Why Arrays Exist and What Problem They Solve in DSA Java - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an array of integers with size 5.

DSA Java

int[] numbers = new int[[1]];

Drag options to blanks, or click blank then click option'

A10

B0

C5

Darray

Attempts:

3 left

💡 Hint

Common Mistakes

Using zero or a variable name instead of a number for the array size.

Forgetting to specify the size when creating the array.

The array size must be a positive integer. Here, 5 means the array can hold 5 integers.

2fill in blank

medium

Complete the code to assign the value 10 to the first element of the array.

DSA Java

numbers[[1]] = 10;

Drag options to blanks, or click blank then click option'

A1

B-1

C5

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 as the first index, which causes skipping the first element.

Using negative indexes which are invalid.

Array indexes start at 0 in Java, so the first element is at index 0.

3fill in blank

hard

Fix the error in the code to print the length of the array.

DSA Java

System.out.println(numbers[1]);

Drag options to blanks, or click blank then click option'

A.length()

B.length

C.size()

D.count

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses like a method call (length()).

Using methods like size() which are for other collections.

In Java, arrays have a field called length (without parentheses) to get their size.

4fill in blank

hard

Fill both blanks to create a loop that prints all elements in the array.

DSA Java

for (int i = [1]; i [2] numbers.length; i++) {
    System.out.println(numbers[i]);
}

Drag options to blanks, or click blank then click option'

A0

B<=

C<

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Starting the loop at 1 and missing the first element.

Using '<=' which causes an index out of bounds error.

The loop starts at index 0 and runs while i is less than the array length to avoid going out of bounds.

5fill in blank

hard

Fill all three blanks to create an array of size 3, assign values, and print the second element.

DSA Java

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

Drag options to blanks, or click blank then click option'

A3

B7

C1

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong indexes like 2 to print the second element.

Assigning values outside the array size.

The array size is 3, the first element is assigned 7, and the second element (index 1) is printed.

Practice

1. Why do arrays exist in programming languages like Java?

easy

A. To perform mathematical calculations faster

B. To store multiple values of the same type in a single variable

C. To create graphical user interfaces

D. To handle user input from the keyboard

Solution

1. Step 1: Understand the purpose of arrays

   Arrays allow storing many values of the same type together, making data management easier.

2. Step 2: Compare with other options

   Options B, C, and D describe unrelated tasks that arrays do not specifically solve.

3. Final Answer:

   To store multiple values of the same type in a single variable -> Option B

4. Quick Check:

   Arrays = store same-type values [OK]

Hint: Arrays group same-type data in one place [OK]

Common Mistakes:

• Thinking arrays perform calculations
• Confusing arrays with UI components
• Believing arrays handle user input directly

2. Which of the following is the correct way to declare an array of integers in Java?

easy

A. int numbers[] = new array();

B. array int numbers;

C. int[] numbers;

D. int numbers = new int[];

Solution

1. Step 1: Recall Java array declaration syntax

   In Java, arrays are declared with the type followed by square brackets, e.g., int[] numbers;

2. Step 2: Check each option

   int[] numbers; matches correct syntax. Options A, B, and D have syntax errors or incorrect keywords.

3. Final Answer:

   int[] numbers; -> Option C

4. Quick Check:

   Java array declaration = int[] name [OK]

Hint: Use type[] name to declare arrays in Java [OK]

Common Mistakes:

• Using 'array' keyword which doesn't exist
• Placing brackets after variable name incorrectly
• Missing 'new' keyword when initializing

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

int[] arr = new int[3];
arr[0] = 5;
arr[1] = 10;
arr[2] = 15;
System.out.println(arr[1]);

medium

A. 10

B. ArrayIndexOutOfBoundsException

C. 15

D. 5

Solution

1. Step 1: Understand array indexing

   Array indexes start at 0, so arr[1] refers to the second element.

2. Step 2: Check assigned values

   arr[1] was assigned 10, so printing arr[1] outputs 10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   arr[1] = 10 [OK]

Hint: Remember arrays start at index 0 in Java [OK]

Common Mistakes:

• Confusing index 1 with index 0
• Expecting output 5 or 15 incorrectly
• Thinking array size causes exception here

4. Identify the error in the following Java code snippet:

int[] numbers = new int[3];
numbers[3] = 7;

medium

A. Runtime error: ArrayIndexOutOfBoundsException

B. Syntax error: missing semicolon

C. Logical error: wrong variable name

D. No error, code runs fine

Solution

1. Step 1: Check array size and indexing

   The array 'numbers' has size 3, so valid indexes are 0, 1, and 2.

2. Step 2: Analyze the assignment

   Assigning to numbers[3] is out of bounds and causes a runtime exception.

3. Final Answer:

   Runtime error: ArrayIndexOutOfBoundsException -> Option A

4. Quick Check:

   Index 3 invalid for size 3 array [OK]

Hint: Array indexes go from 0 to length-1 [OK]

Common Mistakes:

• Assuming index 3 is valid for size 3 array
• Confusing syntax errors with runtime errors
• Ignoring zero-based indexing

5. You want to store the daily temperatures for a week and then calculate the average temperature. Which problem does using an array solve here?

hard

A. It prevents storing duplicate temperature values

B. It automatically calculates the average temperature

C. It sorts the temperatures from lowest to highest

D. It allows storing all temperatures in one place with easy access by day index

Solution

1. Step 1: Identify the data storage need

   We need to store multiple temperature values for each day of the week.

2. Step 2: Understand array benefits

   Arrays let us keep all values together and access each by its index (day number).

3. Step 3: Evaluate other options

   Arrays do not calculate averages, sort data, or prevent duplicates automatically.

4. Final Answer:

   It allows storing all temperatures in one place with easy access by day index -> Option D

5. Quick Check:

   Arrays = grouped storage with index access [OK]

Hint: Arrays store related data with index access [OK]

Common Mistakes:

• Thinking arrays perform calculations automatically
• Assuming arrays sort data by default
• Believing arrays prevent duplicates