Loop through a node list - c

Question

I'm fairly new to coding in c and have stumbled upon a problem. The code below is what I have so far and the problem at hand is how to looping through the node job_list.
We have two nodes, the first one creates a basic job post with a reference number (an integer) and job name (a string - an array of characters) as parameters, and the second is a list that holds job posts with the job_node being one parameter and then the second one being a standard struct job_list * next parameter.

Answer 1

I think you mixed few things up..
singly link list (that's what i understand you are up to) is not a fixed size data structure. it even not an increasing space data structure. link list use exactly the amount of memory you want to hold in it.
link list is a list of nodes. you can create list - and it will be an empty list.
then, you can add nodes with your desired data and every node you insert - create a new node (on the heap = dynamically allocated) which will contain your data and will be linked to the list.
every data node in the list will have a node_next it will point to, and node_prev which will point at this particular node(head and tail will not have this 2 nodes. head will point at node_next but will not have node_prev which point to it, and tail only have node_prev which will point to it).
so, if you want to create list you will have to dynamically allocate the space of the list which contain 2 nodes: head and tail(or end).
this is an example of create function:
slist_t *SlistCreate(void)
{
slist_t *list = malloc(sizeof(slist_t));
slist_iter_t dummy = NULL;
if(NULL == list)
{
return (NULL);
}
dummy = malloc(sizeof(node_t));
if(NULL == dummy)
{
free(list);
return (NULL);
}
dummy->next = NULL;
dummy->data = (void *)list;
list->head = dummy;
list->tail = dummy;
return (list);
}
then you will be able to insert nodes before particular node or after - that's depend on how you will implement this:
if you want to implement insert before:
you will have to encapsulate the list struct and prevent the user from sending your head to insert before.
or you can check every time if your insert function get the head of the list and update the head of the list (you can insert after and copy the head data to the head->next node and then use the head for the data the user wanted to be at the head of the list.
if you want to implement insert after:
you will have to check if the user sent you the tail (also called dummy_node cause it point to dummy_node->next = NULL), and use the same method i mentioned before, only the opposite way..
this is an example of insert before - in this example i used slist_iter_t which is pointer to node_t - the user isn't exposed to any of this structs (not to the list struct or the nodes struct):
slist_iter_t SlistInsert(slist_iter_t iterator, const void *data)
{
slist_t *ptr = NULL;
slist_iter_t dup = (slist_iter_t)malloc(sizeof(node_t));
if(NULL == dup)
{
return(NULL);
}
assert(NULL != iterator);
dup->data = iterator->data;
dup->next = iterator->next;
iterator->data = (void *)data;
iterator->next = dup;
if(NULL == dup->next)
{
ptr = (slist_t *)dup->data;
ptr->tail = dup;
}
return (iterator);
}
so, for using this DS you will have to write create function, insert function and destroy function (you will have to free all your dynamically allocated memory).
you may add more function such as remove, search data, clear list, is empty and so on. if you choose to encapsulate this implementation and hide from the user the struct of the list and the struct of the node, you will have to add more function such as get data function, get next node, and more..
you mentioned you need to insert if this data doesn't exist in the list so you can send from insert function to find function.
your function needs to look something like this:
struct job_list {
struct job_node * front;
struct job_list * next;
};
struct job_node {
int reference_number;
char * job_name;
struct job_node *next;
};
for your first function:
struct job_node *JobListCreate(void)
{
struct job_node *list = malloc(sizeof(struct job_node));
struct node_job dummy = NULL;
if(NULL == list)
{
return (NULL);
}
dummy = malloc(sizeof(node_t));
if(NULL == dummy)
{
free(list);
return (NULL);
}
dummy->next = NULL;
dummy->data = (void *)list;
list->head = dummy;
list->tail = dummy;
return (list);
}
for your second function:
void JobListInsertInFront(struct job_node *list, int reference_number, char * job_name)
{
slist_t *ptr = NULL;
struct node_job dup = NULL;
assert(NULL != list);
dup = (struct node_job)malloc(sizeof(node_t));
if(NULL == dup)
{
printf("Allocation failed\n");
return;
}
dup->reference_number = list->head->reference_number;
dup->job_name = list->head->job_name;
dup->next = list->head->next;
list->head->reference_number = reference_number;
list->head->job_name = job_name;
list->head->next = dup;
return;
}
and for the last function:
bool JobListInsertIfNotExist(struct job_node *list, int reference_number, char * job_name)
{
slist_t *ptr = NULL;
struct node_job dup = NULL;
assert(NULL != list);
while (NULL != dup)
{
if (dup->reference_number == reference_number && dup->job_name == job_name)
{
return false;
}
dup = dup->next;
}
dup = (struct node_job)malloc(sizeof(node_t));
if(NULL == dup)
{
printf("Allocation failed\n");
return;
}
dup->reference_number = list->head->reference_number;
dup->job_name = list->head->job_name;
dup->next = list->head->next;
list->head->reference_number = reference_number;
list->head->job_name = job_name;
list->head->next = dup;
return true;
}

Answer 2

As Jack Lilhammers pointed out in the comments, your code is very complex and there are a lot of mistakes in it, so I wrote down some general functions that you can then modify accordingly.
This is the basic struct, that we are going to work with:
struct node {
int data;
struct node *next;
};
Create a new node
Then this is how you'd create a new node:
#include <stdio.h>
#include <stdlib.h>
struct node *new_node(int data, struct node *next)
{
struct node *new = malloc(sizeof *new);
if (!new) {
printf("Error: memory allocation failed");
exit(EXIT_FAILURE);
}
new->data = data;
new->next = next;
return new;
}
You would initially call the function like this:
struct node *head = new_node(5, NULL);
Check if a node exists
Normally you would do something like this to check if a node with specific data exists in the linked list:
#include <stdbool.h>
/* Return whether or not the node exists */
bool node_exists(struct node *head, int data)
{
struct node *cursor = head;
while (cursor != NULL) {
if (cursor->data == data)
return true;
cursor = cursor->next;
}
return false;
}
Insert new node at the end
If you want to insert a new node at the end of the linked list, this is how it works:
void insert_last(struct node *head, struct node *new)
{
struct node **cursor = &head;
while ((*cursor) != NULL)
cursor = &(*cursor)->next;
*cursor = new;
}
Insert new node if nonexistent
You can combine the last two functions to only insert a new node at the end of the linked list, if the data doesn't already exist:
#include <stdbool.h>
/*
* Return whether or not the node exists. If it exists,
* insert the new node at the end of the linked list.
*/
bool new_insert_last(struct node *head, struct node *new)
{
struct node **cursor = &head;
while ((*cursor) != NULL) {
if ((*cursor)->data == new->data)
return true;
cursor = &(*cursor)->next;
}
*cursor = new;
return false;
}
This function could be called like this:
new_insert_last(head, new_node(3, NULL));
I have created a GitLab Snippet, so you can see the functions in action.