Enqueue Operation in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Enqueue in Circular Queue

What is the output of the following C code after performing two enqueue operations on an initially empty circular queue of size 3?

DSA C

#include <stdio.h>
#define SIZE 3

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

void enqueue(int value) {
    if ((rear + 1) % SIZE == front) {
        printf("Queue is full\n");
        return;
    }
    if (front == -1) front = 0;
    rear = (rear + 1) % SIZE;
    queue[rear] = value;
}

void printQueue() {
    if (front == -1) {
        printf("Queue is empty\n");
        return;
    }
    int i = front;
    while (1) {
        printf("%d ", queue[i]);
        if (i == rear) break;
        i = (i + 1) % SIZE;
    }
    printf("\n");
}

int main() {
    enqueue(10);
    enqueue(20);
    printQueue();
    return 0;
}

A Queue is empty

B 20 10

C Queue is full

D 10 20

💡 Hint

Remember how circular queue updates rear and front indices.

The queue starts empty with front and rear at -1. After enqueue(10), front and rear become 0, queue[0] = 10. After enqueue(20), rear moves to 1, queue[1] = 20. Printing from front to rear outputs '10 20'.

❓ Predict Output

intermediate

State of Queue After Enqueue Operations

Given the following C code for a queue implemented with an array, what will be the printed queue after these enqueue operations?

DSA C

#include <stdio.h>
#define MAX 4

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

void enqueue(int val) {
    if (rear == MAX - 1) {
        printf("Queue is full\n");
        return;
    }
    if (front == -1) front = 0;
    rear++;
    queue[rear] = val;
}

void printQueue() {
    if (front == -1) {
        printf("Queue is empty\n");
        return;
    }
    for (int i = front; i <= rear; i++) {
        printf("%d ", queue[i]);
    }
    printf("\n");
}

int main() {
    enqueue(5);
    enqueue(15);
    enqueue(25);
    printQueue();
    return 0;
}

A Queue is full

B 15 25

C 5 15 25

D Queue is empty

💡 Hint

Check how rear and front indices change after each enqueue.

Initially front and rear are -1. After enqueue(5), front=0, rear=0, queue[0]=5. Then enqueue(15) sets rear=1, queue[1]=15. Then enqueue(25) sets rear=2, queue[2]=25. Printing from front to rear prints '5 15 25'.

🔧 Debug

advanced

Identify the Bug in Enqueue Function

What error will occur when running this enqueue function in C for a queue implemented with an array?

DSA C

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

void enqueue(int val) {
    if (rear == SIZE) {
        printf("Queue is full\n");
        return;
    }
    if (front == -1) front = 0;
    rear++;
    queue[rear] = val;
}

A Array index out of bounds error

B Queue is full message printed prematurely

C No error, works fine

D Compilation error due to missing semicolon

💡 Hint

Check the condition that stops enqueue when queue is full.

The condition 'rear == SIZE' allows rear to become SIZE, which is out of array bounds (0 to SIZE-1). This causes an array index out of bounds error when queue[rear] is accessed.

❓ Predict Output

advanced

Output After Multiple Enqueue and Dequeue in Circular Queue

What will be the output of the queue after these operations on a circular queue of size 4?

DSA C

#include <stdio.h>
#define SIZE 4

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

void enqueue(int val) {
    if ((rear + 1) % SIZE == front) {
        printf("Queue is full\n");
        return;
    }
    if (front == -1) front = 0;
    rear = (rear + 1) % SIZE;
    queue[rear] = val;
}

void dequeue() {
    if (front == -1) {
        printf("Queue is empty\n");
        return;
    }
    if (front == rear) {
        front = rear = -1;
    } else {
        front = (front + 1) % SIZE;
    }
}

void printQueue() {
    if (front == -1) {
        printf("Queue is empty\n");
        return;
    }
    int i = front;
    while (1) {
        printf("%d ", queue[i]);
        if (i == rear) break;
        i = (i + 1) % SIZE;
    }
    printf("\n");
}

int main() {
    enqueue(1);
    enqueue(2);
    enqueue(3);
    dequeue();
    enqueue(4);
    printQueue();
    return 0;
}

A 1 2 3 4

B 2 3 4

C Queue is full

D Queue is empty

💡 Hint

Remember how front and rear move in circular queue after dequeue and enqueue.

Initially enqueue 1,2,3 fills queue at indices 0,1,2. Dequeue moves front from 0 to 1. Enqueue 4 moves rear from 2 to 3. Printing from front=1 to rear=3 outputs '2 3 4'.

🧠 Conceptual

expert

Maximum Number of Enqueue Operations in Circular Queue

Consider a circular queue implemented with an array of size N. If the queue is initially empty, what is the maximum number of enqueue operations that can be performed before the queue is full?

A N - 1

B N

C N + 1

D N / 2

💡 Hint

Think about how circular queue uses one slot to distinguish full from empty.

In a circular queue, one slot is kept empty to differentiate between full and empty states. So, maximum elements stored is N - 1.