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.