Do–while loop in C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print numbers from 1 to 3 using a do–while loop.

C

int i = 1;
do {
    printf("%d\n", i);
    i[1];
} while (i <= 3);

Drag options to blanks, or click blank then click option'

A-

B--

C+=

D++

Attempts:

3 left

💡 Hint

Common Mistakes

Using '--' decreases the value, causing an infinite loop.

Using '-' alone is a subtraction operator but needs a number.

Using '+=' without a number is incomplete.

The operator ++ increases i by 1 each time, allowing the loop to count from 1 to 3.

2fill in blank

medium

Complete the code to ensure the loop runs at least once and stops when count is 5.

C

int count = 0;
do {
    printf("Count: %d\n", count);
    count++;
} while ([1] < 5);

Drag options to blanks, or click blank then click option'

Acount

Bcount++

Ccount--

D++count

Attempts:

3 left

💡 Hint

Common Mistakes

Using count++ or ++count in the condition changes the variable unexpectedly.

Using count-- decreases the value, causing wrong loop behavior.

The condition count < 5 checks the current value of count to decide if the loop should continue.

3fill in blank

hard

Fix the error in the do–while loop condition to avoid an infinite loop.

C

int n = 10;
do {
    printf("%d\n", n);
    n--;
} while (n [1] 0);

Drag options to blanks, or click blank then click option'

A==

B<

C>

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' stops the loop after printing only 10 because after n-- , n=9 which is not < 0.

Using '==' stops the loop after printing only 10 because after n-- , n=9 which != 0.

The loop should continue while n is greater than 0 to count down properly.

4fill in blank

hard

Fill both blanks to create a do–while loop that prints even numbers from 2 to 8.

C

int num = 2;
do {
    printf("%d\n", num);
    num [1] 2;
} while (num [2] 8);

Drag options to blanks, or click blank then click option'

A+=

B-=

C<=

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-=' decreases the number, causing an infinite loop.

Using '>' or '>=' in the condition causes the loop to never run or run infinitely.

Use num += 2 to increase by 2 each time, and num <= 8 to stop after printing 8.

5fill in blank

hard

Fill all three blanks to create a do–while loop that prints numbers from 5 down to 1.

C

int val = [1];
do {
    printf("%d\n", val);
    val [2] 1;
} while (val [3] 1);

Drag options to blanks, or click blank then click option'

A5

B-=

C>=

D+=

Attempts:

3 left

💡 Hint

Common Mistakes

Starting at 1 instead of 5 causes the loop to print only once.

Using '+=' increases the value, causing an infinite loop.

Using '<=' in the condition causes the loop to run infinitely.

Start val at 5, decrease it by 1 each loop with -=, and continue while val >= 1.

Practice

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

easy

A. A do-while loop cannot be used with integers.

B. A do-while loop checks the condition before executing the code block.

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

D. A do-while loop does not require a semicolon after the while condition.

Solution

1. Step 1: Understand the execution order of loops

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

2. Step 2: Identify the behavior of do-while loop

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

3. Final Answer:

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

4. Quick Check:

   do-while runs once first = A [OK]

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

Common Mistakes:

• Thinking do-while checks condition first
• Believing semicolon after while is optional
• Confusing do-while with while loop behavior

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

easy

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

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

C. do { /* code */ } while condition

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

Solution

1. Step 1: Recall the correct syntax structure

   The do-while loop requires braces around the code block and a semicolon after the while condition in parentheses.

2. Step 2: Compare options to syntax

   do { /* code */ } while (condition); matches the correct syntax: do { /* code */ } while (condition); with braces and semicolon.

3. Final Answer:

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

4. Quick Check:

   Semicolon after while(condition) = B [OK]

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

Common Mistakes:

• Omitting semicolon after while condition
• Using parentheses instead of braces for code block
• Placing while before do

3. What will be the output of the following C code?

int i = 3;
do {
    printf("%d ", i);
    i--;
} while (i > 0);

medium

A. 1 2 3

B. 2 1 0

C. 3 2 1 0

D. 3 2 1

Solution

1. Step 1: Trace the loop iterations

   Initially, i = 3. The loop prints i, then decrements i by 1. It repeats while i > 0.

2. Step 2: List printed values

   First iteration: prints 3, i becomes 2
   Second iteration: prints 2, i becomes 1
   Third iteration: prints 1, i becomes 0
   Loop stops because i is not > 0.

3. Final Answer:

   3 2 1 -> Option D

4. Quick Check:

   Prints 3 down to 1 = A [OK]

Hint: Count printed values before condition fails [OK]

Common Mistakes:

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

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

int x = 5;
do {
    printf("%d", x);
    x--;
} while x > 0;

medium

A. The loop condition should be inside braces.

B. Missing parentheses around the while condition.

C. Variable x is not initialized.

D. Missing semicolon after the do block.

Solution

1. Step 1: Check syntax of the while condition

   The while condition must be enclosed in parentheses, like while (x > 0);.

2. Step 2: Identify the error in the code

   The code has while x > 0; missing parentheses, causing a syntax error.

3. Final Answer:

   Missing parentheses around the while condition. -> Option B

4. Quick Check:

   while(condition) needs parentheses = D [OK]

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

Common Mistakes:

• Omitting parentheses in while condition
• Forgetting semicolon after while statement
• Misplacing braces around condition

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?

hard

A. int num;
do {
printf("Enter a positive number: ");
scanf("%d", &num);
} while (num <= 0);

B. int num = 0;
do {
scanf("%d", &num);
printf("Enter a positive number: ");
} while (num > 0);

C. int num;
do {
scanf("%d", &num);
} while (num < 0);

D. int num;
do {
printf("Enter a positive number: ");
scanf("%d", &num);
} while (num < 0);

Solution

1. Step 1: Understand the requirement

   The loop must ask for input at least once and repeat while the number is not positive (<= 0).

2. Step 2: Analyze each option

   int num; do { printf("Enter a positive number: "); scanf("%d", &num); } while (num <= 0); prompts before input and repeats while number is <= 0, which matches the requirement.
   int num = 0; do { scanf("%d", &num); printf("Enter a positive number: "); } while (num > 0); prints prompt after input and repeats while number > 0, which is incorrect.
   int num; do { scanf("%d", &num); } while (num < 0); repeats while number < 0, missing zero case.
   int num; do { printf("Enter a positive number: "); scanf("%d", &num); } while (num < 0); repeats while number < 0, missing zero case.

3. Final Answer:

   int num; do { printf("Enter a positive number: "); scanf("%d", &num); } while (num <= 0); -> Option A

4. Quick Check:

   Repeat while number not positive = C [OK]

Hint: Repeat while input ≤ 0, prompt before input [OK]

Common Mistakes:

• Checking wrong condition (e.g., num < 0 instead of <= 0)
• Prompting after input instead of before
• Not handling zero as invalid input