For loop syntax in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the basic syntax of a for loop in Java?

A for loop in Java has three parts inside parentheses: initialization, condition, and update. It looks like this:

for (int i = 0; i < 5; i++) {
    // code to repeat
}

beginner

What does the 'initialization' part in a for loop do?

Initialization sets up a starting point before the loop runs. Usually, it creates and sets a variable, like int i = 0;.

beginner

What happens if the condition in a for loop is false at the start?

If the condition is false at the start, the loop body does not run even once. The program skips the loop and continues after it.

beginner

What is the role of the update part in a for loop?

The update part changes the loop variable after each run. For example, i++ adds 1 to i each time, moving the loop forward.

beginner

Can you write a for loop that prints numbers from 1 to 3 in Java?

Yes! Here's how:

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

This prints:
1
2
3

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

A The initialization

B The loop body

C The update

D The condition

The condition is checked before each loop run. If it's true, the loop runs; if false, it stops.

What does this for loop do?

for (int i = 0; i < 3; i++) { System.out.print(i); }

A Prints nothing

B Prints 1 2 3

C Prints 0 1 2

D Prints 0 1 2 3

The loop starts at 0 and runs while i is less than 3, printing 0, 1, and 2.

What happens if you forget to update the loop variable in a for loop?

A The loop runs forever (infinite loop)

B The loop runs once

C The loop never runs

D The program crashes immediately

Without updating, the condition may never become false, causing an infinite loop.

Which part of the for loop is optional in Java?

A Initialization

B Condition

C Update

D None, all are required

Initialization, condition, and update can all be omitted in Java, but usually at least the condition is needed to avoid infinite loops.

What is printed by this code?

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

A 0 1 2 3 4

B 5 4 3 2 1

C 1 2 3 4 5

D Nothing

The loop starts at 5 and decreases i by 1 each time until i is 1.

Explain the three parts of a for loop in Java and their roles.

Think about starting value, when to stop, and how to change the counter.

Write a simple for loop in Java that prints the numbers 1 to 5.

Use System.out.println inside the loop.

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