How to Dynamically Resize a C++ Array?

In C++, an array is a collection of elements of the same type placed in contiguous memory locations. In this article, we will learn how to dynamically resize an array in C++.

Example:

Input: 
myArray = {10, 20, 30, 40, 50};
Output:
Resized array: 10 20 30 40 50 0 0 0 0 0 

Resizing a Dynamic Array in C++

In C++, the size of an array is static, but we can also create a dynamic array in C++ using the new and delete operators. However, there is no direct method to specifically resize these dynamic arrays. To do that, we have to allocate a new memory block of the new size, copy all the elements of the previous array, and then delete the previous array.

Approach

• Create a new array with the desired size.
• Use the std::copy function from the <algorithm> library to copy the elements from the old array to the new one.
• When all the elements are copied, delete the old array to free up the memory it was using.
• Finally, assign the pointer of the old array to the new array.

C++ Program to Resize a Dynamic Array in C++

The below example demonstrates how we can dynamically resize an array in C++.

// C++ Program to illustrate how to dynamically resize an
// array
#include <algorithm>
#include <iostream>
using namespace std;

int main()
{
    // Initialize an int array
    int* arr = new int[5]{ 10, 20, 30, 40, 50 };
    int size = 5;

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

    // create new array of the new size
    int newSize = 10;
    int* newArr = new int[newSize];

    // copy all the elements
    copy(arr, arr + size, newArr);

    // deallocate the previous array and assing the pointer
    // to the previous array to the new one
    delete[] arr;
    arr = newArr;

    // printing array
    cout << "Resized array: ";
    for (int i = 0; i < newSize; i++) {
        cout << arr[i] << " ";
    }
    cout << endl;

    return 0;
}

Original array: 10 20 30 40 50 
Resized array: 10 20 30 40 50 0 0 0 0 0 

Time Complexity: O(N)
Auxiliary Space: O(N), here N is the new size after resizing

Note: The above method is not recommended if you want to perform frequent resizing because it has high time complexity. For such cases use std::vector which handles resizing automatically.