have i finally understood this or is it another fluke

i have this code

Code:

---

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

typedef struct Node
{
    int age;
    struct Node *next;
}Node;

typedef struct LinkedList
{
    struct Node *head;
    struct Node *tail;
}LinkedList;

enum Flag {FALSE = 0, TRUE = 1};

void initalize_list(LinkedList *list);
struct Node *create_head(void);
struct Node *get_memory(void);
int get_age(void);
void print_list(const LinkedList list);
void delete_list(LinkedList *list);
void append_to_list(LinkedList *list);
void prepend_to_list(LinkedList *list);
void delete_node(LinkedList *list);
int find_node(int age, Node *current, Node *previous);

int main()
{
    //declare my list
    LinkedList list;

    //list.head = create_head();
    initalize_list(&list);
    append_to_list(&list);
    append_to_list(&list);
    prepend_to_list(&list);
    prepend_to_list(&list);
    print_list(list);
    delete_node(&list);
    print_list(list);
    delete_list(&list);

    return 0;
}

void initalize_list(LinkedList *list)
{
    list->head = NULL;
    list->tail = NULL;

    list->head = create_head();
    list->head->next = NULL;
    list->tail = list->head;

    list->head->age = get_age();
}

struct Node *create_head(void)
{
    Node *head_tmp;

    head_tmp = get_memory();
    if (!head_tmp)
    {
        fprintf(stderr, "error allocating memory\n");
        exit(EXIT_FAILURE);// if it cant allocate memory for the head node at startup somthing really wrong so bug out
    }

    return head_tmp;
}

struct Node *get_memory(void)
{
    return malloc(sizeof(Node));
}

int get_age(void)
{
    int age;

    printf("PLease enter an age: ");
    scanf(" %d", &age);

    return age;
}

void print_list(const LinkedList list)
{
    int count = 1;
    Node *tmp = list.head;

    while (tmp)
    {
        printf("person%d's age is %d\n", count, tmp->age);
        count++;
        tmp = tmp->next;
    }
}

void delete_list(LinkedList *list)
{
    int count = 1;
    Node *tmp;

    printf("freeing memory.....\n");
    while (list->head)
    {
        tmp = list->head->next;
        printf("freeing element %d\n", count);
        free(list->head);
        list->head = tmp;
        count++;
    }
    printf("memory freed\n");
}

void append_to_list(LinkedList *list)
{
    Node *tmp_node = NULL;

    tmp_node = get_memory();

    if(!tmp_node)
    {
        fprintf(stderr, "Unable to get memory for node to append\n");
        return;
    }

    tmp_node->age = get_age();
    list->tail->next = tmp_node; //set the tail's next pointer to point at the new node
    list->tail = tmp_node; //set tail to the new node;

    // if its the second node in the list list->head->next will be NULL so set the next pointer to point at tmp_node as well
    if (!list->head->next)
    {
        list->head->next = tmp_node;
    }
}

void prepend_to_list(LinkedList *list)
{
    Node *tmp_node = NULL;

    tmp_node = get_memory();

    if(!tmp_node)
    {
        fprintf(stderr, "Unable to get memory for node to prepend\n");
        return;
    }

    tmp_node->age = get_age();
    tmp_node->next = list->head; //set tmp_node's next pointer to point at the current head pointer
    list->head = tmp_node; //set head to point to the new head/node
}

void delete_node(LinkedList *list)
{
    int age;
    Node *current = list->head, *previous = NULL;

    printf("Please enter the age you wish to delete: ");
    scanf(" %d", &age);

    if (find_node(age, current, previous))
    {
        if (!previous) // current is pointing at the head
        {
            list->head = current->next;
            printf("freeing head node...\n");
            free(current);
            printf("head node freed\n");
        }
        else if(!current->next) // current is pointing at the tail (the last node)
        {
            list->tail = previous;
            previous->next = NULL;
            printf("freeing tail node...\n");
            free(current);
            printf("tail node freed\n");
        }
        else //its somewhere in the middle of the list
        {
            previous->next = current->next;
            printf("freeing a node in the middle...\n");
            free(current);
            printf("middle node freed\n");
        }
    }
    else
    {
        printf("Invalid age as no node found!\n");
    }
}

int find_node(int age, Node *current, Node *previous)
{
    while (current)
    {
        if (current->age == age)
        {
            return TRUE;
        }
        previous = current;
        current = current->next;
    }
    return FALSE;
}

---

find node doesn't update the current and previous pointers in delete_node.
It dawned on me that again i was passing a pointer to something that was expecting an address not a pointer therefore i realised that i needed a double pointer (a pointer to a pointer) I made the followng changes

Code:

---

void delete_node(LinkedList *list)
{
    int age;
    Node *current = list->head, *previous = NULL;

    printf("Please enter the age you wish to delete: ");
    scanf(" %d", &age);

    if (find_node(age, &current, &previous))
    {
        if (!previous) // current is pointing at the head
        {
            list->head = current->next;
            printf("freeing head node...\n");
            free(current);
            printf("head node freed\n");
        }
        else if(!current->next) // current is pointing at the tail (the last node)
        {
            list->tail = previous;
            list->tail->next = NULL;
            printf("freeing tail node...\n");
            free(current);
            printf("tail node freed\n");
        }
        else //its somewhere in the middle of the list
        {
            previous->next = current->next;
            printf("freeing a node in the middle...\n");
            free(current);
            printf("middle node freed\n");
        }
    }
    else
    {
        printf("Invalid age as no node found!\n");
    }
}

int find_node(int age, Node **current, Node **previous)
{
    while (*current)
    {
        if ((*current)->age == age)
        {
            return TRUE;
        }
        *previous = *current;
        *current = (*current)->next;
    }
    return FALSE;
}

---

and hey presto it works!!(as far as i can test it)
coop

ok there appears to be an issue with my if else if else statement. if i have the following in main

Code:

---

int main()
{
    //declare my list
    LinkedList list;

    //list.head = create_head();
    initalize_list(&list);
    append_to_list(&list);
    append_to_list(&list);
    //prepend_to_list(&list);
    //prepend_to_list(&list);
    print_list(list);
    delete_node(&list);
    print_list(list);
    append_to_list(&list);
    //prepend_to_list(&list);
    print_list(list);
    delete_list(&list);

    return 0;
}

---

if i delete a middle node or the head node i get an infinite loop when i print the list again however i don't if i delete the tail node.

however if i run this version of main (append commented out and prepend uncommented)

Code:

---

int main()
{
    //declare my list
    LinkedList list;

    //list.head = create_head();
    initalize_list(&list);
    append_to_list(&list);
    append_to_list(&list);
    //prepend_to_list(&list);
    //prepend_to_list(&list);
    print_list(list);
    delete_node(&list);
    print_list(list);
    //append_to_list(&list);
    prepend_to_list(&list);
    print_list(list);
    delete_list(&list);

    return 0;
}

---

everything works fine. also if i remove the delete node call everything works fine with both prepend and append after the print list call .

ok i have fixed the issue here is the working function

Code:

---

void delete_node(LinkedList *list)
{
    int age;
    Node *current = list->head, *previous = NULL;

    printf("Please enter the age you wish to delete: ");
    scanf(" %d", &age);

    if (find_node(age, &current, &previous))
    {
        if (!previous) // current is pointing at the head
        {
            list->head = current->next;
            current->next = NULL;
            printf("freeing head node...\n");
            free(current);
            printf("head node freed\n");
        }
        else if(!current->next) // current is pointing at the tail (the last node)
        {
            list->tail = previous;
            list->tail->next = NULL;
            printf("freeing tail node...\n");
            free(current);
            printf("tail node freed\n");
        }
        else //its somewhere in the middle of the list
        {
            previous->next = current->next;
            current->next = NULL;
            printf("freeing a node in the middle...\n");
            free(current);
            printf("middle node freed\n");
        }
    }
    else
    {
        printf("Invalid age as no node found!\n");
    }
}

---

however im a little confused as to why
taking the case of deleting the head node as an example
line 174 i have list->head = current->next; ie i am saying the head pointer should point at the node current->next is pointing at. so head is now pointing at the new head node therefore the list is complete it has a new head node and the tail node and intermediate nodes are untouched. so why do i need line 175????? and why does not having line 175 only affect appending to the list especially when we haven't touched that end of the list

coop

You need to remember where the list starts.

list->head is the start of the list. Whenever we look at the list we start here.

Deleting the list->head element is very similar to a line of people buying tickets and having the person at the front of the line saying, "Actually, I need to visit my Mum that day" and leaving the line. The second person in the line becomes the new start.

i.e. - In the event that you wish to delete "list->head"
1st => "list->head" or "current" (from the top of the function)
2nd => "current->next"

You then make "list->head" = "current->next"
So...
1st => "current"
2nd => "current->next" or "list->head"

"current" is deleted, so you now have...
1st => "current->next" or "list->head"

And then the function exits...