Counter-based while loop in Java - Step-by-Step Execution

Concept Flow - Counter-based while loop

Initialize counter = 0

↓

Check: counter < limit?

No→EXIT

Yes↓

Execute loop body

↓

Update counter = counter + 1

↩Back to Check

Start with a counter, check if it is less than a limit, run the loop body if yes, then increase the counter and repeat until the condition is false.

Execution Sample

Java

int counter = 0;
while (counter < 3) {
    System.out.println(counter);
    counter++;
}

Print numbers 0, 1, 2 by increasing counter until it reaches 3.

Execution Table

Step	counter	Condition (counter < 3)	Action	Output
1	0	true	Print 0, counter++	0
2	1	true	Print 1, counter++	1
3	2	true	Print 2, counter++	2
4	3	false	Exit loop

💡 counter reaches 3, condition 3 < 3 is false, loop stops

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
counter	0	1	2	3	3

Key Moments - 2 Insights

Why does the loop stop when counter equals 3?

Because the condition counter < 3 becomes false at step 4, so the loop exits as shown in the execution_table row 4.

When is the counter variable increased?

The counter is increased after printing the current value inside the loop body, as seen in the Action column of the execution_table rows 1 to 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of counter at step 2?

A 2

B 0

C 1

D 3

💡 Hint

Check the 'counter' column in execution_table row 2.

At which step does the loop condition become false?

A Step 4

B Step 2

C Step 3

D Step 1

💡 Hint

Look at the 'Condition' column in execution_table row 4.

If we change the limit from 3 to 5, how many times will the loop run?

A 3 times

B 5 times

C 4 times

D 6 times

💡 Hint

The loop runs while counter < limit, so it runs limit times starting from 0.

Concept Snapshot

Counter-based while loop syntax:
int counter = 0;
while (counter < limit) {
    // loop body
    counter++;
}
Loop runs while condition is true, counter updates each iteration.

Full Transcript

This example shows a counter-based while loop in Java. We start with counter at 0. Each time the loop checks if counter is less than 3. If yes, it prints the counter and then increases it by 1. When counter reaches 3, the condition becomes false and the loop stops. The execution table tracks each step, showing counter values, condition results, actions, and outputs. The variable tracker shows how counter changes after each iteration. Key moments clarify why the loop stops and when the counter increases. The quiz tests understanding of counter values, loop exit step, and effect of changing the limit.

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