Mastering C++ Arrays: Declaration, Initialization, and Practical Examples

C++ Arrays

This tutorial covers C++ arrays—how to declare, initialize, access, and manipulate them—using practical code examples.

In C++, an array is a single variable that stores a fixed-size sequence of elements of the same type. For instance, a class with 27 students might store their grades in one array:

double grade[27];

Here, grade can hold up to 27 double values. Once an array is declared, its size and element type are immutable.

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C++ Array Declaration

dataType arrayName[arraySize];

For example:

int x[6];

Explanation:

• int – element type
• x – array name
• 6 – number of elements

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Accessing Elements in a C++ Array

Each array element is identified by an index, starting at 0. You can retrieve or modify an element using:

// syntax to access array elements
array[index];

Consider the array x above.

Key Points

• Indices begin at 0 (e.g., x[0] is the first element).
• For an array of size n, the last index is n-1 (here x[5]).
• Array elements are stored in consecutive memory addresses. If x[0] starts at 2120d, then x[1] is at 2124d, x[2] at 2128d, etc., because an int occupies 4 bytes.

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C++ Array Initialization

Arrays can be initialized at declaration:

// declare and initialize an array
int x[6] = {19, 10, 8, 17, 9, 15};

Alternatively, let the compiler deduce the size:

// declare and initialize an array
int x[] = {19, 10, 8, 17, 9, 15};

If fewer elements are provided than the array size, the remaining elements are zero‑initialized in C++.

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Inserting and Printing Array Elements

int mark[5] = {19, 10, 8, 17, 9};

// change 4th element to 9
mark[3] = 9;

// take input from the user and store it at third position
cin >> mark[2];

// take input from the user and insert at ith position
cin >> mark[i-1];

// print first element of the array
cout << mark[0];

// print ith element of the array
cout >> mark[i-1];

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Example 1: Displaying Array Elements

#include <iostream>
using namespace std;

int main() {
    int numbers[5] = {7, 5, 6, 12, 35};

    cout << "The numbers are: ";

    // Print array elements using a range‑based for loop
    for (const int &n : numbers) {
        cout << n << "  ";
    }

    cout << "\nThe numbers are: ";

    // Print array elements using a traditional for loop
    for (int i = 0; i < 5; ++i) {
        cout << numbers[i] << "  ";
    }

    return 0;
}

Output

The numbers are: 7  5  6  12  35
The numbers are: 7  5  6  12  35

Both loops produce the same result. The range‑based loop is preferred when you only need to read values, as it avoids unnecessary copying.

Note: Using const int &n is more efficient than int n because it passes elements by reference and prevents accidental modification.

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Example 2: Reading User Input into an Array

#include <iostream>
using namespace std;

int main() {
    int numbers[5];

    cout << "Enter 5 numbers: " << endl;

    // Store user input into the array
    for (int i = 0; i < 5; ++i) {
        cin >> numbers[i];
    }

    cout << "The numbers are: ";

    // Print the array elements
    for (int n = 0; n < 5; ++n) {
        cout << numbers[n] << "  ";
    }

    return 0;
}

Output

Enter 5 numbers: 
11
12
13
14
15
The numbers are: 11  12  13  14  15

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Example 3: Sum and Average of Array Elements

#include <iostream>
using namespace std;

int main() {
    double numbers[] = {7, 5, 6, 12, 35, 27};

    double sum = 0;
    double count = 0;
    double average;

    cout << "The numbers are: ";

    // Print elements and calculate sum using a range‑based for loop
    for (const double &n : numbers) {
        cout << n << "  ";
        sum += n;
        ++count;
    }

    cout << "\nTheir Sum = " << sum << endl;

    average = sum / count;
    cout << "Their Average = " << average << endl;

    return 0;
}

Output

The numbers are: 7  5  6  12  35  27
Their Sum = 92
Their Average = 15.3333

This example demonstrates how a range‑based loop simplifies traversal when the array size is not known at compile time.

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C++ Array Out‑of‑Bounds Access

Arrays with size 10 expose valid indices from 0 to 9. Accessing index 10 or higher triggers undefined behavior, which may manifest as crashes or corrupted data.