Array initialization in C++

Introduction

We use array initialization to create a list of values stored together in memory. It helps us organize data so we can use it easily later.

When you want to store multiple numbers or items of the same type together.

When you know the values in advance and want to set them at the start.

When you want to create an empty array with a fixed size to fill later.

When you want to quickly access elements by their position number.

When you want to loop through a group of values efficiently.

Syntax

C++

class ArrayExample {
public:
    int numbers[5];  // Declare an array of 5 integers

    void initialize() {
        // Method 1: Initialize with values at declaration
        int fixedNumbers[5] = {1, 2, 3, 4, 5};

        // Method 2: Initialize all elements to zero
        int zeros[5] = {};

        // Method 3: Initialize some elements, rest are zero
        int partial[5] = {10, 20};

        // Method 4: Initialize using a loop
        for (int i = 0; i < 5; i++) {
            numbers[i] = i * 10;
        }
    }
};

Arrays have fixed size, so you must decide how many elements it holds when you declare it.

If you initialize fewer elements than the size, the rest are set to zero automatically.

Examples

This creates an array of 3 integers all set to zero.

C++

int emptyArray[3] = {};

An array with one element initialized to 42.

C++

int oneElement[1] = {42};

Array with first element 5, last element 10, and middle elements zero.

C++

int startEnd[4] = {5, 0, 0, 10};

Initialize array elements using a loop, values 1 to 5.

C++

int loopArray[5];
for (int i = 0; i < 5; i++) {
    loopArray[i] = i + 1;
}

Sample Program

This program shows two ways to initialize arrays: directly with values and by filling with a loop. It prints both arrays to show the results.

C++

#include <iostream>

int main() {
    // Declare and initialize an array with values
    int fixedNumbers[5] = {1, 2, 3, 4, 5};

    // Declare an empty array and fill it with a loop
    int loopNumbers[5];
    for (int index = 0; index < 5; index++) {
        loopNumbers[index] = (index + 1) * 10;
    }

    // Print fixedNumbers
    std::cout << "fixedNumbers: ";
    for (int index = 0; index < 5; index++) {
        std::cout << fixedNumbers[index] << " ";
    }
    std::cout << std::endl;

    // Print loopNumbers
    std::cout << "loopNumbers: ";
    for (int index = 0; index < 5; index++) {
        std::cout << loopNumbers[index] << " ";
    }
    std::cout << std::endl;

    return 0;
}

Important Notes

Time complexity to initialize with a loop is O(n), where n is the array size.

Space complexity is O(n) because the array stores n elements.

Common mistake: forgetting to initialize elements, which can leave garbage values if not zero-initialized.

Use direct initialization when values are known; use loops when values depend on calculations.

Summary

Arrays hold multiple values of the same type in a fixed size.

You can initialize arrays directly with values or fill them later using loops.

Uninitialized elements default to zero if you use empty braces {}.