Singly Linked Lists in C

gistfile1.c

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

struct listNode{
    int id;
    char name[10];
    struct listNode *nextPtr;
};

typedef struct listNode ListNode;
ListNode *head, *previousPtr, *newNode, *p;

void printMenu(void);
void createLinkedList();
void traverseTheList();
void insertNode();
void deleteNode();

int main()
{
    int opt;
    printMenu();
    createLinkedList();
    printf("Choice: ");
    scanf("%d",&opt);

    while(opt != 4){
        switch(opt){
            case 1:insertNode();
                   break;
            case 2:deleteNode();
                   break;
            case 3:traverseTheList();
                   break;
            default:printf("Invalid key.\n");
                    break;
        }

        printMenu();
        printf("Choice: ");
        scanf("%d",&opt);

    }

    return 0;
}

void printMenu(){
    printf("Enter your choice:\n\n 1-Insert an element into the list.\n 2-Delete an element from the list\n 3-Traverse the list\n 4-Exit\n\n");
}

void createLinkedList(){
    int nStudent,i;
    printf("Student size: ");
    scanf("%d",&nStudent);

    for(i = 0; i < nStudent; i++){
        if( i == 0){
            p = malloc(sizeof(ListNode));
            head = p;
        }
        else{
            p->nextPtr = malloc(sizeof(ListNode));
            p = p->nextPtr;
        }

        printf("\nID: ");
        scanf("%d",&p->id);
        printf("Name: ");
        scanf("%s",p->name);
    }
    printf("\n\n");
    p->nextPtr = NULL;
}

void traverseTheList(){
    p = head;
    while( p != NULL ){
        printf(" Student ID: %d\n Student name:%s\n",p->id,p->name);
        p = p->nextPtr;
        printf("\n");
    }
}

void insertNode(){
    int recordNo;
    printf("Which ID: ");
    scanf("%d",&recordNo);

    newNode = malloc(sizeof(ListNode));

    printf("\n");
    printf("ID: ");
    scanf("%d",&newNode->id);
    printf("Name: ");
    scanf("%s",newNode->name);
    printf("\n\n");

    p = head;
    if( p->id == recordNo ){
        newNode->nextPtr = p; // Listemizin başına ekliyoruz
        head = newNode;       // p(Gezici pointerimiz) head'i gösteriyor.
    }                         // Artık head yeni düğüm oldu.
    else{
        while( p->nextPtr != NULL && p->id != recordNo){
            previousPtr = p;
            p = p->nextPtr;
        }
        if( p == NULL ){ //Sondan eleman ekle (NULL DEGER)
            previousPtr->nextPtr = newNode;
            newNode->nextPtr = NULL;
        }
        else if( p->id == recordNo ){
            newNode->nextPtr = p;
            previousPtr->nextPtr = newNode;
        }
    }
}

void deleteNode(){

    int recordNo;
    printf("Which ID: ");
    scanf("%d",&recordNo);

    p = head;
    if( p->id == recordNo ){
        head = p->nextPtr;
        free(p);
    }
    else{
        while(p->nextPtr != NULL && p->id != recordNo){
            previousPtr = p;
            p = p->nextPtr;
        }
        if( p == NULL ){
            printf("No elements");
        }
        else if(p->id == recordNo){
            previousPtr->nextPtr = p->nextPtr;
            free(p);
        }
    }
}