C++ Arrays

An array is a collection of elements of the same type placed in contiguous memory locations. It allows you to store multiple values under a single name and access them using an index.
Arrays are one of the most basic and commonly used data structures in C++ programming

#include <iostream>
using namespace std;

int main() {
    // declaring and initializing an array of size 5
    int arr[5] = {2, 4, 8, 12, 16};

    // printing array elements
    for (int i = 0; i < 5; i++) {
        cout << arr[i] << " ";
    }

    return 0;
}

2 4 8 12 16 

Explanation:

• int arr[5] declares an array of 5 integers.
• The elements are initialized with {2, 4, 8, 12, 16}.
• The for loop is used to iterate over the array and print each element.
• Array indices in C++ start from 0, so arr[0] refers to the first element, and arr[4] refers to the last one in this case.

Create an Array

We can create/declare an array by simply specifying the data type first and then the name of the array with its size inside [] square brackets(better known as array subscript operator).

data_type array_name [size]

This statement will create an array with name array_name that can store size elements of given data_type. Once the array is declared, its size cannot be changed.

Example:

int arr[5];

This will create an array with name arr that can store 5 integers.

When we declared an array, the elements of array do not contain any valid value.

Initialize the Array

Initialization means assigning initial values to array elements. We can initialize the array with values enclosed in curly braces '{}' are assigned to the array. For example:

int arr[5] = {2, 4, 8, 12, 16};

These values will be assigned sequentially. It means that the first element (index 0) will be 10, second will be 20, and so on. The number of values in the list cannot be more than the size of the array. But they can be less that the size. This is called partial initialization.

int arr[5] = {2, 4, 8};

The size of the array can be skipped if the size should be same as the number of values.

int arr[] = {2, 4, 8, 12, 16};

Moreover, all the elements can be easily initialized to 0 as shown below:

int arr[5] = {0};

This method only works for 0, but not for any other value.

Note: The value assigned should be of the same type of the array elements specified in the declaration

Access Array Elements

Elements of an array can be accessed by their position (called index) in the sequence. In C++, indexes of an array starts from 0 instead of 1. We just have to pass this index inside the [] square brackets with the array name as shown:

array_name[index];

It is important to note that index cannot be negative or greater than size of the array minus 1. (0 ≤ index ≤ size - 1). Also, it can also be any expression that results in valid index value.

Example:

#include <iostream>
using namespace std;

int main() {
    int arr[] = {2, 4, 8, 12, 16};
    
    // Accessing fourth element
    cout << arr[3] << " ";
    
    // Accessing first element
    cout << arr[0];
    
    return 0;
}

12 2

Update Array Elements

To change the element at a particular index in an array, just use the = assignment operator with new value as right hand expression while accessing the array element.

array_name[index] =  value;

Example:

#include <iostream>
using namespace std;

int main() {
    int arr[] = {2, 4, 8, 12, 16};
    
    // Updating first element
    arr[0] = 90;
    cout << arr[0] << endl;
    
    return 0;
}

90

Traverse Array

Traversing means visiting each element one by one. The advantage of array is that it can be easily traversed by using a loop with loop variable that runs from 0 to size - 1. We use this loop variable as index of the array and access each element one by one sequentially.

Example:

#include <iostream>
using namespace std;

int main() {
    int arr[5] = {2, 4, 8, 12, 16};
    
    // Traversing and printing arr
    for (int i = 0; i < 5; i++)
        cout << arr[i] << " ";
    
    return 0;
}

2 4 8 12 16 

There are more methods to traverse array that are listed here - Traverse an Array in C++

Size of Array

The size of the array refers to the number of elements that can be stored in the array. The array does not contain the information about its size but we can extract the size using sizeof() operator.

#include <iostream>
using namespace std;

int main() {
    char arr[] = {'a', 'b', 'c', 'd', 'f'};

    // Size of one element of an array
    cout << "Size of arr[0]: " << sizeof(arr[0])
    << endl;

    // Size of  'arr'
    cout << "Size of arr: " << sizeof(arr) << endl;

    // Length of an array
    int n = sizeof(arr) / sizeof(arr[0]);

    cout << "Length of an array: " << n << endl;

    return 0;
}

Size of arr[0]: 1
Size of arr: 5
Length of an array: 5

To know more methods, refer to the article - Length of Array in C++

Arrays and Pointers

In C++, arrays and pointers are closely related to each other. The array name can be treated as a constant pointer that stored the memory address of the first element of the array.

#include <iostream>
using namespace std;

int main() {
    int arr[5];
    
    // Printing array name
    cout << arr << endl;
    
    // Printing address of first element
    cout << &arr[0];

    return 0;
}

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Internally, arrays operators are performed using pointer arithmetic. So, we can do almost any array operation using by using pointer to the first element. For example, we can access all the elements of an array using pointer to the first element.

#include <iostream>
using namespace std;

int main() {
    int arr[] = {2, 4, 8, 12, 16};

    // Define a pointer to first element
    int* ptr = arr;

    for (int i = 0; i < 5; i++)
        cout << *(ptr + i) << " ";

    return 0;
}

2 4 8 12 16 

Refer to this article to know more - Relationship Between Pointer and Array

Pass Array to Function

Arrays are passed to functions using pointers, as the array name decays to a pointer to the first element. So, we also need to pass the size of the array to the function.

#include <iostream>
using namespace std;

// Function that takes array as argument
void printArray(int arr[], int n) {
    for (int i = 0; i < n; i++)
        cout << arr[i] << " ";
}

int main() {
    int arr[] = {2, 4, 8, 12, 16};
    int n = sizeof(arr) / sizeof(arr[0]);
    
    // Passing array
    printArray(arr, n);
    return 0;
}

2 4 8 12 16 

There are also other ways to pass array to functions. Refer to the article - Pass Array to Functions

Multidimensional Arrays

In the above examples, we saw 1D (one dimensional) array. This array's size can only increase in a single direction (called dimension). C provides the feature to have as many dimensions as desired for an array. Arrays declared with more than one dimension are called multidimensional arrays.

Create Multidimensional Array

data_type array_name [size1][size2]...

where size1, size2 ... are the sizes of each dimension.

The complexity of the array operations increases exponentially with increase in dimensions.
Some commonly used multidimensional arrays are:

• Two-Dimensional Array: It is an array that has exactly two dimensions. It can be visualized in the form of rows and columns organized in a two-dimensional plane.
• Three-Dimensional Array: A 3D array has exactly three dimensions. It can be visualized as a collection of 2D arrays stacked on top of each other to create the third dimension.

Properties of Arrays

• An array is a collection of data of the same data type, stored at a contiguous memory location.
• Indexing of an array starts from 0. It means the first element is stored at the 0^th index, the second at 1^st, and so on.
• Elements of an array can be accessed using their indices.
• Once an array is declared its size remains constant throughout the program.
• An array can have multiple dimensions.
• The size of the array in bytes can be determined by the sizeof operator using which we can also find the number of elements in the array.
• We can find the size of the type of elements stored in an array by subtracting adjacent addresses.

Array Practice Problems

The below practice problems provide you some commonly used scenarios and exercises for practicing arrays in C++:

• Print Array in Reverse
• Count the Zeros in an Array
• Search an Element in an Array
• First Repeating Element
• Sum of All Array Elements
• Count Smaller Than X
• Largest Element in Array

Introduction to Arrays in C++

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Introduction to Arrays in C++

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