Common array operations in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an integer array of size 5.

Java

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

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

A1

B10

C0

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 as size causes an array of length zero.

Using 1 or 10 creates arrays of wrong size.

The code declares an integer array named numbers with 5 elements.

2fill in blank

medium

Complete the code to access the third element of the array.

Java

int third = numbers[[1]];

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

A2

B3

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3 causes an ArrayIndexOutOfBoundsException.

Using 1 accesses the second element, not the third.

Array indexes start at 0, so the third element is at index 2.

3fill in blank

hard

Fix the error in the code to correctly print all elements of the array.

Java

for (int i = 0; i < numbers.[1]; i++) {
    System.out.println(numbers[i]);
}

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

Alength()

Bsize()

Clength

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using length() causes a compile error.

Using size() or count are invalid for arrays.

In Java, arrays use length without parentheses to get their size.

4fill in blank

hard

Fill both blanks to create an array of squares for numbers 1 to 5.

Java

int[] squares = new int[5];
for (int [1] = 0; [2] < 5; [1]++) {
    squares[[1]] = ([1] + 1) * ([1] + 1);
}

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

Ai

Bj

Ck

Dx

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for initialization and condition causes errors.

Using variables not declared in the loop.

The variable i is used as the loop counter to fill the array.

5fill in blank

hard

Fill all three blanks to create a new array with elements doubled from the original.

Java

int[] doubled = new int[numbers.[1]];
for (int [2] = 0; [2] < numbers.length; [2]++) {
    doubled[[2]] = numbers[[2]] [3] 2;
}

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

Alength

Bi

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '*' changes the operation.

Using wrong variable names causes errors.

The new array size matches the original's length. The loop uses i as index, and each element is multiplied by 2.

Practice

1. Which of the following correctly declares an array of integers with 5 elements in Java?

easy

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

B. int numbers = new int[5];

C. int numbers[] = new int;

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

Solution

1. Step 1: Understand Java array declaration syntax

   In Java, arrays are declared with the type followed by square brackets, then the variable name, and initialized with 'new' and size.

2. Step 2: Check each option for correct syntax

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

3. Final Answer:

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

4. Quick Check:

   Correct array declaration = int[] numbers = new int[5]; [OK]

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

Common Mistakes:

• Missing square brackets after type
• Not using 'new' keyword
• Incorrect variable initialization

2. Which of the following is the correct way to access the third element of an array named arr in Java?

easy

A. arr(3);

B. arr[2];

C. arr[3];

D. arr.get(2);

Solution

1. Step 1: Recall Java array indexing

   Java arrays use zero-based indexing, so the first element is at index 0, second at 1, third at 2.

2. Step 2: Identify correct syntax for accessing elements

   Elements are accessed with square brackets []. So arr[2] accesses the third element. Options A and D use incorrect syntax, and C accesses the fourth element.

3. Final Answer:

   arr[2]; -> Option B

4. Quick Check:

   Third element index = 2 [OK]

Hint: Arrays start at 0, so third element is arr[2] [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Using 1-based indexing
• Trying to use methods like get() on arrays

3. What is the output of the following Java code?

int[] nums = {1, 2, 3, 4, 5};
int sum = 0;
for (int i = 0; i < nums.length; i++) {
    sum += nums[i];
}
System.out.println(sum);

medium

A. 15

B. 10

C. 5

D. 0

Solution

1. Step 1: Understand the array and loop

   The array nums contains 1, 2, 3, 4, 5. The loop runs from i=0 to i=4 (nums.length is 5).

2. Step 2: Calculate the sum of all elements

   sum = 0 + 1 + 2 + 3 + 4 + 5 = 15.

3. Final Answer:

   15 -> Option A

4. Quick Check:

   Sum of 1 to 5 = 15 [OK]

Hint: Sum array by adding all elements in a loop [OK]

Common Mistakes:

• Off-by-one error in loop
• Starting sum at wrong value
• Using wrong loop condition

4. The following code is intended to print all elements of the array data, but it causes a runtime error. What is the error?

int[] data = {10, 20, 30};
for (int i = 1; i <= data.length; i++) {
    System.out.println(data[i]);
}

medium

A. ArrayIndexOutOfBoundsException because loop goes beyond last index

B. Syntax error due to wrong for loop syntax

C. Prints all elements correctly

D. NullPointerException because array is not initialized

Solution

1. Step 1: Analyze loop indices and array length

   The array has length 3, valid indices are 0, 1, 2. The loop runs i from 1 to 3 inclusive.

2. Step 2: Identify index causing error

   When i = 3, data[3] is accessed, which is out of bounds and causes ArrayIndexOutOfBoundsException.

3. Final Answer:

   ArrayIndexOutOfBoundsException because loop goes beyond last index -> Option A

4. Quick Check:

   Loop index must be less than length [OK]

Hint: Loop indices must be 0 to length-1 for arrays [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Starting loop at 1 instead of 0
• Ignoring zero-based indexing

5. Given an integer array arr, which code snippet correctly counts how many elements are greater than 10?

hard

A. int count = 0;
for (int i = 0; i < arr.length; i++) {
if (arr[i] >= 10) {
count++;
}
}
System.out.println(count);

B. int count = 0;
for (int i = 1; i <= arr.length; i++) {
if (arr[i] > 10) {
count++;
}
}
System.out.println(count);

C. int count = 0;
for (int i = 0; i < arr.length; i++) {
if (arr[i] > 10) {
count++;
}
}
System.out.println(count);

D. int count = 0;
for (int i = 0; i < arr.length; i++) {
if (arr[i] < 10) {
count++;
}
}
System.out.println(count);

Solution

1. Step 1: Understand the counting condition

   We want to count elements strictly greater than 10, so condition is arr[i] > 10.

2. Step 2: Check loop indices and condition correctness

   int count = 0; for (int i = 0; i < arr.length; i++) { if (arr[i] > 10) { count++; } } System.out.println(count); uses correct zero-based indexing and correct condition. int count = 0; for (int i = 1; i <= arr.length; i++) { if (arr[i] > 10) { count++; } } System.out.println(count); has out-of-bounds error. int count = 0; for (int i = 0; i < arr.length; i++) { if (arr[i] >= 10) { count++; } } System.out.println(count); counts elements >= 10 (includes 10). int count = 0; for (int i = 0; i < arr.length; i++) { if (arr[i] < 10) { count++; } } System.out.println(count); counts elements less than 10, which is wrong.

3. Final Answer:

   int count = 0; for (int i = 0; i < arr.length; i++) { if (arr[i] > 10) { count++; } } System.out.println(count); -> Option C

4. Quick Check:

   Count elements with arr[i] > 10 using correct loop [OK]

Hint: Use i < arr.length and condition arr[i] > 10 to count [OK]

Common Mistakes:

• Using <= instead of < in loop
• Using wrong comparison operator
• Starting loop at 1 causing index errors