How to convert a linking list to an dynamic array - c

Question

I created an function in c as a linked list but i can't understand how convert it to a linked list.
My Question is how to convert an linked list to an dynamic array. This function adds elements to an linked list i want to be able to add elements to an dynamic array. i don't know if the struct blockhead_node is right like i said i'm still learning c. I do not understand dynamic arrays so i'm trying to create an program that uses them so i may understand them better. i want to create an add function that adds elements to the front or end of the list. This is what i have:
//this struct i'm trying to use for dynamic array
struct blockhead_node
{
float x, y;
float dx, dy;
long color;
int size; // slots used so far
int capacity; // total available slots
int *data; // array of integers we're storing
};
//this struct is for the linked list
struct blockhead_node
{
float x,y;
float dx, dy;
long color;
int size;
struct blockhead_node * next;
};
void add(struct blockhead_node ** blockhead_list) // double pointer because we can't modify the list it self
{
while((*blockhead_list)!=NULL)
{
blockhead_list=&(*blockhead_list)->next;
}
(*blockhead_list) = (struct blockhead_node*) malloc(sizeof(struct blockhead_node));
(*blockhead_list)->x = rand()%screen_width + (*blockhead_list)->size;
//(*blockhead_list)->x = 400;
//
//look up how to create an random floating point number
//
(*blockhead_list)->dx = ((float)rand()/(float)(10000));
(*blockhead_list)->y = rand()%screen_height + (*blockhead_list)->size;
(*blockhead_list)->dy = ((float)rand()/(float)(10000));
(*blockhead_list)->size = rand()%100;
(*blockhead_list)->next = NULL;
if((*blockhead_list)->x + (*blockhead_list)->size > screen_width)
{
(*blockhead_list)->x = screen_width - (*blockhead_list)->size;
}
if((*blockhead_list)->y + (*blockhead_list)->size > screen_height)
{
(*blockhead_list)->y = screen_height - (*blockhead_list)->size;
}
}

Answer 1

Using a dynamic array is not much different than using a linked-list. The primary difference is you are responsible for keeping track of size and realloc your data array when size == capacity (or when capacity == 0 for a new stuct).
Other than that you are either allocating per-node with a linked-list, whereas with a dynamic array you can allocate in a more efficient manner by allocating by some multiple of the current capacity when size == capacity. For arrays of completely unknown size you can either start with capacity = 2 or 8 and then either increase by some multiple (e.g. 3/2 or 2, etc..) or some fixed amount (8, 16, etc..) each time a reallocation is needed. I generally just double the current capacity, e.g. starting a 2, reallocating storage in data for 4, 8, 16, 32, ... integers as more numbers are added to data. You can do it any way you like -- as long as you keep track of your size and capacity.
A short example may help. Let's start with just a simple struct where data is the only dynamic member we are concerned about. For example:
#define CAP 2 /* initial capacity for new struct */
typedef struct {
size_t size,
capacity;
int *data;
} blkhd_node;
Where size is your "slots used so far" and capacity is your "total available slots" and data your "array of integers we're storing".
The add function is fairly trivial. All you need to do is check whether you need to realloc (e.g. this is a struct not used yet where capacity == 0 or all the slots are used and size == capacity). Since realloc can be used for both new and resizing the allocation, it is all you need. The scheme is simple, if this is a new struct, we will allocate for CAP number of int, otherwise if size == capacity we will allocated 2 * capacity number of int.
Whenever we realloc we do so with a temporary pointer! Why? When realloc fails it returns NULL. If you realloc with the original pointer (e.g. data = realloc (data, newsize); and NULL is returned, you overwrite your original pointer address for data with NULL and your ability to reach (or free) the original block of memory is lost -- creating a memory leak. By using a temporary pointer, we can validate whether realloc succeeds before we assign the new address to data. Importantly, if realloc does fail -- then our existing integers pointed to by data are still fine, so we are free to use our original allocation in the event of realloc failure. Important to remember.
We also need a way to indicate success/failure of or add function. Here, you are not adding a new node, so returning a pointer to a new node isn't an option. In this case a simple int return of 0 for failure of 1 for success is just as good as anything else.
With that, an add function could be as simple as:
int add (blkhd_node *b, int v)
{
/* realloc if (1) new struct or (2) size == capacity */
if (!b->capacity || b->size == b->capacity) {
size_t newsize;
if (!b->capacity) /* new stuct, set size = CAP */
newsize = CAP;
else if (b->size == b->capacity) /* otherwise double size */
newsize = b->capacity * 2;
/* alway realloc with a temporary pointer */
void *tmp = realloc (b->data, newsize * sizeof *b->data);
if (!tmp) { /* validate reallocation */
perror ("realloc_b->data");
return 0; /* return failure */
}
b->data = tmp; /* assign new block of mem to data */
b->capacity = newsize; /* set capacity to newsize */
}
b->data[b->size++] = v; /* set data value to v */
return 1; /* return success */
}
Note: since you are initializing your struct and both size and capacity will be 0, you can dispense with separate checks for capacity == 0 and size == capacity and use a ternary to set newsize. This is arguably the less-readable way to go and a good compiler will optimize both identically, but for sake of completeness, you could replace the code setting newsize with:
/* realloc if (1) new struct or (2) size == capacity */
if (b->size == b->capacity) {
/* set newsize */
size_t newsize = b->capacity ? b->capacity * 2 : CAP;
Given the option, your choice should always be for the more readable and maintainable code. You do your job, and let the compiler worry about the rest.
(you can do a dynamic array of struct blockhead_node in exactly the same way -- just account for the number of structs you have in the array in addition to the size, capacity for data in each -- that is left to you)
So, let's see if our scheme of originally allocating for 2-int will allow us to add 100-int to our data array with a short example putting it altogether:
#include <stdio.h>
#include <stdlib.h>
#define CAP 2 /* initial capacity for new struct */
typedef struct {
size_t size,
capacity;
int *data;
} blkhd_node;
int add (blkhd_node *b, int v)
{
/* realloc if (1) new struct or (2) size == capacity */
if (!b->capacity || b->size == b->capacity) {
size_t newsize;
if (!b->capacity) /* new stuct, set size = CAP */
newsize = CAP;
else if (b->size == b->capacity) /* otherwise double size */
newsize = b->capacity * 2;
/* alway realloc with a temporary pointer */
void *tmp = realloc (b->data, newsize * sizeof *b->data);
if (!tmp) { /* validate reallocation */
perror ("realloc_b->data");
return 0; /* return failure */
}
b->data = tmp; /* assign new block of mem to data */
b->capacity = newsize; /* set capacity to newsize */
}
b->data[b->size++] = v; /* set data value to v */
return 1; /* return success */
}
void prndata (blkhd_node *b)
{
for (size_t i = 0; i < b->size; i++) {
if (i && i % 10 == 0)
putchar ('\n');
printf (" %3d", b->data[i]);
}
putchar ('\n');
}
int main (void) {
blkhd_node b = { .size = 0 }; /* declare/initialize struct */
for (int i = 0; i < 100; i++) /* add 100 data values */
if (!add (&b, i + 1))
break; /* don't exit, all prior data still good */
/* output results */
printf ("\nsize : %zu\ncapacity : %zu\n\n", b.size, b.capacity);
prndata (&b);
free (b.data); /* don't forget to free what you allocate */
return 0;
}
Example Use/Output
$ ./bin/dynarrayint
size : 100
capacity : 128
1 2 3 4 5 6 7 8 9 10
11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30
31 32 33 34 35 36 37 38 39 40
41 42 43 44 45 46 47 48 49 50
51 52 53 54 55 56 57 58 59 60
61 62 63 64 65 66 67 68 69 70
71 72 73 74 75 76 77 78 79 80
81 82 83 84 85 86 87 88 89 90
91 92 93 94 95 96 97 98 99 100
Memory Use/Error Check
In any code you write that dynamically allocates memory, you have 2 responsibilities regarding any block of memory allocated: (1) always preserve a pointer to the starting address for the block of memory so, (2) it can be freed when it is no longer needed.
It is imperative that you use a memory error checking program to insure you do not attempt to access memory or write beyond/outside the bounds of your allocated block, attempt to read or base a conditional jump on an uninitialized value, and finally, to confirm that you free all the memory you have allocated.
For Linux valgrind is the normal choice. There are similar memory checkers for every platform. They are all simple to use, just run your program through it.
$ valgrind ./bin/dynarrayint
==12589== Memcheck, a memory error detector
==12589== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==12589== Using Valgrind-3.12.0 and LibVEX; rerun with -h for copyright info
==12589== Command: ./bin/dynarrayint
==12589==
size : 100
capacity : 128
1 2 3 4 5 6 7 8 9 10
11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30
31 32 33 34 35 36 37 38 39 40
41 42 43 44 45 46 47 48 49 50
51 52 53 54 55 56 57 58 59 60
61 62 63 64 65 66 67 68 69 70
71 72 73 74 75 76 77 78 79 80
81 82 83 84 85 86 87 88 89 90
91 92 93 94 95 96 97 98 99 100
==12589==
==12589== HEAP SUMMARY:
==12589== in use at exit: 0 bytes in 0 blocks
==12589== total heap usage: 7 allocs, 7 frees, 1,016 bytes allocated
==12589==
==12589== All heap blocks were freed -- no leaks are possible
==12589==
==12589== For counts of detected and suppressed errors, rerun with: -v
==12589== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
Always confirm that you have freed all memory you have allocated and that there are no memory errors.
Look things over and let me know if you have further questions.