Queue Concept and FIFO Principle in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an element to the queue.

DSA C

void enqueue(int value) {
    if (rear == MAX - 1) {
        printf("Queue is full\n");
    } else {
        rear = rear [1] 1;
        queue[rear] = value;
    }
}

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

A/

B-

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' instead of '+' causes the rear to move backward.

Using '*' or '/' operators is incorrect for index increment.

To add an element, we increase the rear index by 1 using '+'.

2fill in blank

medium

Complete the code to remove an element from the queue.

DSA C

int dequeue() {
    if (front > rear) {
        printf("Queue is empty\n");
        return -1;
    } else {
        int value = queue[front];
        front = front [1] 1;
        return value;
    }
}

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

A-

B*

C+

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' instead of '+' causes the front to move backward.

Using '*' or '/' operators is incorrect for index increment.

To remove an element, we increase the front index by 1 using '+'.

3fill in blank

hard

Fix the error in the condition that checks if the queue is empty.

DSA C

if (front [1] rear) {
    printf("Queue is empty\n");
}

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

A<

B>

C==

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' or '==' causes wrong empty queue detection.

Using '!=' is incorrect for empty condition.

The queue is empty when front is greater than rear, so use '>'.

4fill in blank

hard

Fill both blanks to correctly initialize the queue indices.

DSA C

int front = [1];
int rear = [2];

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

A-1

B0

C1

DMAX

Attempts:

3 left

💡 Hint

Common Mistakes

Setting both to -1 causes front > rear to fail initially (-1 not > -1).

Using MAX is out of bounds.

front starts at 0 and rear at -1 to indicate an empty queue.

5fill in blank

hard

Fill the blanks to check if the queue is full.

DSA C

if (rear [1] MAX [2] 1) {
    printf("Queue is full\n");
}

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

A==

B-

C+

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '-' causes wrong index calculation.

Using '>' instead of '==' causes incorrect full check.

The queue is full when rear equals MAX - 1 (rear == MAX - 1).

Practice

1. What does FIFO stand for in the context of a queue?

easy

A. First Input, First Operation

B. First In, Final Out

C. Fast Input, Fast Output

D. First In, First Out

Solution

1. Step 1: Understand FIFO meaning

   FIFO means the first element added to the queue is the first one to be removed.

2. Step 2: Match the acronym

   First In, First Out exactly describes this behavior.

3. Final Answer:

   First In, First Out -> Option D

4. Quick Check:

   FIFO = First In, First Out [OK]

Hint: Remember queue removes oldest item first [OK]

Common Mistakes:

• Confusing FIFO with LIFO
• Thinking FIFO means last added first removed
• Mixing up acronym letters

2. Which of the following is the correct syntax to enqueue an element `x` into a queue named `q` in C?

easy

A. push(q, x);

B. enqueue(q, x);

C. q.enqueue(x);

D. q.add(x);

Solution

1. Step 1: Identify common C queue operation

   In C, queue operations are usually functions like enqueue(queue, element).

2. Step 2: Check options for C style

   enqueue(q, x); uses function call syntax typical in C. Others use object-oriented syntax not common in C.

3. Final Answer:

   enqueue(q, x); -> Option B

4. Quick Check:

   C queue enqueue = enqueue(q, x); [OK]

Hint: C uses function calls, not dot notation [OK]

Common Mistakes:

• Using object-oriented syntax in C
• Confusing push with enqueue
• Using wrong function names

3. Consider the following C code snippet using a queue:

enqueue(q, 10);
enqueue(q, 20);
dequeue(q);
enqueue(q, 30);
dequeue(q);

What is the next element to be dequeued?

medium

A. 30

B. 10

C. 20

D. Queue is empty

Solution

1. Step 1: Trace enqueue and dequeue operations

   Queue after enqueue 10 and 20: [10, 20]. Dequeue removes 10, queue: [20]. Enqueue 30: [20, 30]. Dequeue removes 20, queue: [30].

2. Step 2: Identify next element to dequeue

   Next element is the front of queue, which is 30.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   Queue front after operations = 30 [OK]

Hint: Follow FIFO order step-by-step [OK]

Common Mistakes:

• Assuming last enqueued is dequeued next
• Forgetting previous dequeues
• Mixing enqueue and dequeue order

4. Identify the error in the following C code snippet for queue operations:

enqueue(q, 5);
dequeue(q);
dequeue(q);

Assuming the queue was initially empty.

medium

A. Second dequeue causes underflow error

B. No error, code runs fine

C. First enqueue is incorrect syntax

D. Queue overflow error occurs

Solution

1. Step 1: Analyze queue state after operations

   Initially empty queue. Enqueue 5 adds one element. First dequeue removes it, queue empty again.

2. Step 2: Check second dequeue

   Second dequeue tries to remove from empty queue, causing underflow error.

3. Final Answer:

   Second dequeue causes underflow error -> Option A

4. Quick Check:

   Dequeue empty queue = underflow error [OK]

Hint: Check if queue is empty before dequeue [OK]

Common Mistakes:

• Ignoring empty queue before dequeue
• Confusing overflow with underflow
• Assuming enqueue syntax error

5. You have a queue that can hold a maximum of 3 elements. The following operations are performed:

enqueue(q, 1);
enqueue(q, 2);
enqueue(q, 3);
dequeue(q);
enqueue(q, 4);
dequeue(q);
dequeue(q);

What is the state of the queue after these operations?

hard

A. Queue is empty

B. 3 -> 4 -> null

C. 4 -> null

D. 2 -> 3 -> 4 -> null

Solution

1. Step 1: Track queue after each operation

   Enqueue 1,2,3: queue = [1,2,3]. Dequeue removes 1: queue = [2,3]. Enqueue 4: queue = [2,3,4]. Dequeue removes 2: queue = [3,4]. Dequeue removes 3: queue = [4].

2. Step 2: Final queue state

   Only element left is 4, so queue is 4 -> null.

3. Final Answer:

   4 -> null -> Option C

4. Quick Check:

   Final queue front = 4 [OK]

Hint: Follow enqueue and dequeue stepwise, watch max size [OK]

Common Mistakes:

• Forgetting max size limit
• Miscounting dequeues
• Assuming queue empty incorrectly