Array Access and Update at Index in DSA Java - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Array Update and Access

What is the output of the following Java code snippet?

DSA Java

public class Main {
    public static void main(String[] args) {
        int[] arr = {5, 10, 15, 20};
        arr[2] = arr[1] + arr[3];
        System.out.print(arr[2] + ", " + arr[0] + ", " + arr[3]);
    }
}

A 15, 10, 20

B 25, 5, 20

C 30, 5, 20

D 30, 10, 20

💡 Hint

Remember that arr[2] is updated by adding arr[1] and arr[3].

arr[2] = arr[1] + arr[3] = 10 + 20 = 30. The output prints arr[2], arr[0], arr[3] which are 30, 5, 20 respectively.

❓ Predict Output

intermediate

Resulting Array After Multiple Updates

What is the state of the array after executing this Java code?

DSA Java

public class Main {
    public static void main(String[] args) {
        int[] nums = {1, 2, 3, 4, 5};
        nums[0] = nums[4];
        nums[4] = nums[2];
        nums[2] = nums[1];
        for (int num : nums) {
            System.out.print(num + " ");
        }
    }
}

A 5 2 1 4 3

B 5 2 3 4 1

C 1 2 3 4 5

D 5 2 2 4 3

💡 Hint

Track each assignment step carefully in order.

Step 1: nums[0] = nums[4] → nums[0] = 5 Step 2: nums[4] = nums[2] → nums[4] = 3 Step 3: nums[2] = nums[1] → nums[2] = 2 Final array: {5, 2, 2, 4, 3}

🔧 Debug

advanced

Identify the Error in Array Update

What error will this Java code produce when run?

DSA Java

public class Main {
    public static void main(String[] args) {
        int[] data = new int[3];
        data[0] = 10;
        data[1] = 20;
        data[3] = 30;
        System.out.println(data[2]);
    }
}

A NullPointerException

B ArrayIndexOutOfBoundsException

C No error, prints 0

D Compilation error

💡 Hint

Check the array size and the index used for assignment.

The array has size 3 with valid indices 0,1,2. Accessing data[3] causes ArrayIndexOutOfBoundsException.

❓ Predict Output

advanced

Output After Conditional Array Update

What is the output of this Java program?

DSA Java

public class Main {
    public static void main(String[] args) {
        int[] arr = {2, 4, 6, 8};
        for (int i = 0; i < arr.length; i++) {
            if (i % 2 == 0) {
                arr[i] = arr[i] * 2;
            } else {
                arr[i] = arr[i] + 1;
            }
        }
        for (int val : arr) {
            System.out.print(val + " ");
        }
    }
}

A 4 5 12 9

B 2 5 6 9

C 4 4 12 8

D 4 5 6 9

💡 Hint

Multiply elements at even indices by 2, add 1 to odd indices.

Index 0 (even): 2*2=4 Index 1 (odd): 4+1=5 Index 2 (even): 6*2=12 Index 3 (odd): 8+1=9 Output: 4 5 12 9

🧠 Conceptual

expert

Array Update and Access Complexity

Consider an array of size n. What is the time complexity of updating an element at a specific index and then accessing it?

A O(1) for update and O(1) for access

B O(n) for update and O(1) for access

C O(1) for update and O(n) for access

D O(n) for update and O(n) for access

💡 Hint

Think about how arrays store elements and how indexing works.

Arrays allow direct access to any index in constant time, so both update and access are O(1).

Practice

1. What does the expression arr[2] do in a Java array named arr?

easy

A. Deletes the element at index 2

B. Adds a new element at index 2

C. Accesses the element at index 2 of the array

D. Returns the length of the array

Solution

1. Step 1: Understand array indexing

   In Java, arrays use zero-based indexing, so arr[2] accesses the third element.

2. Step 2: Identify the operation

   The expression reads the value stored at index 2 without modifying the array.

3. Final Answer:

   Accesses the element at index 2 of the array -> Option C

4. Quick Check:

   arr[2] means get element at index 2 [OK]

Hint: Remember arrays start at index 0, so arr[2] is third element [OK]

Common Mistakes:

• Thinking arr[2] adds or deletes elements
• Confusing index with array length
• Assuming indexing starts at 1

2. Which of the following is the correct syntax to update the element at index 1 of an integer array nums to 10?

easy

A. nums(1) = 10;

B. nums[1] = 10;

C. nums{1} = 10;

D. nums.1 = 10;

Solution

1. Step 1: Recall Java array update syntax

   In Java, array elements are accessed and updated using square brackets: array[index] = value;

2. Step 2: Match correct syntax

   Only nums[1] = 10; uses correct brackets and assignment.

3. Final Answer:

   nums[1] = 10; -> Option B

4. Quick Check:

   Use square brackets for array index update [OK]

Hint: Use square brackets [] to access or update array elements [OK]

Common Mistakes:

• Using parentheses () instead of brackets []
• Using curly braces {} which are invalid for arrays
• Trying to access with dot notation

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

int[] arr = {5, 10, 15, 20};
arr[2] = 25;
System.out.println(arr[2]);

medium

A. 25

B. 15

C. 20

D. 10

Solution

1. Step 1: Understand initial array values

   The array arr starts as [5, 10, 15, 20].

2. Step 2: Update element at index 2

   We set arr[2] = 25; changing the third element from 15 to 25.

3. Step 3: Print updated element

   System.out.println(arr[2]); prints the updated value 25.

4. Final Answer:

   25 -> Option A

5. Quick Check:

   arr[2] updated to 25, so output is 25 [OK]

Hint: Remember updates change the element before printing [OK]

Common Mistakes:

• Printing old value before update
• Confusing index 2 with index 3
• Assuming array is unchanged

4. Identify the error in the following code snippet:

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

medium

A. ArrayIndexOutOfBoundsException at runtime

B. Syntax error due to missing semicolon

C. Compilation error: invalid array declaration

D. No error, code runs fine

Solution

1. Step 1: Check array size and valid indices

   The array numbers has length 3, valid indices are 0, 1, 2.

2. Step 2: Analyze index used for update

   Attempting numbers[3] = 4; accesses index 3, which is out of bounds.

3. Step 3: Identify error type

   This causes a runtime ArrayIndexOutOfBoundsException.

4. Final Answer:

   ArrayIndexOutOfBoundsException at runtime -> Option A

5. Quick Check:

   Index 3 invalid for length 3 array [OK]

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

Common Mistakes:

• Assuming index 3 is valid for length 3 array
• Confusing compile-time and runtime errors
• Ignoring zero-based indexing

5. Given the array int[] data = {2, 4, 6, 8, 10};, which code snippet correctly doubles the value at index 3 and then prints the entire array?

hard

A. data[4] = data[4] * 2;
for(int i = 0; i < data.length; i++) {
System.out.print(data[i] + " ");
}

B. data[3] *= 2;
for(int i = 1; i <= data.length; i++) {
System.out.print(data[i] + " ");
}

C. data[3] = data[3] + 2;
for(int i = 0; i < data.length; i++) {
System.out.print(data[i] + " ");
}

D. data[3] = data[3] * 2;
for(int i = 0; i < data.length; i++) {
System.out.print(data[i] + " ");
}

Solution

1. Step 1: Identify correct index and operation

   Index 3 corresponds to the fourth element (value 8). Doubling means multiplying by 2.

2. Step 2: Check loop correctness for printing

   The loop must run from 0 to data.length - 1 to print all elements without error.

3. Step 3: Verify each option

   data[3] = data[3] * 2; for(int i = 0; i < data.length; i++) { System.out.print(data[i] + " "); } correctly doubles data[3] and loops from 0 to length-1. data[3] *= 2; for(int i = 1; i <= data.length; i++) { System.out.print(data[i] + " "); } uses invalid loop bounds and will cause runtime error. data[3] = data[3] + 2; for(int i = 0; i < data.length; i++) { System.out.print(data[i] + " "); } adds 2 instead of doubling. data[4] = data[4] * 2; for(int i = 0; i < data.length; i++) { System.out.print(data[i] + " "); } doubles wrong index (4).

4. Final Answer:

   data[3] = data[3] * 2; for(int i = 0; i < data.length; i++) { System.out.print(data[i] + " "); } -> Option D

5. Quick Check:

   Correct index and loop bounds ensure proper update and print [OK]

Hint: Use index 3 for 4th element; loop from 0 to length-1 [OK]

Common Mistakes:

• Using wrong index for update
• Incorrect loop bounds causing errors
• Adding instead of multiplying