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);

int main(int argc, char *argv[]) {
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++) {
do {
printf("Enter a number:\n");
} while (1 != scanf("%d", &numToAdd));

printf("Current list after your inserted node: \n");
}

return 0;
}

void print_list (struct Node *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;

}

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:

1. The list is empty, so we need to add a new node. In which case, our memory looks like this where HEAD is a pointer to the first node:

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;

1. 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; PDF - Download C Language for free