linked list && structures - c

Question

I would like to know if design of my program is correct, as well as to understand if my commented area is doing what it supposed to be doing. I get these compile errors that are associated probably with commented segments of my code, and I would lie to receive some help. THANKS!
part1.c:15:6: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'insert'
part1.c: In function 'main':
part1.c:43:14: error: incompatible types when assigning to type 'struct point' from type 'int'
part1.c:49:44: error: invalid type argument of '->' (have 'struct point')
part1.c:49:59: error: invalid type argument of '->' (have 'struct point')
part1.c:55:5: error: incompatible type for argument 1 of 'free'
/usr/include/stdlib.h:488:13: note: expected 'void *' but argument is of type 'struct point'
char *chars[3]= {"a","b","c"};
int nums[3]= {5,8,9};
struct point {char *letter;
int number;
struct point *next;};
struct point* insert(struct point list[],char *rqdLetters, int rqdNums)
{
struct point *new;
new = (struct point*)malloc(sizeof(struct point));
if(new == NULL)
fprintf(stderr,"error!");
new->letter = rqdLetters;
new->number = rqdNums;
new->next = head;
head = new;
//not sure if i'm returning the a pointer to the start of new list
return head;
}
int main(int argc, char **argv)
{
//not sure if i need to declare these here or in the insert
struct point list[3];
struct point *head = NULL;
struct point *next;
struct point *new;
int i;
for (i = 0; i < 3; i++)
{
//return result put back into the pointer to the start of the list
head[i] = insert(list[i], chars[i], nums[i]);
}
int j;
for(j = 0; j < 3; j++)
{
printf("letter %s and number %d\n", list[j]->letter, list[j]->number);
}
int z;
for(z = 0; z < 3; z++)
{
free(list[z]);
}
return 0;
}

Answer 1

At a glance, there are several issues with your code. Firstly, you're not declaring your variables correctly.
new = list;
should be:
struct point* new;
Your function signature also looks a little suspect. If you're returning a pointer to your data structure, it should be something like:
struct point* insert(...) { ... }
At a more general level, I does seem like your idea of a linked list may be a little off. To represent a list, you should only need to hold on to the head and tail of the list, instead of keep an array of your points.
It usually helps if you create a data structure to hold these pointers. You can then pass this structure around to functions that operate on the list e.g. the insert() function.
As a quick example (untested):
struct node {
struct node *next;
char letter;
int number;
}
struct list {
struct node *head;
struct node *tail;
}
/* create a new list */
struct list* list_new(void) {
struct list *L = malloc(sizeof(struct list));
L->head = NULL;
L->tail = NULL;
}
/* add a new node to the list */
void list_insert(struct list *list, char in_letter, int in_number) {
struct node *node = malloc(sizeof(struct node));
node->letter = in_letter;
node->number = in_number;
node->next = NULL;
if (list->head == NULL) { /* empty list */
list->head = node;
list->tail = node;
} else { /* append to list */
list->tail->next = node;
list->tail = node;
}
}
You can then use it as such:
int i;
char chars[3]= {"a","b","c"};
int nums[3]= {5,8,9};
struct list *mylist = list_new();
for (i = 0; i < 3; i++)
{
list_insert(mylist, chars[i], nums[i]);
}
In response to:
... and i am not sure if i am supposed to declare it inside insert or main, i did in the main however
This depends on where you intend to use the variables and the intended lifespan of these variables. As stated in the comments above, you might want to polish up on your understanding of scoping rules.