Thread: list of structs

hi guys i'm having the following problem with my code.
I have a struct that represents a list :

Code:

struct list_of_elements{
   
    int number_of_elements;  //counter of list elements
    A* head;  //1st element of the list
    A* tail;    //last element of the list

};

So from the above my list contains elements of type A which is another struct.

Code:

struct A{

   int telephone_num;
   char* name;

};

When I initialize the list here's what I do:

Code:

struct list_of_elements *loe = (struct list_of_elements*) malloc ( 5*  sizeof(struct A*)); //I want the max size of the list to be 5 elements	
	
	for(int j=0;j<5;j++){
		
		list_of_elements[i] = NULL;
	}
	
	list_of_elements->head=NULL; 
	list_of_elements->tail=NULL;  
	list_of_elements->number_of_elements=0;

I have 2 questions:

a)Is the initialization of Block correct as it contains an element of type char* which at the moment is undefined?

b)Is the use of malloc correct?In general I'm a bit confused with the whole thing...

Thanks in advance!

Originally Posted by quo

hi guys i'm having the following problem with my code.
I have a struct that represents a list :

Wait, which one do you mean: an array or a list? You seem to be mixing the two. Your description and first structure indicates a list, but your code tries to use it like an array.

This,

Code:

struct list_of_elements {
    int number_of_elements;
    struct element *head;
    struct element *tail;
};

is a fairly typical list handle, but I'd expect the elements to look like this:

Code:

struct element {
    struct element *next;
    int telephone_number;
    char *name;
};

Note the struct element *next. That's what makes above a singly-linked list element: it contains a pointer to the next element in list. (If you have a second one, say struct element *prev, pointing to the previous element in the list, then you have a doubly-linked list.)

On the other hand, if you want an array, the two structures are typically something like

Code:

struct array_of_elements {
    int elements_allocated;
    int elements_used;
    struct element *element;
};

struct element {
    int telephone_number;
    char *name;
};

Which is it, an array or a list?

Originally Posted by Nominal Animal

Which is it, an array or a list?

Yes I must have been a bit confused..No I certainly want a list of elements.

So for a list how should I initilize it?

Originally Posted by quo

b)Is the use of malloc correct?In general I'm a bit confused with the whole thing...

No, it isn't.
There is an idiom which makes allocating memory straight forward:

Code:

ptr = malloc(sizeof(*ptr));

This line allocates the right amount of memory for an object "ptr" points to regardless of the type of the object.
Here are some examples:

Code:

int *ptr;
ptr = malloc(sizeof(*ptr));   // allocates memory for one int
...
double *ptr;
ptr = malloc(sizeof(*ptr));   // allocates memory for one double
...
struct my_struct *ptr;
ptr = malloc(sizeof(*ptr));   // allocates memory for one struct my_struct

The malloc()-line is always the same. Pretty simple, isn't it?

If you want to allocate memory for an array, just multiply sizeof() with the number of elements:

Code:

int *ptr;
ptr = malloc(sizeof(*ptr) * 5);   // allocates memory for an array of 5 ints

Bye, Andreas

Originally Posted by quo

So for a list how should I initilize it?

You usually start with a head node and then add the other nodes when you need them. You have to allocate memory for each node separately.

I've the impression you don't really know how lists work, do you?

There are two articles about linked lists on this site:
FAQ > Linked List Issues - Cprogramming.com
Linked Lists in C Tutorial - Cprogramming.com

Bye, Andreas

Originally Posted by AndiPersti

You usually start with a head node and then add the other nodes when you need them. You have to allocate memory for each node separately.

I've the impression you don't really know how lists work, do you?

There are two articles about linked lists on this site:
FAQ > Linked List Issues - Cprogramming.com
Linked Lists in C Tutorial - Cprogramming.com

Bye, Andreas

Ok thank you I think I've understood now.
I have one more question
When it comes to arrays and I want to allocate an array of structs then how should I do it ?
Like this:

Code:

struct A **a = (struct A **) malloc ( 4 *  sizeof(struct A*));

or like this:

Code:

struct A *a = (struct A*) malloc ( 4 *  sizeof(struct A*));

Both allocate an array of pointers type struct A .Right?
What's the difference?And for both next step is this?

Code:

for(int i=0;i<4;i++){
 A[i].element1=0;
}

Originally Posted by laserlight

For what you want, you should write:

Code:

struct A *a = malloc(4 * sizeof(*a));

Now, space is allocated for 4 struct A objects, and a[0] is the first struct A object, assuming that malloc does not return a null pointer.

Thank you for your answer.So if I do what you suggest then I do not need to do this as well for every object right?

Code:

a[i] = (A *) malloc (sizeof(struct A));

This is what I did for option2 from the 2 options I've mentioned in my previous post