DATA STRUCTURES PROGRAM • QUEUES & DEQUES

Implement a Circular Queue Using an Array

Learn how to implement a circular queue using an array using a clear C program.

IntermediateQueueFIFOfront/rear

PROBLEM UNDERSTANDING

Input and expected output

Sample input

No input required

Sample output

Removed = 10
20 30 40

COMPLETE C PROGRAM

Complete C implementation

#include <stdio.h>
#include <stdlib.h>

#define CAPACITY 5
struct Queue { int values[CAPACITY]; int front; int rear; int count; };
void enqueue(struct Queue *queue, int value)
{
    if (queue->count == CAPACITY) exit(EXIT_FAILURE);
    queue->values[queue->rear] = value;
    queue->rear = (queue->rear + 1) % CAPACITY;
    queue->count++;
}
int dequeue(struct Queue *queue)
{
    if (queue->count == 0) exit(EXIT_FAILURE);
    int value = queue->values[queue->front];
    queue->front = (queue->front + 1) % CAPACITY;
    queue->count--;
    return value;
}

int main(void)
{
    struct Queue queue = {{0}, 0, 0, 0};
    enqueue(&queue, 10); enqueue(&queue, 20); enqueue(&queue, 30);
    printf("Removed = %d\n", dequeue(&queue));
    enqueue(&queue, 40);
    while (queue.count > 0) printf("%d ", dequeue(&queue));
    putchar('\n');
    return 0;
}

CURRENT STEP

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EXPECTED OUTPUT FOR THE SAMPLE

Removed = 10
20 30 40

0%Step 0 of 0

PROGRAM EXPLANATION

Algorithm and explanation

Read the required input values.
Wrap front and rear with modulo so released array positions can be reused.
Display the computed result.

Wrap front and rear with modulo so released array positions can be reused.

EFFICIENCY

Time and space complexity

Time complexity

O(1) per operation

Auxiliary space

O(n)

DEBUGGING CHECKLIST

Common mistakes

Check this

Use the correct format specifier for every variable.

Check this

Initialize variables before using their values.

Check this

Check braces, semicolons and input order carefully.

Try it yourself

Practice: Run the program with the sample input, predict its output, and then test one boundary case of your own.