DATA STRUCTURES PROGRAM • QUEUES & DEQUES

Implement a Deque with a Circular Array

Learn how to implement a deque with a circular array using a clear C program.

IntermediateQueueFIFOfront/rear

PROBLEM UNDERSTANDING

Input and expected output

Sample input

No input required

Sample output

Front = 5
Rear = 20

COMPLETE C PROGRAM

Complete C implementation

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

void push_front(int deque[], int *front, int *count, int value)
{
    if (*count == 6) exit(EXIT_FAILURE);
    *front = (*front + 5) % 6;
    deque[*front] = value; (*count)++;
}
void push_back(int deque[], int *front, int *count, int value)
{
    if (*count == 6) exit(EXIT_FAILURE);
    deque[(*front + *count) % 6] = value; (*count)++;
}

int main(void)
{
    int deque[6] = {0}, front = 0, count = 0;
    push_back(deque, &front, &count, 10);
    push_back(deque, &front, &count, 20);
    push_front(deque, &front, &count, 5);
    printf("Front = %d\n", deque[front]);
    int rear = (front + count - 1) % 6;
    printf("Rear = %d\n", deque[rear]);
    return 0;
}

CURRENT STEP

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EXPECTED OUTPUT FOR THE SAMPLE

Front = 5
Rear = 20

0%Step 0 of 0

PROGRAM EXPLANATION

Algorithm and explanation

Read the required input values.
Use circular indices to allow insertion and removal at both ends.
Display the computed result.

Use circular indices to allow insertion and removal at both ends.

EFFICIENCY

Time and space complexity

Time complexity

O(1) per operation

Auxiliary space

O(n)

DEBUGGING CHECKLIST

Common mistakes

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Use the correct format specifier for every variable.

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Initialize variables before using their values.

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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.