Array traversal in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested array traversal

What is the output of the following Java code that traverses a 2D array?

Java

public class Main {
    public static void main(String[] args) {
        int[][] matrix = {{1, 2}, {3, 4}};
        int sum = 0;
        for (int i = 0; i < matrix.length; i++) {
            for (int j = 0; j < matrix[i].length; j++) {
                sum += matrix[i][j];
            }
        }
        System.out.println(sum);
    }
}

A 10

B 24

C 0

D Compilation error

💡 Hint

Think about adding all elements in the 2D array.

The code sums all elements: 1 + 2 + 3 + 4 = 10.

❓ Predict Output

intermediate

Output of enhanced for-loop traversal

What will be printed by this Java code that uses an enhanced for-loop to traverse an array?

Java

public class Main {
    public static void main(String[] args) {
        String[] fruits = {"apple", "banana", "cherry"};
        for (String fruit : fruits) {
            System.out.print(fruit.charAt(0));
        }
    }
}

A Compilation error

B applebananacherry

C abc\n

D abc

💡 Hint

Look at the first character of each string.

The code prints the first character of each fruit: 'a', 'b', 'c'.

🧠 Conceptual

advanced

Understanding array traversal with break statement

What is the output of this Java code that traverses an array and uses a break statement?

Java

public class Main {
    public static void main(String[] args) {
        int[] numbers = {5, 10, 15, 20};
        int total = 0;
        for (int num : numbers) {
            if (num == 15) {
                break;
            }
            total += num;
        }
        System.out.println(total);
    }
}

A 0

B 30

C 15

D 50

💡 Hint

The loop stops when it reaches 15.

The loop adds 5 and 10, then breaks before adding 15, so total is 15.

🔧 Debug

advanced

Identify the error in array traversal

What error will this Java code produce when run?

Java

public class Main {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3};
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i] + " ");
        }
    }
}

A ArrayIndexOutOfBoundsException

B NullPointerException

C Compilation error

D No output, infinite loop

💡 Hint

Check the loop condition and array indices.

The loop tries to access arr[3], which is out of bounds (max index 2).

🚀 Application

expert

Calculate the product of all even numbers in an array

What is the output of this Java program that multiplies all even numbers in an array?

Java

public class Main {
    public static void main(String[] args) {
        int[] nums = {2, 3, 4, 5, 6};
        int product = 1;
        for (int n : nums) {
            if (n % 2 == 0) {
                product *= n;
            }
        }
        System.out.println(product);
    }
}

A 120

B 48

C 0

D 15

💡 Hint

Multiply only even numbers: 2, 4, and 6.

2 * 4 = 8, 8 * 6 = 48, so output is 48.

Practice

1. What does array traversal mean in Java?
int[] arr = {1, 2, 3};

easy

A. Creating a new array with the same elements

B. Visiting each element of the array one by one using a loop

C. Sorting the array elements in ascending order

D. Deleting elements from the array

Solution

1. Step 1: Understand array traversal meaning

   Array traversal means going through each element in the array one by one.

2. Step 2: Match with given options

   Only Visiting each element of the array one by one using a loop describes visiting each element using a loop, which is traversal.

3. Final Answer:

   Visiting each element of the array one by one using a loop -> Option B

4. Quick Check:

   Array traversal = visiting elements one by one [OK]

Hint: Traversal means visiting all elements step-by-step [OK]

Common Mistakes:

• Confusing traversal with sorting
• Thinking traversal means deleting elements
• Mixing traversal with creating new arrays

2. Which of the following is the correct syntax to traverse an array int[] nums in Java?

easy

A. for(int i = 0; i < nums.length; i++) { System.out.println(nums[i]); }

B. for(int i = 1; i <= nums.length; i++) { System.out.println(nums[i]); }

C. for(int i = 0; i <= nums.length; i++) { System.out.println(nums[i]); }

D. for(int i = 0; i < nums.length - 1; i++) { System.out.println(nums[i]); }

Solution

1. Step 1: Check loop start and end conditions

   The loop should start at 0 and run while i is less than nums.length to cover all indices.

2. Step 2: Validate array access inside loop

   Accessing nums[i] is correct if i is within 0 to length-1. for(int i = 0; i < nums.length; i++) { System.out.println(nums[i]); } satisfies this.

3. Final Answer:

   for(int i = 0; i < nums.length; i++) { System.out.println(nums[i]); } -> Option A

4. Quick Check:

   Loop from 0 to length-1 = correct traversal [OK]

Hint: Loop from 0 to less than length for full traversal [OK]

Common Mistakes:

• Starting loop at 1 instead of 0
• Using <= length causing index out of bounds
• Stopping loop early at length - 1

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

int[] arr = {5, 10, 15};
for(int i = 0; i < arr.length; i++) {
  System.out.print(arr[i] + " ");
}

medium

A. 5 10 15

B. 15 10 5

C. 5 10 15 0

D. Error: ArrayIndexOutOfBoundsException

Solution

1. Step 1: Understand the loop iteration

   The loop runs from i=0 to i=2 (arr.length=3), printing arr[i] each time.

2. Step 2: Check printed values

   Values printed are arr[0]=5, arr[1]=10, arr[2]=15, each followed by a space.

3. Final Answer:

   5 10 15 -> Option A

4. Quick Check:

   Print elements in order = 5 10 15 [OK]

Hint: Prints elements in order from index 0 to length-1 [OK]

Common Mistakes:

• Assuming reverse order output
• Expecting extra zero at end
• Thinking loop causes index error

4. Identify the error in this array traversal code:

int[] data = {2, 4, 6};
for(int i = 0; i <= data.length; i++) {
  System.out.println(data[i]);
}

medium

A. System.out.println should be outside the loop

B. Array declaration is incorrect

C. Loop condition should be i < data.length, not <= data.length

D. No error, code runs fine

Solution

1. Step 1: Analyze loop condition

   The loop runs while i <= data.length, which means i can be equal to data.length (3).

2. Step 2: Check array index access

   Accessing data[3] causes ArrayIndexOutOfBoundsException because valid indices are 0 to 2.

3. Final Answer:

   Loop condition should be i < data.length, not <= data.length -> Option C

4. Quick Check:

   Index must be less than length to avoid error [OK]

Hint: Use i < length, not i <= length in loops [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Ignoring array index limits
• Thinking loop runs without error

5. Given an array int[] nums = {3, 6, 9, 12, 15};, write code to print only the even numbers by traversing the array.

hard

A. for(int i = 0; i < nums.length - 1; i++) { System.out.print(nums[i] + " "); }

B. for(int i = 1; i <= nums.length; i++) { if(nums[i] % 2 == 0) System.out.print(nums[i] + " "); }

C. for(int i = 0; i < nums.length; i++) { if(nums[i] % 2 != 0) System.out.print(nums[i] + " "); }

D. for(int i = 0; i < nums.length; i++) { if(nums[i] % 2 == 0) System.out.print(nums[i] + " "); }

Solution

1. Step 1: Traverse the array correctly

   Loop from i=0 to i < nums.length to visit all elements safely.

2. Step 2: Check condition for even numbers

   Use nums[i] % 2 == 0 to find even numbers and print them.

3. Final Answer:

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

4. Quick Check:

   Traverse all and print only even numbers [OK]

Hint: Check nums[i] % 2 == 0 inside loop to print evens [OK]

Common Mistakes:

• Using wrong loop bounds causing errors
• Printing odd numbers instead of even
• Missing condition inside loop