pointer_to_pointer.c

//
//  main.c
//  Explanation of pointer to pointer by deleting a node from linked list.
//
//  Created by Afsal Thaj on 10/3/19.
//  Copyright © 2019 Afsal Thaj. All rights reserved.
// 
//

// The concept discuss about pointers to pointers: https://www.eskimo.com/~scs/cclass/int/sx8.html
#include <stdio.h>

int main(int argc, const char * argv[]) {
    // declaring a variable as 5;
    int i = 5;
    // delcaring a pointer ip that points to the address of the variable i
    int *ip = &i;
    
    // Now let's define a pointer to pointer and try and dereference it.
    int **ipp;
    
    // ipp now has the address of the pointer that points to the variable i.
    ipp = &ip;
    
    printf("the value when deferencing a pointer %i to pointer should be another address: %i \n.", ipp, *ipp);
    
    printf("the value when dereferencing a pointer to pointer will be: %i\n", **ipp);
    
    struct linkedList
    {
        int item;
        struct linkedList *next;
    };
    
    struct linkedList innerList;
    struct linkedList innerInnerList;
    innerInnerList.item = 3;
    struct linkedList ll;
    innerList.item = 2;
    innerList.next = &innerInnerList;
    ll.item = 1;
    ll.next = &innerList;
    
    
    struct linkedList *lp, *previousList;
    
  // Let's delete an element from linked list with the value 1.

  // Approach 1
  for (lp = &ll; lp != NULL; lp = (*lp).next)
    {
        if ((*lp).item == 2)
        {
            if (lp == &ll) {
                ll = *(*lp).next;
                break;
            }
            else {
                (*previousList).next = (*lp).next;
                 break;
            }
        }

        previousList = lp;
    }
    
     printf("The result list is %i, %i\n", ll.item, *(ll.next));
    
    // Approach 2 - Pointer to Pointer
    
    // Let's define new list
    struct linkedList list;

    struct linkedList a;
    struct linkedList b;
    b.item = 3;
    a.item = 2;
    a.next = &b;
    list.item = 1;
    list.next = &a;
    
    struct linkedList **lpp;
    struct linkedList *lp1 = &list;
    
    for(lpp = &lp1; lpp!=NULL; lpp=&((**lpp).next)) {
        if ((**lpp).item == 2) {
            *lpp = (**lpp).next;
            break;
        }
    }
    
    // Note that list remain unchanged, and lp1 is the reference to the linked list with the deleted element
    
    printf("The result list is %i, %i\n", (*lp1).item, *((*lp1).next));

}