C all functions of singly-linked list!

#include<stdio.h>
#include<malloc.h>

    struct node{
        int data;
        struct node *next;
    }*a, *b, *c, *d, *head, *tail, *current;


//1. Function to Create the linked list with 4 nodes.
    void create(){
        a = (struct node*)malloc(sizeof(struct node));
        b = (struct node*)malloc(sizeof(struct node));
        c = (struct node*)malloc(sizeof(struct node));
        d = (struct node*)malloc(sizeof(struct node));

        a->data = 10;
        a->next = b;

        b->data = 20;
        b->next = c;

        c->data = 30;
        c->next = d;

        d->data = 40;
        d->next = NULL;

        head = a;
        tail = d;
    }


    void display(){
        current = head;
        while(current != NULL){
            printf("%d ", current->data);
            current = current->next;
        }
    }


//2. Function to Insert a new node at the beginning.
    void appendStart(){
        struct node *n;
        n = (struct node*)malloc(sizeof(struct node));

        n->data = 5;
        n->next = head;

        head = n;
    }


//3. Function to Insert a new node at the end.
    void appendLast(){
        struct node *n;
        n = (struct node*)malloc(sizeof(struct node));

        n->data = 50;
        n->next = NULL;

        tail->next = n;
        tail = n;
    }



// 4. Function to Insert a new node after the 3rd node.
    void appendAtThird(){
        current = head;
        struct node *newNode;
        newNode = (struct node*)malloc(sizeof(struct node));
        newNode->data = 25;

        int i = 1;
        for(i=1; i<3; i++){
            current = current->next;
        }
        newNode->next = current->next;
        current->next = newNode;
    }


//5. Function to create Insert a new node after a node by searching the data with a specific key.
    void appendAfterKey(int key){
        current = head;
        while(current->data != key){
            current = current->next;
        }

        struct node *newNode;
        newNode = (struct node*)malloc(sizeof(struct node));
        newNode->data = 121;
        newNode->next = current->next;
        current->next = newNode;
    }


//6. Function to Insert a new node before a node by searching the data with a specific key.
    void appendBeforeKey(int key){
        current = head;

        int i=0;
        while(current->data != key){
            current = current->next;
            i++;
        }

        current = head;
        int j=1;
        for(j=1; j<i; j++){
            current = current->next;
        }


        struct node *newNode;
        newNode = (struct node*)malloc(sizeof(struct node));
        newNode->data = 121;
        newNode->next = current->next;
        current->next = newNode;
    }



// 7. Function to Update data of a node after searching the node and replace by a inputted value.
    void updateByKey(int key, int val){
        current = head;
        while(current->data != key){
            current = current->next;
        }
        current->data = val;
    }

// 8. Delete first node
    void deleteFirst(){
        current = head;
        head = current->next;
    }

// 9. Delete last
    void deleteLast(){
        current = head;
        while(current->next != tail){
            current = current->next;
        }
        current->next = NULL;
    }


//10. Delete at a key
    void deleteAtKey(int key){
        current = head;
        while(current->next->data != key){
            current=current->next;
        }
        current->next = current->next->next;

    }


//11. Delete after key
    void deleteAfterKey(int key){
        current = head;
        while(current->data != key){
            current=current->next;
        }
        current->next = current->next->next;
    }

//12. Delete the previous node after searching with a specific key.
    void deleteBeforeKey(int key){
        current = head;
        while(current->next->data != key){
            current = current->next;
        }
        current->next = current->next->next;
    }

int main(){

    printf("Creating the Node Structure: ");
    printf("\n");
    create();
    display();

    printf("\n\n");
    printf("Appending Node at Starting: \n");
    appendStart();
    display();

    printf("\n\n");
    printf("Appending at Last: \n");
    appendLast();
    display();

    printf("\n\n");
    printf("Appanding after third Node: \n");
    appendAtThird();
    display();

    printf("\n\n");
    printf("Appending After key 10: \n");
    appendAfterKey(10);
    display();

    printf("\n\n");
    printf("Appending Before key 40: \n");
    appendBeforeKey(40);
    display();

    printf("\n\n");
    printf("Updating at key 40: \n");
    updateByKey(40, 333);
    display();

    printf("\n\n");
    printf("Delete first Node: \n");
    deleteFirst();
    display();

    printf("\n\n");
    printf("Delete Last Node: \n");
    deleteLast();
    display();

    printf("\n\n");
    printf("Delete At Key 20: \n");
    deleteAtKey(20);
    display();

    printf("\n\n");
    printf("Delete After Key 30: \n");
    deleteAfterKey(30);
    display();

    printf("\n\n");
    printf("Delete Before Key 25: \n");
    deleteBeforeKey(25);
    display();



return 0;
}