One-dimensional arrays in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an integer array named numbers with size 5.

Java

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

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

A0

B10

C5

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 as size which creates an empty array.

Forgetting to specify the size inside the brackets.

The code declares an integer array named numbers with size 5, which means it can hold 5 integer values.

2fill in blank

medium

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

Java

numbers[[1]] = 10;

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

A0

B1

C5

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 as the first index which causes off-by-one errors.

Using negative indices which are invalid.

Array indices in Java start at 0, 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 numbers.

Java

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

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

A.length

B.size()

C.length()

D.count

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses like a method call (e.g., length()).

Using methods like size() which are for collections, not arrays.

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

4fill in blank

hard

Fill both blanks to create an array of strings named fruits with 3 elements and assign "Apple" to the first element.

Java

String[] fruits = new String[[1]];
fruits[[2]] = "Apple";

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

A3

B1

C0

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 as the first index instead of 0.

Creating an array with wrong size.

The array fruits is created with size 3, and the first element at index 0 is assigned "Apple".

5fill in blank

hard

Fill all three blanks to create an integer array named scores, assign 100 to the last element, and print it.

Java

int[] scores = new int[[1]];
scores[[2]] = 100;
System.out.println(scores[3]);

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

A4

B3

C[scores.length - 1]

D[3]

Attempts:

3 left

💡 Hint

Common Mistakes

Using an index equal to the size which causes an error.

Printing the array variable without index.

The array scores has size 4, the last element is at index 3, and it is accessed using scores[scores.length - 1] to print the last element.

Practice

1.

What is the starting index of a one-dimensional array in Java?

easy

A. 1

B. 0

C. -1

D. Depends on the array size

Solution

1. Step 1: Understand Java array indexing

   Java arrays always start indexing from 0, meaning the first element is at index 0.

2. Step 2: Confirm the starting index

   Since the first element is at index 0, the starting index is 0 regardless of array size.

3. Final Answer:

   0 -> Option B

4. Quick Check:

   Array index starts at 0 [OK]

Hint: Remember, Java arrays always start at zero index [OK]

Common Mistakes:

• Thinking arrays start at 1
• Assuming negative indexes
• Confusing with other languages

2.

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

?

easy

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

B. int arr = new int[5];

C. int arr[] = new int();

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

Solution

1. Step 1: Check array declaration syntax

   In Java, arrays are declared with type[], for example int[]. The new keyword creates the array with size.

2. Step 2: Validate each option

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

3. Final Answer:

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

4. Quick Check:

   Correct array declaration uses new int[size] [OK]

Hint: Use 'type[] name = new type[size];' for arrays [OK]

Common Mistakes:

• Missing new keyword
• Wrong brackets placement
• Using parentheses instead of brackets

3.

What will be the output of the following Java code?

int[] numbers = {10, 20, 30, 40};
System.out.println(numbers[2]);

medium

A. 40

B. 20

C. 30

D. ArrayIndexOutOfBoundsException

Solution

1. Step 1: Identify the array elements and indexes

   The array 'numbers' has elements: index 0=10, 1=20, 2=30, 3=40.

2. Step 2: Access the element at index 2

   numbers[2] refers to the third element, which is 30.

3. Final Answer:

   30 -> Option C

4. Quick Check:

   Index 2 value is 30 [OK]

Hint: Remember array index starts at 0, so 2 means third element [OK]

Common Mistakes:

• Confusing index 2 with 1 or 3
• Expecting 40 instead of 30
• Thinking index starts at 1

4.

Find the error in the following code snippet:

int[] arr = new int[3];
arr[3] = 10;
System.out.println(arr[3]);

medium

A. Index 3 is out of bounds for array of size 3

B. Missing initialization of array elements

C. Array size should be 4

D. Syntax error in array declaration

Solution

1. Step 1: Check array size and valid indexes

   The array size is 3, so valid indexes are 0, 1, and 2.

2. Step 2: Identify the invalid index usage

   Accessing arr[3] is invalid and causes ArrayIndexOutOfBoundsException.

3. Final Answer:

   Index 3 is out of bounds for array of size 3 -> Option A

4. Quick Check:

   Array indexes go from 0 to size-1 [OK]

Hint: Array indexes must be less than size [OK]

Common Mistakes:

• Using index equal to size
• Assuming array auto-expands
• Ignoring runtime exceptions

5.

Given an integer array arr of size 5 initialized as {2, 4, 6, 8, 10}, which code snippet correctly doubles each element's value?

?

hard

A. for(int i = 0; i < arr.length; i++) { arr[i] *= 2; }

B. for(int i = 0; i <= arr.length; i++) { arr[i] = arr[i] * 2; }

C. for(int i = 1; i < arr.length; i++) { arr[i] = arr[i] * 2; }

D. for(int i = 0; i < arr.length - 1; i++) { arr[i] = arr[i] * 2; }

Solution

1. Step 1: Understand array length and loop bounds

   The array length is 5, so valid indexes are 0 to 4. Loop must run from 0 to less than length.

2. Step 2: Evaluate each option's loop condition

   for(int i = 0; i <= arr.length; i++) { arr[i] = arr[i] * 2; } uses i <= arr.length which causes out of bounds error at i=5. for(int i = 0; i < arr.length; i++) { arr[i] *= 2; } correctly uses i < arr.length. for(int i = 1; i < arr.length; i++) { arr[i] = arr[i] * 2; } starts at 1, missing index 0. for(int i = 0; i < arr.length - 1; i++) { arr[i] = arr[i] * 2; } stops at length-1, missing last element.

3. Final Answer:

   for(int i = 0; i < arr.length; i++) { arr[i] *= 2; } -> Option A

4. Quick Check:

   Loop from 0 to length-1 doubles all elements [OK]

Hint: Use i < arr.length to avoid index errors [OK]

Common Mistakes:

• Using <= arr.length causing errors
• Starting loop at 1 missing first element
• Stopping loop early missing last element