Loop execution flow in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested loops with break

What is the output of the following Java code snippet?

Java

public class Main {
    public static void main(String[] args) {
        for (int i = 1; i <= 3; i++) {
            for (int j = 1; j <= 3; j++) {
                if (j == 2) {
                    break;
                }
                System.out.print(i + "-" + j + " ");
            }
        }
    }
}

A 1-1 1-2 1-3 2-1 2-2 2-3 3-1 3-2 3-3

B 1-1 1-2 2-1 2-2 3-1 3-2

C 1-1 2-1 3-1 3-2

D 1-1 2-1 3-1

💡 Hint

Remember that break exits the innermost loop immediately.

The inner loop breaks when j == 2, so only j=1 prints for each i. Thus output is "1-1 2-1 3-1 ".

❓ Predict Output

intermediate

Output of while loop with continue

What will be printed by this Java code?

Java

public class Main {
    public static void main(String[] args) {
        int i = 0;
        while (i < 5) {
            i++;
            if (i == 3) {
                continue;
            }
            System.out.print(i + " ");
        }
    }
}

A 2 3 4 5

B 1 2 3 4 5

C 1 2 4 5

D 1 2 3 4

💡 Hint

The continue skips the print when i equals 3.

When i is 3, continue skips the print statement, so 3 is not printed. All other values from 1 to 5 print.

🔧 Debug

advanced

Identify the error in for loop increment

What error will this Java code produce when run?

Java

public class Main {
    public static void main(String[] args) {
        for (int i = 0; i < 5; i = i++) {
            System.out.print(i + " ");
        }
    }
}

A Infinite loop printing 0 repeatedly

B Compilation error: invalid increment expression

C Prints 0 1 2 3 4

D Runtime exception: ArrayIndexOutOfBoundsException

💡 Hint

Look carefully at the increment expression i = i++.

The expression i = i++ does not change i because i++ returns the original value before increment. So i stays 0, causing an infinite loop.

❓ Predict Output

advanced

Output of for loop with multiple variables

What is the output of this Java code?

Java

public class Main {
    public static void main(String[] args) {
        for (int i = 0, j = 5; i < j; i++, j--) {
            System.out.print(i + "-" + j + " ");
        }
    }
}

A 0-5 1-4 2-3 3-2

B 0-5 1-4 2-3

C 0-5 1-4 2-3 3-2 4-1

D 0-5 1-4

💡 Hint

The loop stops when i is no longer less than j.

The loop runs while i < j. Iterations: (0,5), (1,4), (2,3). When i=3 and j=2, condition fails, loop stops.

🧠 Conceptual

expert

Number of iterations in nested loops with dependent conditions

Consider the following Java code. How many times will the innermost print statement execute?

Java

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

A 10

B 16

C 20

D 8

💡 Hint

Count how many times j runs for each i.

For i=1, j=1..4 (4 times); i=2, j=2..4 (3 times); i=3, j=3..4 (2 times); i=4, j=4..4 (1 time). Total 4+3+2+1=10.

Practice

1. What does a for loop in Java primarily do?

easy

A. Executes code only once

B. Stops the program immediately

C. Repeats a block of code a specific number of times

D. Creates a new variable

Solution

1. Step 1: Understand the purpose of a for loop and compare options

   A for loop is designed to repeat a block of code multiple times based on a condition. Only "Repeats a block of code a specific number of times" correctly describes it.

2. Final Answer:

   Repeats a block of code a specific number of times -> Option C

3. Quick Check:

   for loop = repeat code [OK]

Hint: For loops repeat code a set number of times [OK]

Common Mistakes:

• Thinking for loops run only once
• Confusing loops with variable declarations
• Believing loops stop the program

2. Which of the following is the correct syntax for a basic while loop in Java?

easy

A. while (condition); { // code }

B. while condition { // code }

C. while: (condition) { // code }

D. while (condition) { // code }

Solution

1. Step 1: Recall Java while loop syntax and check options

   The correct syntax requires parentheses around the condition and braces for the code block. Only "while (condition) { // code }" matches: while (condition) { // code }.

2. Final Answer:

   while (condition) { // code } -> Option D

3. Quick Check:

   while loop syntax = parentheses + braces [OK]

Hint: While loops need parentheses and braces [OK]

Common Mistakes:

• Omitting parentheses around condition
• Adding semicolon after while condition
• Using colon instead of parentheses

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

int count = 0;
while (count < 3) {
  System.out.print(count + " ");
  count++;
}

medium

A. 0 1 2

B. 1 2 3

C. 0 1 2 3

D. 3 2 1

Solution

1. Step 1: Analyze the loop condition and trace iterations

   The loop runs while count is less than 3, starting from 0. It prints 0, then increments to 1; prints 1, increments to 2; prints 2, increments to 3; then stops because 3 is not less than 3.

2. Final Answer:

   0 1 2 -> Option A

3. Quick Check:

   Prints 0,1,2 then stops [OK]

Hint: Count starts at 0 and stops before 3 [OK]

Common Mistakes:

• Including 3 in output
• Starting count from 1
• Printing in reverse order

4. Identify the error in this loop code:

int i = 0;
while (i < 5) {
  System.out.println(i);
}

medium

A. Missing increment of i inside the loop

B. Incorrect loop condition syntax

C. Missing semicolon after while

D. Variable i should be declared inside the loop

Solution

1. Step 1: Check loop control variable and consequences

   The variable i is initialized but never changed inside the loop. Without incrementing i, the condition i < 5 is always true, causing an infinite loop.

2. Final Answer:

   Missing increment of i inside the loop -> Option A

3. Quick Check:

   Loop needs variable update to stop [OK]

Hint: Always update loop variable to avoid infinite loops [OK]

Common Mistakes:

• Forgetting to increment loop variable
• Misplacing semicolons
• Declaring variable inside loop unnecessarily

5. Consider this code snippet:

int sum = 0;
for (int i = 1; i <= 5; i++) {
  if (i == 3) continue;
  sum += i;
}
System.out.println(sum);

What is the output and why?

hard

A. 15 because all numbers 1 to 5 are added

B. 12 because number 3 is skipped in addition

C. 10 because only numbers less than 3 are added

D. Error due to incorrect use of continue

Solution

1. Step 1: Trace the loop and effect of continue

   The loop runs from 1 to 5. When i equals 3, continue skips the addition for that iteration. Sum = 1 + 2 + 4 + 5 = 12 because 3 is skipped.

2. Final Answer:

   12 because number 3 is skipped in addition -> Option B

3. Quick Check:

   continue skips 3, sum = 12 [OK]

Hint: Continue skips current loop iteration [OK]

Common Mistakes:

• Adding 3 despite continue
• Thinking continue causes error
• Summing only numbers less than 3