Do–while loop in Java - Time & Space Complexity

Time Complexity: Do-while loop

O(n)

Understanding Time Complexity

We want to understand how the time it takes to run a do-while loop changes as the input size grows.

How many times does the loop run when the input gets bigger?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

int i = 0;
do {
    System.out.println(i);
    i++;
} while (i < n);
    

This code prints numbers from 0 up to n-1 using a do-while loop.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The do-while loop runs the print statement repeatedly.
• How many times: It runs exactly n times, once for each number from 0 to n-1.

How Execution Grows With Input

As n grows, the number of times the loop runs grows the same way.

Input Size (n)	Approx. Operations
10	10
100	100
1000	1000

Pattern observation: The operations increase directly with n, so doubling n doubles the work.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the loop grows in a straight line with the size of n.

Common Mistake

[X] Wrong: "The do-while loop always runs only once, so time is constant."

[OK] Correct: The loop runs at least once, but if n is bigger, it runs many times, so time grows with n.

Interview Connect

Understanding how loops like do-while grow with input size helps you explain code efficiency clearly and confidently.

Self-Check

"What if we changed the loop to stop when i <= n instead of i < n? How would the time complexity change?"

Practice

1.

What is the main difference between a do-while loop and a while loop in Java?

easy

A. A do-while loop executes the code block at least once before checking the condition.

B. A while loop executes the code block at least once before checking the condition.

C. A do-while loop never checks the condition.

D. A while loop always executes the code block twice.

Solution

1. Step 1: Understand do-while loop behavior

   A do-while loop runs the code block first, then checks the condition to decide if it should repeat.

2. Step 2: Compare with while loop

   A while loop checks the condition first before running the code block, so it may not run at all if the condition is false initially.

3. Final Answer:

   A do-while loop executes the code block at least once before checking the condition. -> Option A

4. Quick Check:

   do-while runs once before condition check [OK]

Hint: Remember: do-while runs code first, while checks condition first [OK]

Common Mistakes:

• Thinking while loop always runs code once
• Believing do-while skips condition check
• Confusing order of execution and condition check

2.

Which of the following is the correct syntax for a do-while loop in Java?

?

easy

A. do { // code } while (condition);

B. do (condition) { // code };

C. while (condition) { // code } do;

D. do { // code } until (condition);

Solution

1. Step 1: Recall correct do-while syntax

   The correct syntax starts with do, then the code block in braces, followed by while (condition); with a semicolon.

2. Step 2: Identify the correct option

   do { // code } while (condition); matches the correct syntax exactly. Others have wrong order, missing semicolon, or wrong keywords.

3. Final Answer:

   do { // code } while (condition); -> Option A

4. Quick Check:

   Syntax: do { } while (condition); [OK]

Hint: Look for 'do { } while (condition);' with semicolon [OK]

Common Mistakes:

• Missing semicolon after while condition
• Using 'until' instead of 'while'
• Placing condition before code block

3.

What will be the output of the following Java code?

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

medium

A. 1 2 3 4

B. 2 3 4

C. 1 2 3

D. No output

Solution

1. Step 1: Trace the loop iterations

   Initially, count = 1. The loop prints count, then increments it. It repeats while count <= 3.

2. Step 2: List printed values

   Iterations print: 1 (count=1), 2 (count=2), 3 (count=3). When count becomes 4, condition fails and loop stops.

3. Final Answer:

   1 2 3 -> Option C

4. Quick Check:

   Prints 1 to 3 inclusive [OK]

Hint: Count starts at 1, loop runs while count ≤ 3 [OK]

Common Mistakes:

• Thinking loop prints 4 because of increment
• Confusing pre-increment and post-increment
• Assuming no output if condition false initially

4.

Identify the error in this do-while loop code snippet:

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

medium

A. Loop should use while instead of do-while

B. Condition should be inside braces

C. Variable i must be declared inside the loop

D. Missing semicolon after i++

Solution

1. Step 1: Check syntax inside the loop

   The statement i++ is missing a semicolon at the end, which causes a syntax error.

2. Step 2: Verify other parts

   The condition is correctly placed after the loop with a semicolon. Variable declaration outside is valid. Loop type is correct.

3. Final Answer:

   Missing semicolon after i++ -> Option D

4. Quick Check:

   Every statement needs a semicolon [OK]

Hint: Check for missing semicolons inside loop body [OK]

Common Mistakes:

• Forgetting semicolon after increment
• Misplacing condition inside braces
• Thinking variable must be declared inside loop

5.

You want to write a do-while loop that asks the user to enter a positive number and repeats until the user enters a positive number. Which code snippet correctly implements this?

Scanner sc = new Scanner(System.in);
int num;
do {
    System.out.print("Enter a positive number: ");
    num = sc.nextInt();
} while (???);

hard

A. num > 0

B. num <= 0

C. num == 0

D. num < 0

Solution

1. Step 1: Understand the loop goal

   The loop should repeat while the number is not positive, meaning while it is zero or negative.

2. Step 2: Choose the correct condition

   The condition num <= 0 ensures the loop repeats until the user enters a positive number.

3. Final Answer:

   num <= 0 -> Option B

4. Quick Check:

   Repeat while number is not positive [OK]

Hint: Loop while input is zero or negative to get positive number [OK]

Common Mistakes:

• Using condition that stops too early
• Checking for positive instead of non-positive
• Confusing < and <= operators