For loop syntax in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a basic for loop

What is the output of this Java code?

Java

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

A 0 1 2 3

B 0 1 2

C 1 2 3

D 1 2 3 4

💡 Hint

Remember the loop runs while i is less than 3, starting from 0.

The loop starts at 0 and runs while i is less than 3, so it prints 0, 1, and 2 with spaces.

🧠 Conceptual

intermediate

Understanding for loop components

Which part of the for loop controls how many times the loop runs?

A The condition part

B The initialization part

C The update part

D The body of the loop

💡 Hint

Think about what decides if the loop continues or stops.

The condition part is checked before each iteration; if false, the loop stops.

❓ Predict Output

advanced

Output with multiple variables in for loop

What is the output of this Java code?

Java

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

A 0-3 1-2 2-1

B 0-3 1-2 2-1 3-0

C 1-2 2-1 3-0

D 0-3 1-2 2-1 3-0 4--1

💡 Hint

Both i and j change each loop; watch their values carefully.

The loop runs 3 times with i from 0 to 2 and j from 3 down to 1, printing pairs.

🔧 Debug

advanced

Identify the error in this for loop

What error does this Java code produce?

Java

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

A No error, code runs fine

B SyntaxError: Missing semicolon after initialization

C SyntaxError: Missing curly braces around loop body

D SyntaxError: Missing parentheses around for loop header

💡 Hint

Check the syntax of the for loop header carefully.

The for loop header must be enclosed in parentheses. Missing them causes a syntax error.

🚀 Application

expert

Number of iterations in nested for loops

How many times does the inner loop run in total when this code executes?

Java

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

A 16

B 8

C 10

D 12

💡 Hint

Count how many times the inner loop runs for each i from 1 to 4.

The inner loop runs 1 + 2 + 3 + 4 = 10 times in total.

Practice

1.

What is the main purpose of a for loop in Java?

easy

A. To define a new class

B. To make decisions based on conditions

C. To repeat a block of code a specific number of times

D. To store multiple values in a list

Solution

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

   A for loop is used to repeat code multiple times when the number of repetitions is known.

2. Step 2: Compare with other options

   Options A, B, and C describe other programming concepts, not loops.

3. Final Answer:

   To repeat a block of code a specific number of times -> Option C

4. Quick Check:

   For loop = repeat code fixed times [OK]

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

Common Mistakes:

• Confusing for loop with if statement
• Thinking for loop stores data
• Mixing for loop with class definition

2.

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

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

easy

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

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

C. for (int i = 0; i < 5; i++) System.out.println(i)

D. for (int i = 0; i < 5; i++) System.out.println(i); }

Solution

1. Step 1: Check the for loop syntax structure

   The correct syntax requires parentheses around initialization, condition, and increment, and braces to enclose the loop body.

2. Step 2: Validate each option

   for (int i = 0; i < 5; i++) { System.out.println(i); } has correct syntax with parentheses and braces. for int i = 0; i < 5; i++ { System.out.println(i); } misses parentheses. for (int i = 0; i < 5; i++) System.out.println(i) misses semicolon after println(i). for (int i = 0; i < 5; i++) System.out.println(i); } has an extra closing brace.

3. Final Answer:

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

4. Quick Check:

   Correct for loop syntax = for (int i = 0; i < 5; i++) { System.out.println(i); } [OK]

Hint: For loop needs parentheses and braces for multiple statements [OK]

Common Mistakes:

• Omitting parentheses around loop control
• Missing braces for multiple statements
• Extra or missing braces

3.

What will be the output of the following Java code?

for (int i = 1; i <= 3; i++) {
    System.out.print(i + " ");
}

medium

A. 0 1 2

B. 1 2 3

C. 1 2 3 4

D. 3 2 1

Solution

1. Step 1: Analyze the loop range

   The loop starts at i=1 and runs while i is less than or equal to 3, so i takes values 1, 2, and 3.

2. Step 2: Determine the output

   Each iteration prints the current i followed by a space, so output is "1 2 3 "

3. Final Answer:

   1 2 3 -> Option B

4. Quick Check:

   Prints 1 to 3 with spaces = 1 2 3 [OK]

Hint: Check loop start, end, and print inside loop [OK]

Common Mistakes:

• Starting loop at 0 instead of 1
• Including extra number beyond 3
• Printing in reverse order

4.

Identify the error in the following for loop code snippet:

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

medium

A. Extra closing brace } without matching opening brace

B. Incorrect loop condition syntax

C. Missing semicolon after i++

D. Variable i not declared

Solution

1. Step 1: Check loop syntax and braces

   The for loop has no opening brace but has a closing brace, which is unmatched and causes a syntax error.

2. Step 2: Verify other parts

   Semicolons and variable declaration are correct, and the loop condition syntax is valid.

3. Final Answer:

   Extra closing brace } -> Option A

4. Quick Check:

   Unmatched brace causes syntax error = Extra closing brace } without matching opening brace [OK]

Hint: Match every opening brace with a closing brace [OK]

Common Mistakes:

• Leaving unmatched braces
• Forgetting semicolons in loop header
• Not declaring loop variable

5.

How can you write a for loop in Java that prints only even numbers from 2 to 10 inclusive?

hard

A. for (int i = 0; i <= 10; i += 2) { System.out.print(i + " "); }

B. for (int i = 1; i < 10; i++) { if (i % 2 == 0) System.out.print(i + " "); }

C. for (int i = 2; i < 10; i++) { System.out.print(i * 2 + " "); }

D. for (int i = 2; i <= 10; i += 2) { System.out.print(i + " "); }

Solution

1. Step 1: Understand the requirement

   We want to print even numbers from 2 to 10 inclusive, so numbers: 2, 4, 6, 8, 10.

2. Step 2: Analyze each option

   for (int i = 2; i <= 10; i += 2) { System.out.print(i + " "); } starts at 2 and increments by 2 up to 10, printing even numbers correctly. for (int i = 1; i < 10; i++) { if (i % 2 == 0) System.out.print(i + " "); } uses a condition inside the loop but loops only up to 9, missing 10. for (int i = 2; i < 10; i++) { System.out.print(i * 2 + " "); } prints multiples of 2 starting at 4 (2*2), but loop ends before 10 and prints wrong numbers. for (int i = 0; i <= 10; i += 2) { System.out.print(i + " "); } starts at 0, which is not in the required range.

3. Final Answer:

   for (int i = 2; i <= 10; i += 2) { System.out.print(i + " "); } -> Option D

4. Quick Check:

   Increment by 2 from 2 to 10 = for (int i = 2; i <= 10; i += 2) { System.out.print(i + " "); } [OK]

Hint: Use i += 2 starting at 2 for even numbers [OK]

Common Mistakes:

• Starting loop at 0 instead of 2
• Using wrong loop condition
• Multiplying inside print instead of increment