C Programming Code Examples
C > Linked Lists Code Examples
Program to Implement Doubly Linked List using Singly Linked List
/* Program to Implement Doubly Linked List using Singly Linked List
This C Program implements doubly linked list using singly linked list. It makes use of 2 pointers, one points at the current node, other points at the head. When user requests to move back, the pointer from head travels to a previous node of the current pointer. The pointer to previous node is the resulting node. */
#include <stdio.h>
#include <stdlib.h>
struct node
{
int j;
struct node *next;
};
void create(struct node **);
void move (struct node *);
void release(struct node **);
void display(struct node *);
int main()
{
struct node *p = NULL, *q = NULL;
int result, count;
printf("Enter data into the list\n");
create(&p);
printf("Displaying list:\n");
display(p);
move(p);
release (&p);
return 0;
}
void move(struct node *head)
{
struct node *p, *q;
int ch;
p = q = head;
printf("\nPointer at %d\n", head->j);
do
{
printf("Select option:\n1. Move front\n2. Move back\n3. Exit\nYour choice: ");
scanf("%d", &ch);
switch(ch)
{
case 1: if(q->next != NULL)
{
q = q->next;
printf("\nPointer at %d\n", q->j);
}
else
{
printf("\nPointer at last node %d. Cannot move ahead.\n", q->j);
}
break;
case 2: while (p->next != q)
{
p = p->next;
}
if (p == q)
{
printf("\nPointer at first node %d. Cannot move behind.\n", q->j);
}
else
{
q = p;
p = head;
printf("\nPointer at %d\n", q->j);
}
break;
case 3: return;
default: printf("\nInvalid choice entered. Try again\n");
}
} while (1);
}
void create(struct node **head)
{
int c, ch;
struct node *temp, *rear;
do
{
printf("Enter number: ");
scanf("%d", &c);
temp = (struct node *)malloc(sizeof(struct node));
temp->j = c;
temp->next = NULL;
if (*head == NULL)
{
*head = temp;
}
else
{
rear->next = temp;
}
rear = temp;
printf("Do you wish to continue [1/0]: ");
scanf("%d", &ch);
} while (ch != 0);
printf("\n");
}
void display(struct node *head)
{
while (head != NULL)
{
printf("%d\t", head->j);
head = head->next;
}
printf("\n");
}
void release(struct node **head)
{
struct node *temp;
while ((*head) != NULL)
{
temp = *head;
(*head) = (*head)->next;
free(temp);
}
}
Pointers in C are easy and fun to learn. Some C programming tasks are performed more easily with pointers, and other tasks, such as dynamic memory allocation, cannot be performed without using pointers. So it becomes necessary to learn pointers to become a perfect C programmer. Let's start learning them in simple and easy steps. As you know, every variable is a memory location and every memory location has its address defined which can be accessed using ampersand (&) operator, which denotes an address in memory. Consider the following example, which prints the address of the variables defined. A pointer is a variable whose value is the address of another variable, i.e., direct address of the memory location. Like any variable or constant, you must declare a pointer before using it to store any variable address. The general form of a pointer variable declaration is:
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The free() function in C library allows you to release or deallocate the memory blocks which are previously allocated by calloc(), malloc() or realloc() functions. It frees up the memory blocks and returns the memory to heap. It helps freeing the memory in your program which will be available for later use. In C, the memory for variables is automatically deallocated at compile time. For dynamic memory allocation in C, you have to deallocate the memory explicitly. If not done, you may encounter out of memory error.
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The if-else statement is used to perform two operations for a single condition. The if-else statement is an extension to the if statement using which, we can perform two different operations, i.e., one is for the correctness of that condition, and the other is for the incorrectness of the condition. Here, we must notice that if and else block cannot be executed simiulteneously. Using if-else statement is always preferable since it always invokes an otherwise case with every if condition.
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The break is a keyword in C which is used to bring the program control out of the loop. The break statement is used inside loops or switch statement. The break statement breaks the loop one by one, i.e., in the case of nested loops, it breaks the inner loop first and then proceeds to outer loops.
In C, the "main" function is treated the same as every function, it has a return type (and in some cases accepts inputs via parameters). The only difference is that the main function is "called" by the operating system when the user runs the program. Thus the main function is always the first code executed when a program starts. main() function is a user defined, body of the function is defined by the programmer or we can say main() is programmer/user implemented function, whose prototype is predefined in the compiler. Hence we can say that main() in c programming is user defined as well as predefined because it's prototype is predefined. main() is a system (compiler) declared function whose defined by the user, which is invoked automatically by the operating system when program is being executed.
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Writes the C string pointed by format to the standard output (stdout). If format includes format specifiers (subsequences beginning with %), the additional arguments following format are formatted and inserted in the resulting string replacing their respective specifiers. printf format string refers to a control parameter used by a class of functions in the input/output libraries of C programming language. The string is written in a simple template language: characters are usually copied literally into the function's output, but format specifiers, which start with a % character, indicate the location and method to translate a piece of data (such as a number) to characters. "printf" is the name of one of the main C output functions, and stands for "print formatted". printf format strings are complementary to scanf format strings, which provide formatted input (parsing). In both cases these provide simple functionality and fixed format compared to more sophisticated and flexible template engines or parsers,
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