Queue Implementation Using Array in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the front index of the queue.

DSA C

int front = [1];

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

A0

BNULL

C1

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing front to 0 causes confusion between empty and first element.

Using NULL is invalid for integer variables.

The front index is initialized to -1 to indicate the queue is empty.

2fill in blank

medium

Complete the code to check if the queue is full.

DSA C

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

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

Afront

BMAX_SIZE

Crear

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing rear to front instead of MAX_SIZE.

Using size variable which may not be defined.

The queue is full when rear reaches MAX_SIZE - 1, the last index of the array.

3fill in blank

hard

Fix the error in the enqueue function to correctly add an element.

DSA C

void enqueue(int queue[], int *rear, int value) {
    if (*rear == MAX_SIZE - 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 rear to move backward.

Using * or / causes incorrect pointer arithmetic.

To add an element, increment rear by 1 before assigning the value.

4fill in blank

hard

Fill the blank to correctly dequeue an element and update front.

DSA C

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

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

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using - causes front to move backward.

Using * or / causes wrong index calculation.

Increment front by 1 to remove the front element and return it.

5fill in blank

hard

Fill all three blanks to implement a function that prints all elements in the queue.

DSA C

void printQueue(int queue[], int front, int rear) {
    if (front == rear) {
        printf("Queue is empty\n");
    } else {
        for (int i = front [1] 1; i [2] rear; i [3] 1) {
            printf("%d -> ", queue[i]);
        }
        printf("NULL\n");
    }
}

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

A+

B<=

C<

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using <= may cause printing beyond the rear element.

Using - or other operators breaks the loop logic.

The loop starts from front + 1, runs while i < rear, and increments i by 1 each time to print all elements.

Practice

1. What is the main characteristic of a queue implemented using an array?

easy

A. It follows First-In-First-Out (FIFO) order

B. It follows Last-In-First-Out (LIFO) order

C. It allows random access to elements

D. It stores elements in sorted order

Solution

1. Step 1: Understand queue behavior

   A queue is a data structure where the first element added is the first one to be removed, which is FIFO.

2. Step 2: Compare with options

   It follows First-In-First-Out (FIFO) order correctly states FIFO order. It follows Last-In-First-Out (LIFO) order is LIFO (stack), C allows random access (array), D stores sorted (not queue).

3. Final Answer:

   It follows First-In-First-Out (FIFO) order -> Option A

4. Quick Check:

   Queue = FIFO [OK]

Hint: Queue always removes the oldest element first [OK]

Common Mistakes:

• Confusing queue with stack (LIFO)
• Thinking queue allows random access
• Assuming queue sorts elements automatically

2. Which of the following is the correct way to declare a queue array of size 5 in C?

easy

A. queue int[5];

B. int queue[5];

C. int queue();

D. int queue = new int[5];

Solution

1. Step 1: Recall C array declaration syntax

   In C, arrays are declared as type name[size], e.g., int queue[5];

2. Step 2: Check each option

   int queue[5]; matches correct syntax. queue int[5]; is invalid syntax. int queue(); declares a function, not an array. int queue = new int[5]; is Java-style syntax.

3. Final Answer:

   int queue[5]; -> Option B

4. Quick Check:

   C array declaration = int name[size] [OK]

Hint: C arrays use type name[size] syntax [OK]

Common Mistakes:

• Using Java or other language syntax
• Confusing function declaration with array
• Missing semicolon or brackets

3. Consider this C code snippet for a queue implemented using an array of size 3:

int queue[3];
int front = -1, rear = -1;

// Enqueue 10
rear++;
queue[rear] = 10;
if(front == -1) front = 0;

// Enqueue 20
rear++;
queue[rear] = 20;

// Dequeue
int val = queue[front];
front++;

// Enqueue 30
rear++;
queue[rear] = 30;

// Current queue elements from front to rear?

medium

A. 30 -> null

B. 10 -> 20 -> 30 -> null

C. 20 -> 30 -> null

D. 10 -> 30 -> null

Solution

1. Step 1: Trace enqueue and dequeue operations

   Initially front = -1, rear = -1. Enqueue 10: rear=0, queue[0]=10, front=0. Enqueue 20: rear=1, queue[1]=20. Dequeue: val=queue[front]=queue[0]=10, front=1. Enqueue 30: rear=2, queue[2]=30.

2. Step 2: Identify current queue elements

   front=1, rear=2, so elements are queue[1]=20 and queue[2]=30. Hence queue is 20 -> 30 -> null.

3. Final Answer:

   20 -> 30 -> null -> Option C

4. Quick Check:

   After dequeue, front moves forward; queue = 20, 30 [OK]

Hint: After dequeue, front moves forward; queue shows elements from front to rear [OK]

Common Mistakes:

• Not updating front after dequeue
• Including dequeued elements in output
• Confusing front and rear indices

4. Identify the error in this queue enqueue function using array in C:

void enqueue(int queue[], int *rear, int size, int value) {
    if (*rear == size) {
        printf("Queue is full\n");
        return;
    }
    (*rear)++;
    queue[*rear] = value;
}

medium

A. rear should be compared with size - 1, not size

B. rear should be incremented after inserting value

C. Function should return int instead of void

D. queue array should be dynamically allocated

Solution

1. Step 1: Analyze the full condition

   The check if(*rear == size) is wrong. Array indices are 0 to size-1, so full when *rear == size-1 before incrementing. Current check allows overflow.

2. Step 2: Compare options

   rear should be compared with size - 1, not size identifies the correct fix: compare with size-1, not size. Others are incorrect.

3. Final Answer:

   rear should be compared with size - 1, not size -> Option A

4. Quick Check:

   Full: *rear == size-1 [OK]

Hint: Full check: rear == size - 1 [OK]

Common Mistakes:

• Comparing rear == size instead of size-1
• Incorrect overflow condition
• Ignoring zero-based indexing

5. You have a queue implemented using an array of size 4 with front = 1 and rear = 3. After dequeuing two elements and enqueuing one element, what will be the correct front and rear values if you implement a circular queue?

hard

A. front = 2, rear = 0

B. front = 3, rear = 1

C. front = 0, rear = 1

D. front = 3, rear = 0

Solution

1. Step 1: Understand initial state and operations

   Initial: front=1, rear=3, size=4. Dequeue twice: front moves forward twice. Since circular, front = (1 + 2) % 4 = 3.

2. Step 2: Enqueue one element

   rear moves forward by one circularly: rear = (3 + 1) % 4 = 0.

3. Final Answer:

   front = 3, rear = 0 -> Option D

4. Quick Check:

   Circular queue uses modulo for front and rear [OK]

Hint: Use modulo to wrap front and rear in circular queue [OK]

Common Mistakes:

• Not using modulo for circular increment
• Updating front or rear incorrectly
• Confusing front and rear positions