C - passing sub linked list to functions - c

Question

I'm working in this simple program with list but I'm having a bad time passing pointers.
Let's say I have a struct for students
typedef struct student{
char lastname[50];
int age;
int std_id;
struct student * next;
struct student * prev;
}stdn;
and I have another struct for classes
typedef struct class{
char class_id[3];
struct class * next;
struct class * prev;
struct student * stdn_list;
}clss;
so basically I have this list with classes, and each class contain a sublist with students.
so, here is the function creating the class list, and it works!
void create_class_list(clss ** root, clss * node){
clss * root_aux;
if(!(*root)){
(*root) = node;
}
else{
root_aux = (*root);
while(root_aux->next != NULL){
root_aux = root_aux->next;
}
node->prev = root_aux;
root_aux->next = node;
}
}
my problem is when I need to work with the sublist in each node of the class list.
Here is the function in charge of creating the sublist, and it works
void assign_student(clss ** root, stdn * node, char * class_id){
clss * root_aux;
stdn * stdn_aux;
root_aux = (*root);
while(root_aux != NULL){
if(strcmp(root_aux->class_id,class_id) == 0)
break;
root_aux = root_aux->next;
}
if(root_aux != NULL){
if(root_aux->stdn_list == NULL){
root_aux->stdn_list = node;
}
else{
stdn_aux = root_aux->stdn_list;
while(stdn_aux->next != NULL){
stdn_aux = stdn_aux->next;
}
node->prev = stdn_aux;
stdn_aux->next = node;
}
}
}
Basically this function looks for the specific class and add the student to that class.
My problem is when I want to delete a student, or sort the list using an algorithm like bubblesort, here's an example of a function to delete a student.
void delete_student(clss ** root, int stdn_id){
clss * root_aux;
stdn * stdn_aux;
stdn * temp;
int deleted=0;
root_aux = (*root);
while(root_aux != NULL){
stdn_aux = root_aux->stdn_list;
//try with root first//
if(stdn_aux->std_id == stdn_id){
temp = stdn_aux;
stdn_aux = stdn_aux->next;
stdn_aux->prev = NULL;
free(temp);
deleted = 1;
}
//if the student isn't the root
if(deleted == 0){
stdn_aux = stdn_aux->next;
while(stdn_aux != NULL){
if(stdn_aux->std_id == stdn_id){
temp = stdn_aux;
//link the prev element with the next element
stdn_aux->prev->next = stdn_aux->next;
//link the next element with the prev element
stdn_aux->next->prev = stdn_aux->prev;
stdn_aux = stdn_aux->next;
free(temp);
deleted = 1;
break;
}
stdn_aux = stdn_aux->next;
}
}
if(deleted == 1){
break;
}
root_aux = root_aux->next;
}
}
The function just looks like don't delete the element from the list and I'm not sure if is something with how I pass the pointer to the function, or how I create the list in first place.

Answer 1

When you delete a student node that is at the head of the Student List, you need to re-assign to root_aux->stdn_list as you are deleting the node it currently points to. That must be why it appers you are not deleting a student node.
root_aux->stdn_list = stdn_aux->next;
There are other issues with regard to processing that should be wrapped in if statements to prevent the program from core dumping:
Before you begin processing the student list, you need to first check that there is a student list. That is, check that the class variable pointing to the student list - root_aux->stdn_list - is not NULL.
Before doing the following statements, make sure that stdn_aux->next is not NULL, that is, there is stuff beyond the root node you are deleting.
stdn_aux = stdn_aux->next;
stdn_aux->prev = NULL;
Before making this assignment
stdn_aux->next->prev = stdn_aux->prev;
check that stdn_aux->next is not null because this is the last node in the student list.