A dequeue is an ordered set of elements in which elements may be inserted or retrieved from either end. Using an array simulate a dequeue of characters and the operations retrieve left, retrieve right, insert left, insert right. Exceptional conditions such as dequeue full or empty should be indicated. Two pointers (namely, left and right) are needed in the simulation.

#include<stdio.h>
#include<conio.h>

#define MAX 10

void lAdd(int *l, int *q, int *r, int num);
void rAdd(int *l, int *q, int *r, int num);
void show(int *q, int *rside);
int rFetch(int *l, int *q, int *r);
int lFetch(int *l, int *q, int *r);

int main()
{
	int que[MAX];
	int left, right;
	left = right = -1;

	rAdd(&left, que, &right, 10);
	rAdd(&left, que, &right, 20);
	rAdd(&left, que, &right, 30);
	show(que, &right);

	lAdd(&left, que, &right, 5);
	show(que, &right);

	lFetch(&left, que, &right);
	show(que, &right);

	rFetch(&left, que, &right);
	show(que, &right);

	_getch();
	return 0;
}

/*Insertion from right*/
void rAdd(int *lside, int *q, int *rside, int num)
{
	if (*rside == MAX - 1)
	{
		printf("\nDequeue is full, no more insertion is possible.\n");
		return;
	}

	if (*rside == -1)
		*rside = 0;
	else
		(*rside)++;
	q[*rside] = num;
}

/*Insertion from left*/
void lAdd(int *lside, int *q, int *rside, int num)
{
	int i;

	if (*rside == MAX - 1)
	{
		printf("\nDequeue is full, no more insertion is possible.\n");
		return;
	}

	for (i = *rside; i >= 0; i--)
		q[i+1] = q[i];

	q[0] = num;

	if (*lside == -1)
		*lside = 0;
	(*rside)++;
}

/*Dislays the list*/
void show(int *q, int *rside)
{
	int i;

	printf("\n\nList\n");
	for (i = 0; i <= *rside; i++)
		printf("%d\t", q[i]);
	printf("\n\n");
}

/*Retrieval from left*/
int lFetch(int *lside, int *q, int *rside)
{
	int item = q[0], i;
	if (*rside == -1)
	{
		printf("\nList is empty.\n");
		return NULL;
	}

	for (i = 0; i < *rside; i++)
		q[i] = q[i + 1];

	(*rside)--;
	return item;
}

/*Retrieval from right*/
int rFetch(int *lside, int *q, int *rside)
{
	if (*rside == -1)
	{
		printf("\nList is empty.\n");
		return NULL;
	}
	int item = q[*rside];
	(*rside)--;
	return item;
}