Counter-based while loop in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a counter-based while loop?

A counter-based while loop repeats a block of code while a counter variable meets a condition, usually increasing or decreasing the counter each time to eventually stop the loop.

beginner

Why do we need a counter variable in a while loop?

The counter variable helps control how many times the loop runs. It changes each time the loop runs so the loop can stop when the counter reaches a limit.

beginner

What happens if the counter is not updated inside a while loop?

If the counter is not updated, the loop condition may never become false, causing an infinite loop that runs forever.

beginner

Example: What does this code print? int i = 1; while (i <= 3) { System.out.println(i); i++; }

It prints: 1 2 3 The loop starts with i=1 and prints i, then increases i by 1 each time until i is greater than 3.

beginner

How to avoid infinite loops in counter-based while loops?

Always update the counter inside the loop so the condition will eventually become false. Also, check the loop condition carefully before running the loop.

What is the role of the counter in a counter-based while loop?

A To store the output of the loop

B To stop the program immediately

C To control how many times the loop runs

D To print messages inside the loop

The counter controls the number of loop repetitions by changing each time the loop runs.

What happens if you forget to increase the counter inside a while loop?

A The program will crash immediately

B The loop will run forever (infinite loop)

C The loop will run only once

D The counter will automatically increase

Without increasing the counter, the loop condition never becomes false, causing an infinite loop.

Which of these is a correct way to start a counter for a while loop that runs 5 times?

A All of the above

B int count = 5; while (count > 0) { ... count--; }

C int count = 1; while (count <= 5) { ... count++; }

D int count = 0; while (count < 5) { ... count++; }

All these ways correctly use a counter to run the loop 5 times.

What will this code print? int i = 3; while (i > 0) { System.out.println(i); i--; }

A Nothing

B 1 2 3

C 0 1 2

D 3 2 1

The loop prints 3, then 2, then 1, decreasing i each time until i is 0.

Which part of a counter-based while loop changes each time the loop runs?

A The counter variable

B The loop condition

C The program output

D The loop keyword

The counter variable is updated each time to eventually stop the loop.

Explain how a counter-based while loop works and why the counter must be updated inside the loop.

Think about how the loop knows when to stop.

Write a simple Java while loop that prints numbers from 1 to 5 using a counter.

Start with int i = 1; and use i <= 5 as condition.

Practice

1. What is the main purpose of using a counter in a while loop in Java?

easy

A. To create an infinite loop

B. To store the result of the loop

C. To control how many times the loop runs

D. To skip loop iterations

Solution

1. Step 1: Understand the role of a counter

   A counter is a variable that keeps track of how many times the loop has run.

2. Step 2: Identify the purpose in a while loop

   The counter helps stop the loop after a fixed number of runs, preventing infinite loops.

3. Final Answer:

   To control how many times the loop runs -> Option C

4. Quick Check:

   Counter controls loop count = A [OK]

Hint: Counter limits loop repetitions to fixed times [OK]

Common Mistakes:

• Thinking counter stores results
• Assuming counter creates infinite loops
• Believing counter skips iterations

2. Which of the following is the correct way to initialize a counter variable for a while loop in Java?

easy

A. while (count = 0) {}

B. int count = 0;

C. int count == 0;

D. count := 0;

Solution

1. Step 1: Check Java variable initialization syntax

   Java uses int count = 0; to declare and initialize an integer variable.

2. Step 2: Identify incorrect options

   while (count = 0) {} uses assignment in while condition incorrectly, C uses double equals in declaration, D uses wrong assignment operator.

3. Final Answer:

   int count = 0; -> Option B

4. Quick Check:

   Correct Java int initialization = A [OK]

Hint: Use single equals (=) to assign variables in Java [OK]

Common Mistakes:

• Using == instead of = for assignment
• Trying to assign inside while condition
• Using := which is not Java syntax

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

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

medium

A. 1 2 3

B. 0 1 2 3

C. 1 2 3 4

D. 3 2 1

Solution

1. Step 1: Analyze the loop start and condition

   The counter starts at 1 and runs while count is less than or equal to 3.

2. Step 2: Trace the output during each iteration

   It prints 1, then increments to 2, prints 2, increments to 3, prints 3, then increments to 4 and stops.

3. Final Answer:

   1 2 3 -> Option A

4. Quick Check:

   Prints 1 to 3 with spaces = B [OK]

Hint: Count from 1 to 3 prints 1 2 3 [OK]

Common Mistakes:

• Starting count at 0 instead of 1
• Printing after increment causing extra number
• Reversing the order of output

4. Identify the error in this Java code snippet:

int count = 1;
while (count < 5) {
    System.out.println(count);
}

medium

A. Counter variable is not incremented inside the loop

B. Loop condition should be count <= 5

C. Incorrect print statement syntax

D. Missing semicolon after while condition

Solution

1. Step 1: Check loop body for counter update

   The code does not increase count inside the loop, so count stays 1 forever.

2. Step 2: Understand effect of missing increment

   Without increment, the loop never ends, causing an infinite loop.

3. Final Answer:

   Counter variable is not incremented inside the loop -> Option A

4. Quick Check:

   Missing count++ causes infinite loop = D [OK]

Hint: Always update counter inside while loop [OK]

Common Mistakes:

• Forgetting to increment counter
• Changing loop condition incorrectly
• Assuming print syntax is wrong

5. You want to print all even numbers from 2 to 10 using a counter-based while loop in Java. Which code correctly does this?

hard

A. int count = 2;
while (count < 10) {
System.out.print(count + " " );
count += 2;
}

B. int count = 0;
while (count < 10) {
count += 2;
System.out.print(count + " " );
}

C. int count = 2;
while (count < 10) {
System.out.print(count + " " );
count++;
}

D. int count = 1;
while (count <= 10) {
if (count % 2 == 0) {
System.out.print(count + " " );
}
count++;
}

Solution

1. Step 1: Understand the goal to print even numbers 2 to 10

   We need to print 2,4,6,8,10 using a while loop and a counter.

2. Step 2: Analyze each option

   int count = 2; while (count < 10) { System.out.print(count + " " ); count += 2; } increments by 2 starting at 2, prints 2,4,6,8 misses 10. int count = 1; while (count <= 10) { if (count % 2 == 0) { System.out.print(count + " " ); } count++; } starts at 1, checks if even, prints if yes, increments by 1, correct. int count = 0; while (count < 10) { count += 2; System.out.print(count + " " ); } starts at 0, increments before print, prints 2,4,6,8 misses 10. int count = 2; while (count < 10) { System.out.print(count + " " ); count++; } increments by 1 but stops before 10, so misses 10.

3. Step 3: Choose the best option

   int count = 1; while (count <= 10) { if (count % 2 == 0) { System.out.print(count + " " ); } count++; } uses a counter-based loop with condition and checks evenness inside, which is a common pattern.

4. Final Answer:

   int count = 1; while (count <= 10) { if (count % 2 == 0) { System.out.print(count + " " ); } count++; } -> Option D

5. Quick Check:

   Check even inside loop and increment count = C [OK]

Hint: Check even inside loop, increment by 1 for all numbers [OK]

Common Mistakes:

• Incrementing by 1 but missing last number
• Starting count at 0 and printing before increment
• Not checking even condition inside loop