C program to delete first node of Singly Linked List

Required knowledge:

Algorithm to delete first node of Singly Linked List

Algorithm to delete first node of Singly Linked List
%%Input:  head of the linked list
Begin:
    If (head != NULL) then
        toDelete ← head
        head ← head.next
        unalloc (toDelete)
    End if
End

Steps to delete first node of Singly Linked List

1. Copy the address of first node i.e. head node to some temp variable say toDelete.
   
   
   
   
2. Move the head to the second node of the linked list i.e. head = head->next.
   
   
   
   
3. Disconnect the connection of first node to second node.
   
   
   
   
4. Free the memory occupied by the first node.
   
   
   
   

Program to delete first node of Singly Linked List

/**
 * C program to delete first node of Singly Linked List
 */

#include <stdio.h>
#include <stdlib.h>


/* Structure of a node */
struct node {
    int data; //Data part
    struct node *next; //Address part
}*head;



void createList(int n);
void deleteFirstNode();
void displayList();



int main()
{
    int n, choice;

    /*
     * Creates a singly linked list of n nodes
     */
    printf("Enter the total number of nodes: ");
    scanf("%d", &n);
    createList(n);

    printf("\nData in the list \n");
    displayList();

    printf("\nPress 1 to delete first node: ");
    scanf("%d", &choice);

    /* Deletes the first node from list */
    if(choice == 1)
        deleteFirstNode();

    printf("\nData in the list \n");
    displayList();

    return 0;
}



/*
 * Create a list of n nodes
 */
void createList(int n)
{
    struct node *newNode, *temp;
    int data, i;

    head = (struct node *)malloc(sizeof(struct node));

    /*
     * If unable to allocate memory for head node
     */
    if(head == NULL)
    {
        printf("Unable to allocate memory.");
    }
    else
    {
        /*
         * Reads data of node from the user
         */
        printf("Enter the data of node 1: ");
        scanf("%d", &data);

        head->data = data; //Links the data field with data
        head->next = NULL; //Links the address field to NULL

        temp = head;

        /*
         * Creates n nodes and adds to linked list
         */
        for(i=2; i<=n; i++)
        {
            newNode = (struct node *)malloc(sizeof(struct node));

            /* If memory is not allocated for newNode */
            if(newNode == NULL)
            {
                printf("Unable to allocate memory.");
                break;
            }
            else
            {
                printf("Enter the data of node %d: ", i);
                scanf("%d", &data);

                newNode->data = data; //Links the data field of newNode with data
                newNode->next = NULL; //Links the address field of newNode with NULL

                temp->next = newNode; //Links previous node i.e. temp to the newNode
                temp = temp->next;
            }
        }

        printf("SINGLY LINKED LIST CREATED SUCCESSFULLY\n");
    }
}



/*
 * Deletes the first node of the linked list
 */
void deleteFirstNode()
{
    struct node *toDelete;

    if(head == NULL)
    {
        printf("List is already empty.");
    }
    else
    {
        toDelete = head;
        head = head->next;

        printf("\nData deleted = %d\n", toDelete->data);

        /* Clears the memory occupied by first node*/
        free(toDelete);

        printf("SUCCESSFULLY DELETED FIRST NODE FROM LIST\n");
    }
}



/*
 * Displays the entire list
 */
void displayList()
{
    struct node *temp;

    /*
     * If the list is empty i.e. head = NULL
     */
    if(head == NULL)
    {
        printf("List is empty.");
    }
    else
    {
        temp = head;
        while(temp != NULL)
        {
            printf("Data = %d\n", temp->data); //Prints the data of current node
            temp = temp->next; //Advances the position of current node
        }
    }
}