Array Declaration and Initialization in DSA Java - Build from Scratch

Array Declaration and Initialization

📖 Scenario: You are helping a small grocery store keep track of the prices of some fruits they sell. You will create an array to store these prices.

🎯 Goal: Create an array to hold the prices of fruits, set a variable for the number of fruits, fill the array with prices, and then print the prices.

📋 What You'll Learn

Create an integer array called fruitPrices with exactly 5 elements.

Create an integer variable called numberOfFruits and set it to 5.

Assign the prices 10, 15, 7, 20, and 12 to the fruitPrices array in order.

Use a for loop with variable i to print each price in fruitPrices on its own line.

💡 Why This Matters

🌍 Real World

Arrays are used to store lists of related data, like prices of items in a store, so you can easily access and update them.

💼 Career

Understanding arrays is fundamental for programming jobs, as they are used in many applications to manage collections of data efficiently.

1

Declare the array fruitPrices

Create an integer array called fruitPrices with exactly 5 elements.

DSA Java

// Declare an integer array called fruitPrices with 5 elements
// Your code here

Hint

Use int[] fruitPrices = new int[5]; to create an array of 5 integers.

2

Create the variable numberOfFruits

Create an integer variable called numberOfFruits and set it to 5.

DSA Java

int[] fruitPrices = new int[5];
// Create an integer variable numberOfFruits and set it to 5
// Your code here

Hint

Use int numberOfFruits = 5; to create the variable.

3

Assign prices to the fruitPrices array

Assign the prices 10, 15, 7, 20, and 12 to the fruitPrices array in order using their indexes.

DSA Java

int[] fruitPrices = new int[5];
int numberOfFruits = 5;
// Assign prices 10, 15, 7, 20, 12 to fruitPrices array
// Your code here

Hint

Use fruitPrices[index] = value; to assign each price.

4

Print all prices using a for loop

Use a for loop with variable i from 0 to numberOfFruits - 1 to print each price in fruitPrices on its own line.

DSA Java

int[] fruitPrices = new int[5];
int numberOfFruits = 5;
fruitPrices[0] = 10;
fruitPrices[1] = 15;
fruitPrices[2] = 7;
fruitPrices[3] = 20;
fruitPrices[4] = 12;
// Use a for loop to print each price in fruitPrices
// Your code here

Hint

Use for (int i = 0; i < numberOfFruits; i++) and System.out.println(fruitPrices[i]); inside the loop.

Practice

1. Which of the following is the correct way to declare and initialize an integer array of size 5 in Java?

easy

A. int[] arr = new int[5];

B. int arr = new int[5];

C. int arr[] = int[5];

D. int[] arr = int[5];

Solution

1. Step 1: Understand Java array declaration syntax

   In Java, arrays are declared with the type followed by square brackets and initialized with the new keyword and size inside square brackets.

2. Step 2: Check each option for correct syntax

   int[] arr = new int[5]; uses correct syntax: int[] arr = new int[5]; Options A, B, and D have syntax errors or missing keywords.

3. Final Answer:

   int[] arr = new int[5]; -> Option A

4. Quick Check:

   Correct Java array declaration = int[] arr = new int[5]; [OK]

Hint: Use 'new' keyword and specify size in brackets [OK]

Common Mistakes:

• Forgetting 'new' keyword
• Using wrong brackets or missing type
• Declaring array without brackets

2. Which of the following is a valid way to declare and initialize an array with values in Java?

easy

A. int[] arr = new int[] {1, 2, 3, 4};

B. int arr[] = new int{1, 2, 3, 4};

C. int[] arr = new int{1, 2, 3, 4};

D. int arr = {1, 2, 3, 4};

Solution

1. Step 1: Understand array initialization with values

   In Java, to initialize an array with values using 'new', syntax is: int[] arr = new int[] {values};

2. Step 2: Check each option

   int[] arr = new int[] {1, 2, 3, 4};: correct syntax. int arr[] = new int{1, 2, 3, 4};: incorrect declaration style and missing [] after new int. int[] arr = new int{1, 2, 3, 4};: missing [] after new int. int arr = {1, 2, 3, 4};: arr not declared as array.

3. Final Answer:

   int[] arr = new int[] {1, 2, 3, 4}; -> Option A

4. Quick Check:

   Array init with 'new int[]' = int[] arr = new int[] {1, 2, 3, 4}; [OK]

Hint: Use 'new int[]' when initializing separately [OK]

Common Mistakes:

• Omitting 'new' when required
• Using wrong brackets
• Assigning values without array type

3. What will be the output of the following Java code?

int[] arr = new int[3];
arr[0] = 5;
arr[2] = 10;
System.out.println(arr[1]);

medium

A. 5

B. 0

C. 10

D. Compilation error

Solution

1. Step 1: Understand default values in integer arrays

   When an integer array is created with 'new int[3]', all elements are initialized to 0 by default.

2. Step 2: Check assigned values and output

   arr[0] = 5 and arr[2] = 10 are assigned. arr[1] is not assigned, so it remains 0. The print statement outputs arr[1], which is 0.

3. Final Answer:

   0 -> Option B

4. Quick Check:

   Unassigned int array element = 0 [OK]

Hint: Unassigned int array elements default to 0 [OK]

Common Mistakes:

• Assuming unassigned elements have garbage values
• Confusing index positions
• Expecting runtime error for unassigned index

4. Identify the error in the following Java array declaration:

int arr[5];

medium

A. Missing semicolon at the end

B. Array must be declared with curly braces

C. Array size cannot be specified in declaration without initialization

D. Array size must be a variable, not a number

Solution

1. Step 1: Understand Java array declaration rules

   In Java, you cannot specify the size of the array in the declaration alone. Size must be specified during initialization with 'new'.

2. Step 2: Analyze the given code

   int arr[5]; tries to declare an array with size 5 without initialization, which is invalid syntax in Java.

3. Final Answer:

   Array size cannot be specified in declaration without initialization -> Option C

4. Quick Check:

   Size only with 'new' keyword [OK]

Hint: Specify size only with 'new' keyword during initialization [OK]

Common Mistakes:

• Using C-style array declaration
• Trying to declare size without 'new'
• Confusing declaration and initialization

5. Given the following code, what will be the output?

int[] arr = {1, 2, 3};
arr = new int[] {4, 5};
System.out.print(arr.length);

Choose the correct output and explain why.

hard

A. 3

B. Runtime error

C. Compilation error

D. 2

Solution

1. Step 1: Understand array reassignment

   Initially, arr points to an array of length 3 with values {1, 2, 3}. Then arr is reassigned to a new array with values {4, 5} of length 2.

2. Step 2: Check the length property after reassignment

   After reassignment, arr.length reflects the new array length, which is 2.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   Array length after reassignment = 2 [OK]

Hint: Array variable can point to new array; length updates accordingly [OK]

Common Mistakes:

• Assuming original array length remains
• Expecting error on reassignment
• Confusing array contents with length