Array of Struct Pointers (C Programming) Issues - c

Question

So i'm trying to figure out how to do a few different things and I haven't worked with C that much, so any help would be much appreciated.
typedef int data_t;
typedef struct set {
data_t *array;
size_t capacity;
size_t size;
} set_t;
typedef data_t* set_i_t;
#define CLEAR -1
I have gotten this method working which uses malloc and allocates memory:
int set_init( set_t *set, int capacity ){
set->array = (data_t*)malloc(capacity * sizeof(data_t));
if(set->array == NULL){
return 1;
}
else{
set->capacity = capacity;
set->size = 0;
return 0;
}
}
And a method which frees it:
void set_free( set_t *set ){
free(set->array);
set->array = NULL;
set->capacity = set->size = 0;
}
In a separate method i'm trying to set all the values in the set to -1 (CLEAR)
void set_clear( set_t *set){
int i = 0;
for (i = 0; i < set->size; i++){
set->array = CLEAR;
}
set->size = 0;
}
Return the Size of the set:
int set_size( set_t set ) {
return sizeof(set->array);
}
Return the capacity:
int set_capacity( set_t set ) {
int capacity = set->capacity;
return capacity;
}
And then print the set:
void set_print( set_t set ) {
//Honestly don't feel like i'm ready for this one yet.
}
If anyone could walk me through a couple of these or give me a little assistance on how these can work, that would be awesome. Thanks guys!

Answer 1

A good resource is C dynamically growing array
1
You can read about size_t. What is size_t in C?
typedef int data_t;
// Here you are redefining int to data_t this is then used in array.
typedef struct set {
data_t *array;
// The address on heap where the typedef data_t is stored
size_t capacity;
size_t size;
} set_t;
typedef data_t* set_i_t;
// not sure why this is here maybe you use somewhere else
#define CLEAR -1
2
set_free( set_t *set); Looks good to me.
set_init(); yes but no
set_t set_init(int capacity) {
// create it here then return it.
set_t ret;
ret.array = (data_t*)malloc(capacity * sizeof(data_t));
if (ret.array == NULL) return NULL;
ret.capacity = capacity;
ret.size = 0;
return ret;
}
In the calling function
set_t A = set_init(5);
if (A == NULL) fprintf(stderr, "could not alloc memory\n");
// :)
3
void set_clear( set_t *set){
// you pass the address of the struct into the function. you could also use set_i_t
//int i = 0;
// why do this you can do it in the for loop as you can see
// for (i = 0; i < set->size; i++){
for (int i = 0; i < set->size; i++){
//set->array = CLEAR; common mistake
// you are saying the address of the array. aka array[0]
// this is the same as set->(array+i)
set->array[i] = CLEAR;
}
set->size = 0;
}
4 & 5
// looks good but again better ways of doing this.
set_size( set_t set );
set_capacity( set_t set );
Better ways of managing memory such as in the example here. C dynamically growing array
6
read all about printf();
http://www.tutorialspoint.com/c_standard_library/c_function_printf.htm
void set_print( set_t set ) {
// Here you passed the struct in plain and simple no pointer......
// so you will use the '.' not the '->'
// Here we can take a look at printf();
// %d is used to print int variables.
// to start off you know you will have to loop through the array.
for (int i = 0; i < set.size; i++) {
// you know the array must be at least have one item in it.
printf("%d\n", set.array[i]);
// using printf print the data_t aka "int" item in the array
}
}
Hope this helps. G

Answer 2

There were a few places where you defined the function arguments with set_t instead of set_t *.
Your set_size would just return the size of the array pointer (i.e. always 4 or 8), so that needed set->size
Also, set_clear was incorrect [and wouldn't even compile].
I've added some functions and implemented the [dreaded :-)] print function. No worries ...
Anyway, here's the corrected code [please pardon the gratuitous style cleanup]:
#include <stdio.h>
#include <malloc.h>
typedef int data_t;
typedef struct set {
data_t *array; // pointer to set's data
size_t capacity; // total number of data slots
size_t size; // number of slots currently in use
} set_t;
typedef data_t *set_i_t;
#define CLEAR -1
int
set_init(set_t *set, int capacity)
{
set->array = (data_t *) malloc(capacity * sizeof(data_t));
if (set->array == NULL) {
return 1;
}
else {
set->capacity = capacity;
set->size = 0;
return 0;
}
}
// And a method which frees it:
void
set_free(set_t *set)
{
free(set->array);
set->array = NULL;
set->capacity = set->size = 0;
}
// i'm trying to set all the values in the set to -1 (CLEAR)
void
set_clear(set_t *set)
{
int i = 0;
for (i = 0; i < set->size; i++) {
#if 0
set->array = CLEAR;
#else
set->array[i] = CLEAR;
#endif
}
set->size = 0;
}
// Return the Size of the set:
int
set_size(set_t *set)
{
return set->size;
}
// Return the maximum capacity:
int
set_capacity_max(set_t *set)
{
int capacity = set->capacity;
return capacity;
}
// Return the remaining available capacity:
int
set_capacity_avail(set_t *set)
{
int capacity = set->capacity - set->size;
return capacity;
}
// add some data
void
set_append(set_t *set,int val)
{
// NOTES:
// (1) this does _not_ check for overflow against capacity
// (2) when out of capacity, we might increase capacity and do a realloc
// on array
#if 0
if ((set->size + 1) >= set->capacity) {
set->capacity += 100;
set->array = realloc(set->array,sizeof(data_t) * set->capacity);
}
#endif
set->array[set->size++] = val;
}
// And then print the set:
void
set_print(set_t *set)
{
int i;
int len;
// Honestly don't feel like i'm ready for this one yet.
// Relax, no worries ...
len = 0;
for (i = 0; i < set->size; i++) {
len += printf(" %d",set->array[i]);
if (len >= 72) {
printf("\n");
len = 0;
}
}
if (len > 0)
printf("\n");
}
int
main(void)
{
set_t myset;
set_init(&myset,100);
set_append(&myset,17);
set_append(&myset,23);
set_append(&myset,37);
set_print(&myset);
set_free(&myset);
return 0;
}