Why Queue Exists and What Problems It Solves in DSA C - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enqueue an element at the rear of the queue.

DSA C

void enqueue(int queue[], int *rear, int value) {
    queue[++(*rear)] = [1];
}

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

Avalue

Brear

Cqueue

Dfront

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'rear' instead of 'value' to insert the element.

Using 'front' or 'queue' which are not the element to add.

The enqueue function adds the given value at the position after the current rear.

2fill in blank

medium

Complete the code to dequeue an element from the front of the queue.

DSA C

int dequeue(int queue[], int *front, int rear) {
    if (*front == -1 || *front > rear) {
        return -1; // Queue is empty
    }
    return queue[[1]++];
}

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

Arear

B*rear

Cfront

D*front

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'front' without dereferencing, causing wrong index access.

Using 'rear' which points to the end, not the front.

The element at the current front is returned, then front is incremented to remove it from the queue.

3fill in blank

hard

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

DSA C

int isFull(int rear, int maxSize) {
    return rear == [1] - 1;
}

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

AmaxSize + 1

Brear

CmaxSize

Dfront

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing rear with rear or front instead of maxSize.

Using maxSize + 1 which is out of bounds.

The queue is full when rear index reaches maxSize - 1, the last valid position.

4fill in blank

hard

Fill both blanks to correctly initialize the queue pointers.

DSA C

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

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

A-1

B0

C1

DmaxSize

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing front or rear to 0 which may cause off-by-one errors.

Using maxSize which is out of valid index range.

Both front and rear are initialized to -1 to indicate the queue is empty.

5fill in blank

hard

Fill the blanks to implement a circular queue condition to check if the queue is full.

DSA C

int isFull(int front, int rear, int maxSize) {
    return (rear + 1) % [1] == [2];
}

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

AmaxSize

Bfront

Crear

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Not using modulo operation causing index overflow.

Mixing up front and rear in the condition.

In a circular queue (wasting one space to distinguish full from empty), the queue is full when (rear + 1) % maxSize == front.

Practice

1. Why do we use a queue data structure instead of just using an array or list for tasks that need to be processed in order?

easy

A. Because a queue automatically processes elements in the order they arrive (first-in, first-out).

B. Because a queue stores elements in random order for faster access.

C. Because a queue allows jumping to any element quickly.

D. Because a queue stores only unique elements.

Solution

1. Step 1: Understand the order of processing

   A queue processes elements in the order they arrive, known as first-in, first-out (FIFO).

2. Step 2: Compare with other data structures

   Arrays or lists do not enforce FIFO automatically; queues do, which is useful for tasks like printing or scheduling.

3. Final Answer:

   Because a queue automatically processes elements in the order they arrive (first-in, first-out). -> Option A

4. Quick Check:

   Queue = FIFO order [OK]

Hint: Queues process in arrival order, unlike arrays [OK]

Common Mistakes:

• Thinking queues store elements randomly
• Confusing queue with stack (LIFO)
• Assuming queues allow random access

2. Which of the following is the correct way to add an element to the end of a queue in C?

easy

A. queue_push(&q, element);

B. queue_enqueue(&q, element);

C. queue_addFirst(&q, element);

D. queue_insert(&q, 0, element);

Solution

1. Step 1: Identify standard queue operation names

   The standard operation to add an element to a queue is called enqueue.

2. Step 2: Match the correct function name

   Among the options, only queue_enqueue correctly represents adding to the end of the queue.

3. Final Answer:

   queue_enqueue(&q, element); -> Option B

4. Quick Check:

   Enqueue means add to queue end [OK]

Hint: Enqueue adds to queue end, dequeue removes from front [OK]

Common Mistakes:

• Using push which is for stacks
• Using insert with index in queues
• Confusing addFirst with adding to front

3. Consider this C code snippet using a queue:

queue q;
queue_init(&q);
queue_enqueue(&q, 10);
queue_enqueue(&q, 20);
int x = queue_dequeue(&q);
queue_enqueue(&q, 30);
int y = queue_dequeue(&q);

What are the values of x and y after these operations?

medium

A. x = 10, y = 20

B. x = 20, y = 10

C. x = 10, y = 30

D. x = 20, y = 30

Solution

1. Step 1: Trace enqueue and dequeue operations

   First enqueue 10, then 20. Dequeue removes 10 (first in). Then enqueue 30. Next dequeue removes 20 (now first in).

2. Step 2: Assign values to x and y

   x gets 10 from first dequeue, y gets 20 from second dequeue.

3. Final Answer:

   x = 10, y = 20 -> Option A

4. Quick Check:

   FIFO order: 10 then 20 dequeued [OK]

Hint: Dequeue returns oldest enqueued element [OK]

Common Mistakes:

• Mixing order of dequeue results
• Assuming dequeue removes last element
• Forgetting to enqueue before dequeue

4. The following code tries to dequeue from an empty queue:

queue q;
queue_init(&q);
int val = queue_dequeue(&q);

What is the likely problem with this code?

medium

A. It will return the last element instead of error.

B. It will enqueue a default value automatically.

C. It will cause an error or undefined behavior because the queue is empty.

D. It will print the queue contents instead of dequeuing.

Solution

1. Step 1: Understand dequeue on empty queue

   Dequeuing from an empty queue has no element to remove, causing error or undefined behavior.

2. Step 2: Check code behavior

   Since queue is empty after init, dequeue likely causes error or returns invalid value.

3. Final Answer:

   It will cause an error or undefined behavior because the queue is empty. -> Option C

4. Quick Check:

   Dequeue empty queue = error [OK]

Hint: Never dequeue without checking if queue is empty [OK]

Common Mistakes:

• Assuming dequeue returns last element
• Expecting automatic enqueue on dequeue
• Thinking dequeue prints queue

5. A printer receives print jobs in this order: Job1, Job2, Job3. The printer processes jobs in the order they arrive. Which data structure best models this scenario and why?

hard

A. Stack, because it processes the most recent job first.

B. Array, because it stores jobs randomly for quick access.

C. Tree, because it organizes jobs hierarchically.

D. Queue, because it processes jobs in the order they arrive (FIFO).

Solution

1. Step 1: Identify processing order needed

   The printer processes jobs in arrival order, meaning first job received is processed first.

2. Step 2: Match data structure to order

   Queue follows first-in, first-out (FIFO), matching the printer's processing order.

3. Final Answer:

   Queue, because it processes jobs in the order they arrive (FIFO). -> Option D

4. Quick Check:

   Printer jobs = FIFO queue [OK]

Hint: FIFO matches real-world waiting lines like printers [OK]

Common Mistakes:

• Choosing stack which is LIFO
• Thinking array stores order automatically
• Confusing tree with linear processing