Loop initialization, condition, update in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a for-loop with multiple updates

What is the output of the following Java code?

Java

public class Main {
    public static void main(String[] args) {
        int sum = 0;
        for (int i = 1, j = 10; i < j; i += 2, j -= 3) {
            sum += i + j;
        }
        System.out.println(sum);
    }
}

A 21

B 50

C 45

D 55

💡 Hint

Trace the values of i and j in each loop iteration and add their sum.

The for-loop initializes i=1, j=10. Condition: i < j. Updates: i += 2, j -= 3. - Iteration 1: i=1 < j=10 true, sum += 11 → sum=11, update i=3, j=7 - Iteration 2: i=3 < j=7 true, sum += 10 → sum=21, update i=5, j=4 - Next: i=5 < j=4 false, end. Output: 21.

❓ Predict Output

intermediate

Output of a while-loop with complex condition

What is the output of this Java code snippet?

Java

public class Main {
    public static void main(String[] args) {
        int i = 0;
        int count = 0;
        while (i < 10) {
            i += 3;
            if (i % 2 == 0) {
                count++;
            }
        }
        System.out.println(count);
    }
}

A 5

B 3

C 4

D 2

💡 Hint

Check the values of i after each increment and count how many are even.

The while loop checks i < 10 before the body: - i=0 <10: i+=3 → i=3 (odd) - i=3 <10: i=6 (even, count=1) - i=6 <10: i=9 (odd) - i=9 <10: i=12 (even, count=2) - i=12 <10 false. Output: 2.

🔧 Debug

advanced

Identify the error in loop initialization

What error will this Java code produce when compiled?

Java

public class Main {
    public static void main(String[] args) {
        for (int i = 0; i < 5; i++)
            int j = i * 2;
        System.out.println(j);
    }
}

A Compilation error: variable j might not have been initialized

B Compilation error: cannot find symbol j

C Runtime error: NullPointerException

D Output: 8

💡 Hint

Check the scope of variable j declared inside the loop.

Variable j is declared inside the for-loop body and is not accessible outside it. The System.out.println(j) tries to access j outside its scope, causing a compilation error: cannot find symbol j.

❓ Predict Output

advanced

Output of nested for-loops with update expressions

What is the output of this Java program?

Java

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

A 7

B 6

C 4

D 8

💡 Hint

Count how many times the inner loop runs for each i.

Trace inner loop: - i=1: j=1 (count=1), j=3 (count=2) - i=2: j=2 (count=3) - i=3: j=3 (count=4) Total: 4.

🧠 Conceptual

expert

Effect of loop update expression on iteration count

Consider the loop:

for (int i = 0; i < 20; i += 4) { /* body */ }

How many times will the loop body execute?

A 5

B 4

C 6

D 7

💡 Hint

Count how many increments of 4 fit before i reaches 20.

i starts at 0 and increases by 4 each time: 0,4,8,12,16. Next would be 20 which is not less than 20, so loop stops. The loop runs 5 times.

Practice

1. Which part of a for loop in Java sets the starting value of the loop variable?

easy

A. Initialization

B. Condition

C. Update

D. Body

Solution

1. Step 1: Understand the parts of a for loop

   A for loop has three parts: initialization, condition, and update.

2. Step 2: Identify the starting value part

   The initialization part sets the starting value of the loop variable before the loop runs.

3. Final Answer:

   Initialization -> Option A

4. Quick Check:

   Starting value = Initialization [OK]

Hint: Initialization always sets the loop's start value [OK]

Common Mistakes:

• Confusing condition with initialization
• Thinking update sets the start value
• Mixing loop body with initialization

2. Which of the following is the correct syntax for a for loop in Java that counts from 0 to 4?

easy

A. for(int i = 0; i < 4; i++) {}

B. for(int i = 0; i <= 4; i++) {}

C. for(int i = 1; i <= 5; i++) {}

D. for(int i = 0; i != 5; i--) {}

Solution

1. Step 1: Check initialization and condition

   for(int i = 0; i <= 4; i++) {} starts at 0 and runs while i is less than or equal to 4, which covers 0 to 4.

2. Step 2: Check update direction

   for(int i = 0; i <= 4; i++) {} increments i by 1 each time, moving forward correctly.

3. Final Answer:

   for(int i = 0; i <= 4; i++) {} -> Option B

4. Quick Check:

   0 to 4 inclusive = for(int i = 0; i <= 4; i++) {} [OK]

Hint: Use i <= max to include last number [OK]

Common Mistakes:

• Using i < 4 excludes 4
• Starting at 1 misses 0
• Decrementing i causes infinite loop

3. What is the output of this Java code?

for(int i = 1; i < 5; i += 2) {
    System.out.print(i + " ");
}

medium

A. 1 2 3 4

B. 1 3 5

C. 2 4

D. 1 3

Solution

1. Step 1: Trace loop values

   i starts at 1, prints 1, then i increases by 2 to 3, prints 3, then i increases by 2 to 5.

2. Step 2: Check loop condition

   Loop runs while i < 5. When i is 5, condition fails, so loop stops.

3. Final Answer:

   1 3 -> Option D

4. Quick Check:

   Prints 1 and 3 only [OK]

Hint: Add update to i and check condition carefully [OK]

Common Mistakes:

• Assuming loop runs when i equals 5
• Forgetting i increments by 2
• Printing values after loop ends

4. Identify the error in this Java loop:

for(int i = 0; i > 5; i++) {
    System.out.println(i);
}

medium

A. Condition causes loop to never run

B. Update should decrement i

C. Initialization is incorrect

D. Missing loop body braces

Solution

1. Step 1: Analyze the loop condition

   The condition is i > 5, but i starts at 0, so 0 > 5 is false immediately.

2. Step 2: Understand loop execution

   Since condition is false at start, loop body never runs.

3. Final Answer:

   Condition causes loop to never run -> Option A

4. Quick Check:

   False condition stops loop immediately [OK]

Hint: Check if condition is true at start to run loop [OK]

Common Mistakes:

• Thinking loop runs at least once
• Confusing > with < in condition
• Assuming update fixes condition

5. You want to print all even numbers from 2 to 20 using a for loop in Java. Which loop correctly does this?

hard

A. for(int i = 2; i < 20; i++) { if(i % 2 == 0) System.out.print(i + " "); }

B. for(int i = 1; i < 20; i++) { if(i % 2 == 0) System.out.print(i + " "); }

C. for(int i = 2; i <= 20; i += 2) { System.out.print(i + " "); }

D. for(int i = 0; i <= 20; i += 2) { System.out.print(i + " "); }

Solution

1. Step 1: Check initialization and update for even numbers

   for(int i = 2; i <= 20; i += 2) { System.out.print(i + " "); } starts at 2 and increments by 2, printing even numbers directly up to 20.

2. Step 2: Verify condition includes 20

   Condition i <= 20 ensures 20 is included in output.

3. Final Answer:

   for(int i = 2; i <= 20; i += 2) { System.out.print(i + " "); } -> Option C

4. Quick Check:

   Start at 2, step 2, up to 20 [OK]

Hint: Start at first even, add 2 each time [OK]

Common Mistakes:

• Starting at 0 prints unwanted 0
• Using < 20 excludes 20
• Forgetting to increment by 2