Insert at start of list - c

Question

I am writing a simple function in c++.
I get some errors with my code. THERE is no output.
Also I need to call free after remembered where the next index.
I dont know how to do that.
Do I need to free current inside the while loop after printf?
here my code
#include <stdio.h>
#include<stdlib.h>
/* these arrays are just used to give the parameters to 'insert',
to create the 'people' array
*/
#define HOW_MANY 7
char *names[HOW_MANY]= {"Simon", "Suzie", "Alfred", "Chip", "John", "Tim",
"Harriet"};
int ages[HOW_MANY]= {22, 24, 106, 6, 18, 32, 24};
/* declare your struct for a person here */
typedef struct{
char* name;
int age;
struct person *next;
}
person;
static void insert( person *headptr, char *name, int age)
{
person *ptr=malloc(sizeof(person));
if(ptr==NULL) abort();
//assign to structure field
ptr->name = name;
ptr->age = age;
//link new object into the list
ptr->next=headptr;
headptr=ptr;
}
int main(int argc, char **argv)
{
/* declare the people array here */
person *headptr=NULL;
// Make a copy of the pointer to the head item in the list
for (int index=0;index < HOW_MANY;index=index+1)
{
insert(headptr, *(names+index), ages[index]);
}
person *current=NULL;
// current will be set to NULL when it reaches the end
while(current != NULL)
{
// print out the item information
printf("name: %s, age: %i\n",current -> name, current-> age);
// Now move to the next item in the linked list
current= current -> next;
}
}

Answer 1

There are several problems in your code.
Your struct's next field is declared using an unknown type.
Your insert() function is not updating the headptr variable in main().
You get no output because the current variable in main() is initialized to NULL instead of headptr, so there is nothing for the loop to do.
You are leaking the allocated memory.
Try something more like this instead:
#include <stdio.h>
#include <stdlib.h>
/* these arrays are just used to give the parameters to 'insert', to create the 'people' array */
#define HOW_MANY 7
char* names[HOW_MANY] = {"Simon", "Suzie", "Alfred", "Chip", "John", "Tim", "Harriet"};
int ages[HOW_MANY] = {22, 24, 106, 6, 18, 32, 24};
/* declare your struct for a person here */
typedef struct person {
char* name;
int age;
struct person *next;
} person;
static void insert(person **headptr, char *name, int age) {
person *ptr = malloc(sizeof(person));
if (!ptr) abort();
//assign to structure fields
ptr->name = name;
ptr->age = age;
//link new object into the list
ptr->next = *headptr;
*headptr = ptr;
}
int main(int argc, char **argv) {
/* declare the people array here */
person *headptr = NULL;
// insert items at the head of the list
for (int index = 0; index < HOW_MANY; ++index) {
insert(&headptr, names[index], ages[index]);
}
person *current = headptr;
// current will be set to NULL when it reaches the end
while (current) {
// print out the item information
printf("name: %s, age: %i\n", current->name, current->age);
// Now move to the next item in the linked list
current = current->next;
}
// free the items
current = headptr;
while (current) {
person *next = current->next;
free(current);
current = next;
}
return 0;
}

Answer 2

To insert at the front of a linked list, I would suggest that you pass a pointer to the head of the list, not the head, resulting in something like this example (written from memory, but should work):
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
typedef struct list {
struct list *next;
int value;
} list;
void list_push(list **phead, int value) {
list *front = malloc(sizeof(struct list));
front->next = *phead;
front->value = value;
*phead = front;
}
bool list_insert(list **phead, int value, int index) {
list **ptr = phead;
while (index-- > 0) {
if (!*ptr) return false;
ptr = &(*ptr)->next;
}
list_push(ptr, value);
return true;
}
void list_dump(list *head) {
while (head) {
printf("%d\n", head->value);
head = head->next;
}
}
void main(void) {
list *head = NULL; // empty list
list_push(&head, 23);
list_insert(&head, 42, 1);
list_insert(&head, 13, 0);
list_dump(head);
}

Answer 3

You need to modify your insert method like below:-
static person* insert(person *headptr, char *name, int age)
{
if (headptr == NULL) {
headptr = (person*)malloc(sizeof(person));
headptr->name = name;
headptr->age = age;
headptr->next = NULL;
}
else{
person *ptr = (person*)malloc(sizeof(person));
if (ptr == NULL) abort();
//assign to structure field
ptr->name = name;
ptr->age = age;
//link new object into the list
ptr->next = headptr;
headptr = ptr;
}
return headptr;
}
And now you need to call the above method like below:-
// Make a copy of the pointer to the head item in the list
for (int index=0;index < HOW_MANY;index=index+1)
{
headptr = insert(headptr, *(names+index), ages[index]);
}
Now you need to print your list like below:-
person *current=headptr;//you need to point to the first node of the list
// current will be set to NULL when it reaches the end
while(current != NULL)
{
// print out the item information
printf("name: %s, age: %i\n",current -> name, current-> age);
// Now move to the next item in the linked list
current= current -> next;
}