C Language Linked lists Inserting a node at the beginning of a singly linked list
Example
The code below will prompt for numbers and continue to add them to the beginning of a linked list.
/* This program will demonstrate inserting a node at the beginning of a linked list */
#include <stdio.h>
#include <stdlib.h>
struct Node {
int data;
struct Node* next;
};
void insert_node (struct Node **head, int nodeValue);
void print_list (struct Node *head);
int main(int argc, char *argv[]) {
struct Node* headNode;
headNode = NULL; /* Initialize our first node pointer to be NULL. */
size_t listSize, i;
do {
printf("How many numbers would you like to input?\n");
} while(1 != scanf("%zu", &listSize));
for (i = 0; i < listSize; i++) {
int numToAdd;
do {
printf("Enter a number:\n");
} while (1 != scanf("%d", &numToAdd));
insert_node (&headNode, numToAdd);
printf("Current list after your inserted node: \n");
print_list(headNode);
}
return 0;
}
void print_list (struct Node *head) {
struct node* currentNode = head;
/* Iterate through each link. */
while (currentNode != NULL) {
printf("Value: %d\n", currentNode->data);
currentNode = currentNode -> next;
}
}
void insert_node (struct Node **head, int nodeValue) {
struct Node *currentNode = malloc(sizeof *currentNode);
currentNode->data = nodeValue;
currentNode->next = (*head);
*head = currentNode;
}
Explanation for the Insertion of Nodes
In order to understand how we add nodes at the beginning, let's take a look at possible scenarios:
- The list is empty, so we need to add a new node. In which case, our memory looks like this where
HEADis a pointer to the first node:
| HEAD | --> NULL
The line currentNode->next = *headNode; will assign the value of currentNode->next to be NULL since headNode originally starts out at a value of NULL.
Now, we want to set our head node pointer to point to our current node.
----- -------------
|HEAD | --> |CURRENTNODE| --> NULL /* The head node points to the current node */
----- -------------
This is done with *headNode = currentNode;
- The list is already populated; we need to add a new node to the beginning. For the sake of simplicity, let's start out with 1 node:
----- -----------
HEAD --> FIRST NODE --> NULL
----- -----------
With currentNode->next = *headNode, the data structure looks like this:
--------- ----- ---------------------
currentNode --> HEAD --> POINTER TO FIRST NODE --> NULL
--------- ----- ---------------------
Which, obviously needs to be altered since *headNode should point to currentNode.
---- ----------- ---------------
HEAD -> currentNode --> NODE -> NULL
---- ----------- ---------------
This is done with *headNode = currentNode;
