Adding struct pointer to a pointer array - c

Question

I have a problem and I really dont know what to do.
I'am trying to insert "new students" to an student-array. The array contains pointers to the created structs. Can somebody find the error? It adds the student-structs to the array but especially the printing doesnt work.
It would be really helpful, if somebody could help me. :) PS: You can just copy the code.
Here is my code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_HASH 10
typedef struct student
{
unsigned int matnr;
char *name;
struct student *next_student;
} Student;
Student **hash_tabelle[MAX_HASH];
void insert_student (unsigned int matnr, char *name)
{
Student *neuer_student = malloc(sizeof(Student));
neuer_student->name = malloc(sizeof(*name)+1);
neuer_student->matnr = matnr;
strcpy(neuer_student->name, name);
neuer_student->next_student = NULL;
// Index im Hash-Array ermitteln
int hash_index = matnr % 10;
if(hash_tabelle[hash_index] == NULL)
{
neuer_student->next_student = hash_tabelle[hash_index];
hash_tabelle[hash_index] = neuer_student;
}
else
{
while(*hash_tabelle[hash_index] != NULL && (((*hash_tabelle[hash_index])->matnr - neuer_student->matnr) <= 0))
hash_tabelle[hash_index] = &(*hash_tabelle[hash_index])->next_student;
neuer_student->next_student = *hash_tabelle[hash_index];
*hash_tabelle[hash_index] = neuer_student;
}
}
void print_hash_tabelle()
{
for(int i = 0; i != MAX_HASH - 1; i++){
printf("%d)\t", i);
hash_tabelle[i] = &(*hash_tabelle[i])->next_student;
for(; hash_tabelle[i] != NULL; hash_tabelle[i] = &(*hash_tabelle[i])->next_student){
printf("%s (%d)", (&(*hash_tabelle[i])->name), (&(*hash_tabelle[i])->matnr));
}
printf("\t");
}
}
int main()
{
unsigned int matnr;
char name[100];
do
{
printf("Matrikelnummer:\t");
scanf("%d", &matnr);
fflush(stdin);
getchar(); // um das \n aus dem Puffer zu kriegen und rauszuschmeißen
printf("Name:\t\t");
fgets(name, 30, stdin);
insert_student(matnr, name);
}
while (matnr != 0);
print_hash_tabelle();
return 0;
}

Answer 1

Using a hash table is so simple... No need to use dereferencing for a fixed-size array of linked-list pointers.
Step 1 - a hash table is a array of linked-list pointers.
As #BLUEPIXY suggests:
Student *hash_tabelle[MAX_HASH];
Step 2 - to allocate and free each linked-list, initialize each item to NULL.
Otherwise, if(hash_tabelle[hash_index] == NULL) is Undefined
behavior in the function insert_student().
void hash_init()
{
for(int i=0;i<MAX_HASH;i++) {
hash_tabelle[MAX_HASH]=NULL;
}
}
Step 3 - allocate enough char to store the char *name to insert_student().
As # WhozCraig suggests, use strlen().
void insert_student (unsigned int matnr, char *name)
{
Student *neuer_student = malloc(sizeof(Student));
neuer_student->name = malloc(strlen(name)+1);
neuer_student->matnr = matnr;
strcpy(neuer_student->name, name);
neuer_student->next_student = NULL;
Step 4 - add the neuer_student in the hash_tabelle[] (function insert_student())
Warning: the index shall be included in the size of the array
[0..MAX_HASH[. (using 10 instead of MAX_HASH could become a bug).
int hash_index = matnr % MAX_HASH;
When the hash_tabelle[hash_index] is NULL, simple store the
neuer_student. No need to modify neuer_student->next_student.
if(hash_tabelle[hash_index] == NULL)
{
hash_tabelle[hash_index] = neuer_student;
}
Else explore the linked-list of hash_tabelle[hash_index] to store
the neuer_student at the end.
else
{
Student *tmp;
tmp = hash_tabelle[hash_index];
while (tmp->next_student!=NULL) {
tmp = tmp->next_student;
}
tmp->next_student = neuer_student;
}
Step 5 - to print the all items of the hash table (function print_hash_tabelle())
Reuse the same method to explore each linked-list pointer.
Warning: explore all item from 0 to MAX_HASH-1
void print_hash_tabelle()
{
for(int i = 0; i < MAX_HASH; i++){ // ERR != MAX_HASH - 1; i++){
printf("%d)\t", i);
Student *tmp = hash_tabelle[i];
while (tmp!=NULL) {
printf("%s (%d)", tmp->name, tmp->matnr);
tmp = tmp->next_student;
}
printf("\n");
}
}
Step 6 - free the memory of each item of the hash_tabelle[].
Free the allocated string free(tmp->name);.
Remove the current student hash_tabelle[i] = tmp->next_student;
Free the allocated student free(tmp);
Repeat until the end of the linked-list
That's all (no change in the main() except adding a call to hash_free() at the end).
void hash_free()
{
for(int i=0;i<MAX_HASH;i++) {
Student *tmp = hash_tabelle[i];
while (tmp!=NULL) {
free(tmp->name);
hash_tabelle[i] = tmp->next_student;
free(tmp);
tmp = hash_tabelle[i];
}
}
}

Answer 2

like this:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_HASH 10
typedef struct student {
unsigned int matnr;
char *name;
struct student *next_student;
} Student;
Student *hash_tabelle[MAX_HASH];
void insert_student (unsigned int matnr, char *name){
Student *neuer_student = malloc(sizeof(Student));
neuer_student->name = malloc(strlen(name)+1);
strcpy(neuer_student->name, name);
neuer_student->matnr = matnr;
//neuer_student->next_student = NULL;
int hash_index = matnr % MAX_HASH;
Student head = { .next_student = hash_tabelle[hash_index] };
Student *prev = &head, *curr = head.next_student;
while(curr != NULL && curr->matnr <= neuer_student->matnr){
prev = curr;
curr = curr->next_student;
}
neuer_student->next_student = curr;
prev->next_student = neuer_student;
hash_tabelle[hash_index] = head.next_student;
}
void print_hash_tabelle(void){
for(int i = 0; i < MAX_HASH; i++){
printf("%d)\t", i);
for(Student *p = hash_tabelle[i]; p; p = p->next_student){
printf("%s (%d)\t", p->name, p->matnr);
}
printf("\n");
}
}
void free_hash_tabelle(void){
for(int i = 0; i < MAX_HASH; i++){
Student *p = hash_tabelle[i];
while(p){
Student *temp = p->next_student;
free(p->name);
free(p);
p = temp;
}
}
}
int main(void){
int matnr = -1;//for %d of scanf
char name[100];
while(1){
printf("Matrikelnummer(-1 for end input): ");fflush(stdout);
scanf("%d", &matnr);
if(matnr < 0)
break;
while(getchar() != '\n');
printf("Name: ");fflush(stdout);
fgets(name, sizeof name, stdin);
name[strcspn(name, "\n")] = 0;
insert_student(matnr, name);
}
print_hash_tabelle();
free_hash_tabelle();
return 0;
}