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What happens Array is not initialized?

Last Updated : 16 Apr, 2024
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As the array is an important data structure for the coding perspective, there are some conditions that need to be careful. As in any language, there are two different methods to initialize the array:

  • While declaring the array
  • Any time after declaring the array

Shown below are the two ways of initializing an array

Initializing the array at the time of declaration:

Syntax

int arr[] = {1, 2, 3, 4, 5};

Initializing the array after declaration:

Syntax

int arr[5];
for (int i = 0; i < 5; i++)
    arr[i] = i + 1;

Why is there a need to initialize an Array?

If the array is not initialized at the time of declaration or any time after that then it will contain some random values in each memory position. These random values can be of two types: 

1. Default values.

1.1 If the array elements are of object types then the default value is null.

C++
#include <iostream>
#include <vector>

int main() {
    std::vector<int> array(5); // Creating a vector of size 5

    // Iterate over the elements of the vector
    for (int p : array) {
        std::cout << p << " "; // Output each element
    }

    return 0;
}
//This code is contributed by Aman
Java
import java.io.*;

class GFG {
    public static void main(String[] args)
    {
        Integer[] array = new Integer[5];
        for (Integer p : array) {
            System.out.print(p + " ");
        }
    }
}
Python3
# code
arr = [None]*5
print(arr)

#this code is contributed by Akshay Tripathi(akshaytripathi630)
C#
using System;
using System.Collections.Generic;

class Program
{
    static void Main(string[] args)
    {
        List<int> array = new List<int>(5); // Creating a list of size 5

        // Iterate over the elements of the list
      // no output would be generated in c# because array is not initaliazed with any value.
        foreach (int p in array)
        {
            Console.Write(p + " "); // Output each element
        }

        // Ensure the console window stays open in debug mode
        Console.ReadKey();
    }
}
//This code is contributed by Prachi.
JavaScript
let array = new Array(5).fill(0); // Creating an array of size 5 and filling it with zeros

// Iterate over the elements of the array
array.forEach(function (p) {
    process.stdout.write(p + " "); // Output each element on the same line
});

Output
0 0 0 0 0 

1.2. If the array elements are of primitive data types then the default value is 0.

C++
#include <iostream>

int main() {
    int array[5] = {0}; // Initializing an array of size 5 with default values

    for (int p : array) {
        std::cout << p << " "; // Output each element
    }

    return 0;
}
Java
import java.io.*;

class GFG {
    public static void main(String[] args)
    {
        int[] array = new int[5];
        for (int p : array) {
            System.out.print(p + " ");
        }
    }
}
Python3
# code
arr = [0]*5
print(arr)

#this code is contributed by Akshay Tripathi(akshaytripathi630)
C#
using System;

public class GFG
{
   static public void Main (){
        int[] array = new int[5];
        foreach (int p in array)
        {
            Console.Write(p + " ");
        }
    }
}

// This code is contributed by akashish__
Javascript
function main() {
    let array = new Array(5).fill(0);
    output = "";
    for (let p of array) {
        output += p + " ";
    }
    console.log(output);
}

// Run the main function
main();

Output
0 0 0 0 0 

2. Garbage value:

C++
#include <bits/stdc++.h>
using namespace std;

int main()
{
    int array[5];
    for (int p : array)
        cout << p << " ";
    return 0;
}
Java
import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        int[] array = new int[5];
        for (int p : array)
            System.out.print(p + " ");
    }
}
//This code is contributed by Adarsh
Python
if __name__ == "__main__":
    array = [0] * 5
    for p in array:
        print p,
#this code is contributed by Adarsh
C#
using System;

class Program
{
    static void Main(string[] args)
    {
        int[] array = new int[5];
        foreach (int p in array)
            Console.Write(p + " ");
    }
}
//This code is contributed by Monu.
JavaScript
// Main class
class Main {
    // Main method
    static main() {
        let array = new Array(5); // Create an array of size 5
        for (let p of array) // Iterate over each element in the array
            console.log(p + " "); // Print the value of each element
    }
}

// Call the main method
Main.main();

Output
17 0 0 0 4196464 

Default values in Javascript

C++
#include <iostream>

int main() {
    int arr[5] = {0}; // Initialize array with zeros

    // Iterate over the array elements
    for (int x : arr) {
        std::cout << x << std::endl; // Print each element
    }

    return 0;
}
//this code is contributed by Utkarsh.
Java
public class Main {
    public static void main(String[] args) {
        int[] arr = new int[5];

        // Iterate over the array elements
        for (int x : arr) {
            System.out.println(x); // Print each element
        }
    }
}
Python3
arr = [None] * 5  # Define array without initializing it

# Iterate over the array elements
for x in arr:
    print(x)  # Print each element
Javascript
const arr = new Array(5); // Declare array without initializing it

// Iterate over the array elements
arr.forEach(x => {
    console.log(x); // Print each element
});

Output:

undefined
undefined
undefined
undefined
undefined




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