I am new to C and have written this code just to teach myself how to work with linked lists and was hoping someone could review it and give me any comments or tips for efficiency, etc. The code just creates a list, adds and removes nodes from the list, and can reverse the list.
/* Creates a linked list, adds and removes nodes and reverses list */
#include <stdlib.h>
#include <stddef.h>
#include <stdio.h>
typedef bool;
#define false 0
#define true 1
typedef struct node {
int value;
struct node *next;
} node;
node *head = NULL; //Contains entire list
node *current = NULL; //Last item currently in the list
// Initialize list with input value
struct node* create_list(int input)
{
node* ptr;
printf("Creating a LinkedList with head node = %d\n", input);
ptr = (node*)malloc(sizeof(node)); // Allocating 8 bytes of memory for type node pointer
if (ptr == NULL) // Would occur is malloc can't allocate enough memory
{
printf("Node creation failed \n");
return NULL;
}
ptr->value = input;
ptr->next = NULL;
head = current = ptr;
return ptr;
}
// Add input value to the end of the list
struct node* add_to_end(int input)
{
node* ptr;
if (head == NULL)
{
return create_list(input);
}
ptr = (struct node*)malloc(sizeof(struct node));
if (ptr == NULL)
{
printf("Node creation failed \n");
return NULL;
}
else
{
ptr->value = input;
ptr->next = NULL; // End value in list should have NULL next pointer
current -> next = ptr; // Current node contains last value information
current = ptr;
}
printf("%d Added to END of the LinkedList.\n", input);
return head;
}
// Add input value to the head of the list
struct node* add_to_front(int input)
{
node* ptr;
if (head == NULL)
{
return create_list(input);
}
ptr = (struct node*)malloc(sizeof(struct node));
if (ptr == NULL)
{
printf("Node creation failed \n");
return NULL;
}
else
{
ptr->value = input;
ptr->next = head; // Point next value to the previous head
head = ptr;
}
printf("%d Added to HEAD of the LinkedList.\n", input);
return head;
}
// Return the number of items contained in a list
int size_list(node* ptr)
{
int index_count = 0;
while (ptr != NULL)
{
++index_count;
ptr = ptr->next;
}
return index_count;
}
// Add an input value at a user-specified index location in the list (starting from 0 index)
struct node* add_to_list(int input, int index)
{
node* ptr_prev = head;
node* ptr_new;
int index_count; // Used to count size of list
int index_track = 1; // Used to track current index
ptr_new = (struct node*)malloc(sizeof(struct node));
// Check that list exists before adding it in
if (head == NULL)
{
if (index == 0) // Create new list if 0 index is specified
{
add_to_front(input);
}
else
{
printf("Could not insert '%d' at index '%d' in the LinkedList because the list has not been initialized yet.\n", input, index);
return NULL;
}
}
// Count items in list to check whether item can added at specified location
if ((index_count = size_list(head)) < index)
{
printf("Could not insert '%d' at index '%d' in the LinkedList because there are only '%d' nodes in the LinkedList.\n", input, index, index_count);
return NULL;
}
//Go through list -- stop at item before insertion point
while (ptr_prev != NULL)
{
if (index == 0) // Use add_to_front function if user-specified index is 0 (the head of the list)
{
add_to_front(input);
return head;
}
if ((index_track) == index)
{
break;
}
ptr_prev = ptr_prev ->next;
++index_track;
}
ptr_new ->next = ptr_prev ->next; // Change the new node to point to the original's next pointer
ptr_new->value = input;
ptr_prev ->next = ptr_new; // Change the original node to point to the new node
return head;
}
// Verify if the list contains an input value and return the pointer to the value if it exists
struct node* search_list(int input, struct node **prev)
{
node* ptr = head;
node* temp = (node*)malloc(sizeof(node));
bool found = false;
// Search if value to be deleted exists in the list
while (ptr != NULL)
{
if(ptr->value == input)
{
found = true;
break;
}
else
{
temp = ptr;
ptr = ptr ->next;
}
}
// If the value is found in the list return the ptr to it
if(found == true)
{
if(prev)
*prev = temp;
return ptr;
}
else
{
return NULL;
}
}
// Remove an input value from the list
struct node* remove_from_list(int input)
{
node* prev = NULL; // list starting from one item before value to be deleted
node* del = NULL; // pointer to deleted value
// Obtain pointer to the list value to be deleted
del = search_list(input, &prev);
if(del == NULL)
{
printf("Error: '%d' could not be deleted from the LinkedList because it could not be found\n");
return NULL;
}
else
{
if (prev != NULL)
{
prev->next = del->next;
}
if (del == current) // If item to be deleted is last in list, set the current last item as the item before deleted one
{
current = prev;
}
else if (del == head) // If item to be deleted is the head of the list, set the new head as the item following the deleted one
{
head = del ->next;
}
return head;
}
}
// Reverse the order of the list
struct node* reverse_list()
{
node* reverse = NULL;
node* next = NULL;
node* ptr = head;
if (head == NULL)
{
printf("Error: There is no LinkedList to reverse.\n");
return NULL;
}
printf("Reversing order of the LinkedList.\n");
while (ptr != NULL)
{
next = ptr ->next; // Holds the remaining items in the original list
ptr ->next = reverse; // List now points to items in reversed list
reverse = ptr; // Reversed list set equal to the List
ptr = next; // List re-pointed back to hold list
}
head = reverse;
}
// Print the list to the console
void print_list()
{
node* ptr = head;
printf("------------------------------\n");
printf("PRINTING LINKED LIST\n");
while (ptr != NULL)
{
printf("%d\n", ptr->value);
ptr = ptr->next;
}
printf("------------------------------\n");
}
int main()
{
int i;
reverse_list(); //test function error message
for (i = 3; i > 0; --i)
{
add_to_front(i);
}
for (i= 4; i < 7; ++i)
{
add_to_end(i);
}
add_to_list(4,9); //test function error message
add_to_list(4,1);
print_list();
remove_from_list(3);
print_list();
reverse_list();
print_list();
add_to_list(10,0);
print_list();
getchar();
}