Do–while loop in C++ - Time & Space Complexity

Time Complexity: Do-while loop

O(n)

Understanding Time Complexity

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

Specifically, how many times the loop runs affects the total work done.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

int i = 0;
do {
    // some constant time work
    i++;
} while (i < n);
    

This code runs a loop that repeats until the variable i reaches n.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The do-while loop body runs repeatedly.
• How many times: It runs exactly n times, increasing i each time.

How Execution Grows With Input

As n grows, the loop runs more times, so the total work grows in a straight line with n.

Input Size (n)	Approx. Operations
10	About 10 loop runs
100	About 100 loop runs
1000	About 1000 loop runs

Pattern observation: The work grows evenly as n increases.

Final Time Complexity

Time Complexity: O(n)

This means the time taken grows directly in proportion to the input size n.

Common Mistake

[X] Wrong: "The do-while loop always runs at least once, so it takes constant time."

[OK] Correct: The loop runs once or more, but the total number of runs depends on n, so time grows with input size, not constant.

Interview Connect

Understanding how loops like do-while scale helps you explain how programs behave with bigger inputs, a key skill in coding interviews.

Self-Check

"What if the loop increments i by 2 each time instead of 1? How would the time complexity change?"

Practice

1.

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

easy

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

B. while loop always runs the code block at least once.

C. do-while loop checks the condition before running the code block.

D. There is no difference; both behave the same.

Solution

1. Step 1: Understand do-while loop behavior

   A do-while loop executes the code block first, then checks the condition to decide if it repeats.

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:

   do-while loop runs 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, then checks condition [OK]

Common Mistakes:

• Thinking while loop always runs code once
• Confusing order of condition check
• Assuming do-while and while behave the same

2.

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

?

easy

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

B. do ( /* code */ ) while condition;

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

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

Solution

1. Step 1: Recall correct do-while syntax

   The correct syntax uses do { ... } while (condition); with braces and a semicolon after the while condition.

2. Step 2: Identify syntax errors in other options

   do ( /* code */ ) while condition; misses braces and semicolon; C uses incorrect keyword 'until'; A reverses order incorrectly.

3. Final Answer:

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

4. Quick Check:

   Syntax ends with semicolon after while(condition) [OK]

Hint: Look for braces and semicolon after while(condition) [OK]

Common Mistakes:

• Omitting semicolon after while(condition)
• Using 'until' instead of 'while'
• Incorrect order of do and while keywords

3.

What will be the output of the following C++ code?

int i = 3;
do {
    std::cout << i << " ";
    i--;
} while (i > 0);

medium

A. 3 2 1

B. 3 2 1 0

C. 2 1 0

D. 1 2 3

Solution

1. Step 1: Trace the loop iterations

   Initially, i=3. The loop prints 3, then decrements i to 2. Condition i>0 is true, so repeats printing 2, then 1.

2. Step 2: Check when loop stops

   After printing 1, i becomes 0. Condition i>0 is false, loop stops. So output is "3 2 1 "

3. Final Answer:

   3 2 1 -> Option A

4. Quick Check:

   Loop runs while i>0, prints 3,2,1 [OK]

Hint: Count printed values before condition fails [OK]

Common Mistakes:

• Including 0 in output
• Starting count from 2 instead of 3
• Printing in wrong order

4.

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

int x = 5;
do {
    std::cout << x << " ";
    x++;
} while x < 10;

medium

A. The loop should end with a comma, not a semicolon.

B. The variable x should be decremented, not incremented.

C. The do keyword is misspelled.

D. Missing parentheses around the condition in while statement.

Solution

1. Step 1: Check syntax of while condition

   The condition after while must be enclosed in parentheses: while (x < 10);.

2. Step 2: Verify other parts

   Incrementing x is valid; do is spelled correctly; loop ends with semicolon, which is correct.

3. Final Answer:

   Missing parentheses around the condition in while statement. -> Option D

4. Quick Check:

   while condition needs parentheses [OK]

Hint: Always put condition in parentheses after while [OK]

Common Mistakes:

• Forgetting parentheses in while condition
• Confusing semicolon with comma at loop end
• Thinking increment is always wrong

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 number greater than zero. Which code snippet correctly implements this?

?

hard

A. int num;
while (num <= 0) {
std::cout << "Enter positive number: ";
std::cin >> num;
}

B. int num;
do {
std::cout << "Enter positive number: ";
std::cin >> num;
} while (num <= 0);

C. int num = 0;
do {
std::cout << "Enter positive number: ";
std::cin >> num;
} while (num > 0);

D. int num;
do {
std::cout << "Enter positive number: ";
std::cin >> num;
} while (num < 0);

Solution

1. Step 1: Understand the condition to repeat

   The loop should continue while the number is less than or equal to zero, so condition is num <= 0.

2. Step 2: Check each option's condition

   int num; do { std::cout << "Enter positive number: "; std::cin >> num; } while (num <= 0); uses do-while (num <= 0); which repeats until user enters positive number. int num = 0; do { std::cout << "Enter positive number: "; std::cin >> num; } while (num > 0); repeats while number is positive (wrong). int num; while (num <= 0) { std::cout << "Enter positive number: "; std::cin >> num; } uses while loop, not do-while. int num; do { std::cout << "Enter positive number: "; std::cin >> num; } while (num < 0); repeats while number is less than zero, so zero input stops loop incorrectly.

3. Final Answer:

   int num; do { std::cout << "Enter positive number: "; std::cin >> num; } while (num <= 0); -> Option B

4. Quick Check:

   Repeat while input ≤ 0 to ensure positive input [OK]

Hint: Repeat while input is not positive (≤ 0) [OK]

Common Mistakes:

• Using wrong condition sign (> 0 instead of ≤ 0)
• Using while loop instead of do-while
• Stopping loop too early on zero input