for (int num : arr) {
        cout << num << " ";
    }
    cout << endl;

    return 0;
}

Output:

Original array: 64 34 25 12 22 11 90
Sorted array: 11 12 22 25 34 64 90

💻 Efficiency: std::sort guarantees O(n log n) complexity in the average and worst-case scenarios, making it an excellent choice for most sorting needs.

7. Sorting in Descending Order

Sometimes, you might need to sort an array in descending order. Here's how you can modify the std::sort function to achieve this:

#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;

int main() {
    vector<int> arr = {64, 34, 25, 12, 22, 11, 90};

    cout << "Original array: ";
    for (int num : arr) {
        cout << num << " ";
    }
    cout << endl;

    sort(arr.begin(), arr.end(), greater<int>());

    cout << "Sorted array (descending): ";
    for (int num : arr) {
        cout << num << " ";
    }
    cout << endl;

    return 0;
}

Output:

Original array: 64 34 25 12 22 11 90
Sorted array (descending): 90 64 34 25 22 12 11

🔄 Flexibility: The std::sort function allows you to pass a custom comparison function, giving you the flexibility to define your own sorting criteria.

8. Sorting Custom Objects

When working with custom objects, you need to define how these objects should be compared. Let's look at an example where we sort a vector of Student objects based on their grades:

#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
using namespace std;

class Student {
public:
    string name;
    int grade;

    Student(string n, int g) : name(n), grade(g) {}
};

bool compareStudents(const Student &a, const Student &b) {
    return a.grade > b.grade;  // Sort in descending order of grades
}

int main() {
    vector<Student> students = {
        Student("Alice", 85),
        Student("Bob", 92),
        Student("Charlie", 78),
        Student("David", 95)
    };

    cout << "Original order:" << endl;
    for (const auto &student : students) {
        cout << student.name << ": " << student.grade << endl;
    }

    sort(students.begin(), students.end(), compareStudents);

    cout << "\nSorted by grade (descending):" << endl;
    for (const auto &student : students) {
        cout << student.name << ": " << student.grade << endl;
    }

    return 0;
}

Output:

Original order:
Alice: 85
Bob: 92
Charlie: 78
David: 95

Sorted by grade (descending):
David: 95
Bob: 92
Alice: 85
Charlie: 78

🎓 Custom Comparisons: When sorting custom objects, you have full control over the sorting criteria. You can sort based on multiple fields or implement complex comparison logic as needed.

Conclusion

Sorting arrays is a fundamental skill in C++ programming, and understanding various sorting algorithms can significantly enhance your problem-solving abilities. While simple algorithms like Bubble Sort, Selection Sort, and Insertion Sort are great for learning and small datasets, more advanced algorithms like Quick Sort and Merge Sort offer better performance for larger arrays.

For most practical applications, using the C++ Standard Library's std::sort function is recommended due to its efficiency and ease of use. However, understanding the underlying principles of sorting algorithms remains valuable for any C++ developer.

Remember, the choice of sorting algorithm can greatly impact your program's performance, especially when dealing with large datasets. Always consider the nature of your data and the specific requirements of your application when selecting a sorting method.

Happy coding, and may your arrays always be perfectly sorted! 🚀📊