Array of Struct Pointers (C Programming) Issues

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;
}