Thread: error subscripted value is neither an array nor pointer nor vector

and there is an error in my code as i changed the variable from head to head tmp and forgot to update the call to malloc

and there is an error in my code as i changed the variable from head to head tmp and forgot to update the call to malloc

all i want to do is have the function return a pointer to a chunk of memory that is the size of node and assign it to a pointer called head. just like laserlight did in her code. just like every example i have seen dones

Originally Posted by cooper1200

all i want to do is have the function return a pointer to a chunk of memory that is the size of node and assign it to a pointer called head. just like laserlight did in her code. just like every example i have seen dones

Ok. here's an interesting way to do it with polymorphism... Say you have a list_T type defined as:

Code:

// structure to keep track of the list.
typedef struct list_s {
  node_T *head;
  node_T *tail;
} list_T;

And your note_T type defined as:

Code:

// generic node.
typedef struct node_s {
  struct node_s *next;
} node_T;

If all your elements on the list is defined as:

Code:

struct element_s {
  node_T *next; // this pointer MUST be here always;

  // your data (3D vetor coordinates as example)...
  double x, y, z;  
};

You can deal with your list like this:

Code:

// Used to initialize your empty list.
#define EMPTY_LIST { NULL, NULL }

void addhead( list_T *lstp, void *elemp )
{
  node_T *p = elemp;

  p->next = lstp->head;
  lstp->head = p;  // our element will be the new head.

  // if the list is previously empty, we have a new tail.
  if ( ! lstp->tail )
    lstp->tail = p;
}

Of course you'll need to allocate your new element somewhere. Example of use:

Code:

list_T list = EMPTY_LIST;
struct element_s *elemp;

// allocate element:
elemp = malloc( sizeof *elemp );

// fill the data
elemp->x = 1.0f;
elemp->y = 0.0f;
elemp->z = sqrt(2.0);

// insert element in the list:
addhead( &list, elemp );

ok this is getting beyond a joke now

Code:

#include <stdio.h>
#include <stdlib.h>

typedef struct Node
{
    int age;
    struct Node *next;
}Node;

typedef struct LinkedList
{
    struct Node *head;
    struct Node *tail;
}LinkedList;

struct LinkedList *create_head(void);

int main()
{
    //declare my list
    LinkedList list;
    Node data;

    //set both head and tail pointers to null
    list.head = NULL;
    list.tail = NULL;

    list.head = malloc(sizeof(data));
    if (!list.head)
    {
        printf("error allocating memory\n");
        return -1;
    }
    list.tail = list.head;



    return 0;
}

compiles no errors no warnings.

yet all i have done is the same code (literally i cut and pasted it) in a function

Code:

#include <stdio.h>
#include <stdlib.h>

typedef struct Node
{
    int age;
    struct Node *next;
}Node;

typedef struct LinkedList
{
    struct Node *head;
    struct Node *tail;
}LinkedList;

struct LinkedList *create_head(Node data);

int main()
{
    //declare my list
    LinkedList list;
    Node data;

    //set both head and tail pointers to null
    list.head = NULL;
    list.tail = NULL;

    list.head = create_head(data);

    list.tail = list.head;



    return 0;
}
struct LinkedList *create_head(Node data)
{
    LinkedList *head_tmp;

    head_tmp = malloc(sizeof(data));
    if (!head_tmp)
    {
        printf("error allocating memory\n");
    }

    return *head_tmp;
}

produces

Code:

linked list simple/main.c|28|warning: assignment from incompatible pointer type [-Wincompatible-pointer-types]|
linked list simple/main.c||In function ‘create_head’:|
linked list simple/main.c|46|error: incompatible types when returning type ‘LinkedList {aka struct LinkedList}’ but ‘struct LinkedList
linked list simple/main.c|47|warning: control reaches end of non-void function [-Wreturn-type]|

so the way i read all that is it doesn't see the return statement even though it says the return statement is an error, the error on line 46 is because i passed it a linked list and it wanted a linked list. and s to the warning on line 28 god alone knows
||=== Build failed: 1 error(s), 2 warning(s) (0 minute(s), 0 second(s)) ===|

ok i have tried again and managed to change everything so it compiles with no warnings or errors. so i went ahead and added a print and delete list functions

Code:

#include <stdio.h>
#include <stdlib.h>

typedef struct Node
{
    int age;
    struct Node *next;
}Node;

typedef struct LinkedList
{
    struct Node *head;
    struct Node *tail;
}LinkedList;

struct LinkedList *initalize_list(LinkedList *list);
struct Node *create_head(void);
struct Node *get_memory(void);
int get_age(void);
void print_list(LinkedList list);
void delete_list(LinkedList *list);

int main()
{
    //declare my list
    LinkedList *list = NULL;

    list = initalize_list(list);
    print_list(*list);
    delete_list(list);

    return 0;
}

struct LinkedList *initalize_list(LinkedList *list)
{
    list->head = NULL;
    list->tail = NULL;

    list->head = create_head();
    list->head->next = NULL;
    list->tail = list->head;

    list->head->age = get_age();
    return list;

}

struct Node *create_head(void)
{
    Node *head_tmp;

    head_tmp = get_memory();
    if (!head_tmp)
    {
        fprintf(stderr, "error allocating memory\n");
        exit(EXIT_FAILURE);// if it cant allocate memory for the head node at startup somthing really wrong so bug out
    }

    return head_tmp;
}

struct Node *get_memory(void)
{
    return malloc(sizeof(Node));
}

int get_age(void)
{
    int age;

    printf("PLease enter an age: ");
    scanf(" %d", &age);

    return age;
}

void print_list(LinkedList list)
{
    int count = 1;
    Node *tmp = list.head;

    while (tmp)
    {
        printf("person%d's age is %d\n", count, list.head->age);
        count++;
        tmp = list.head->next;
    }
}

void delete_list(LinkedList *list)
{
    int count = 1;
    Node *tmp;


    printf("freeing memory.....\n");
    while (list->head)
    {
        tmp = list->head->next;
        printf("freeing element %d\n", count);
        free(list->head);
        list->head = tmp;
        count++;
    }
    printf("memory freed\n");
}

however when i run it get seg fault on line 37.
coop

ok after hours of searching and re-watching videos i just don't get what i have done wrong. it just will not let me set head to anything be it a null or a pointer derived from malloc so either there is something really wrong or this struct idea holding a pointer to the head and tail just doesn't work.

the whole thing has been an exercise in futility i just don't understand this at all.

Have a closer look at what create_head is returning - Hint: It's not the same as list.head

its returning the pointer from malloc

list.head is a struct Node *
create_head returns a struct _ *

or more precisely it returns head_tmp which is of type node and set to the pointer returned from malloc

and as list_head is of type node there is no miss match if i have it return a type of linked list it whines like a moped with its throttle stuck open

Originally Posted by Click_here

list.head is a struct Node *
create_head returns a struct _ *

how is it i have defined it as struct node

however when i run it get seg fault on line 37.

Code:

 
int main()
{
    //declare my list
    LinkedList *list = NULL;
 
    list = initalize_list(list);
...
 
struct LinkedList *initalize_list(LinkedList *list)
{
    list->head = NULL;
...

You create a list pointer and assign NULL to that pointer, then pass that NULL pointer into your initialize_list() function, next you try to assign NULL to a field of a NULL pointer object, result CRASH. You need to assign memory to that pointer before you try to use it.

it crashes when i try to assign memory as well

however i didn't realise that linkedlist *list = null is the same as linked list *list ...... list->head = null list->tail = null