Making an array dynamic

Hey there everyone, I'm doing a project for school and I have it completely working right now. There's 1 part of the project that I don't know how to do...Make the array that's inside of a struct, dynamic.

If anybody could help me, I'd greatly appreciate it.

Code:

---

typedef struct
{
        VectorItemT items[DEFAULT_CAPACITY];
        int size;
        int capacity;
}VectorT;

---

Code:

---

VectorT *newVector()
{
        VectorT *ptr_malloc = (VectorT*)malloc(sizeof(VectorT));
        ptr_malloc->size = 0;
        ptr_malloc->capacity = 5;
       
        return ptr_malloc;
}

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These are in separate files.... Anybody got a clue where to start?
I've read several tutorials & other posts, it just doesn't click when I read them.
What else do you need to know?

Thanks ahead of time.

you do not need to cast malloc in C

2.

Code:

---

typedef struct
{
        VectorItemT* items;
        size_t allocated_capacity;
        int size;
        int capacity;
}VectorT;


VectorT *newVector()
{
        VectorT *ptr_malloc = malloc(sizeof(VectorT));
        if(!ptr_malloc) return NULL;

        ptr_alloc->items = malloc(DEFAULT_CAPACITY * sizeof *ptr_alloc->items );
        if(!ptr_malloc->items)
                ptr_malloc-> allocated_capacity = 0;
        else
                ptr_malloc-> allocated_capacity = DEFAULT_CAPACITY;

        ptr_malloc->size = 0;
        ptr_malloc->capacity = 5;
        return ptr_malloc;
}

---

Thanks so much!

Would you mind going over this step-by-step so I know what's going on?

and did you just miss type ptr_malloc a couple times?

Code:

---

VectorT *newVector()
{
        VectorT *ptr_malloc = malloc(sizeof(VectorT));
        if(!ptr_malloc) return NULL;
/////  \/ ptr_malloc?                                        \/ptr_malloc?
        ptr_alloc->items = malloc(DEFAULT_CAPACITY * sizeof *ptr_alloc->items );
        if(!ptr_malloc->items)
                ptr_malloc-> allocated_capacity = 0;
        else
                ptr_malloc-> allocated_capacity = DEFAULT_CAPACITY;

        ptr_malloc->size = 0;
        ptr_malloc->capacity = 5;
        return ptr_malloc;
}

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Quote:

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Originally Posted by madmax2006

Thanks so much!

Would you mind going over this step-by-step so I know what's going on?

and did you just miss type ptr_malloc a couple times?

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m key is too lazy on my keyboard

Code:

---

VectorT *newVector()
{
        VectorT *ptr_malloc = malloc(sizeof(VectorT));
        if(!ptr_malloc) return NULL; /* you need to check the return value of malloc */
        ptr_malloc->items = malloc(DEFAULT_CAPACITY * sizeof *ptr_malloc->items ); /* allocate DEFAULT_CAPACITY items
                * size of each is sizeof *ptr_malloc->items -
                * if you change the type of the variable you do not need to change the sizeof operator as in your malloc */
        if(!ptr_malloc->items)
                ptr_malloc-> allocated_capacity = 0; /* failed to allocate array */
        else
                ptr_malloc-> allocated_capacity = DEFAULT_CAPACITY; /* store the array size to prevent buffer overrun,
                * if we will need more than this -
                * use realloc to increase size and update this var accordingly */

        ptr_malloc->size = 0;
        ptr_malloc->capacity = 5;
        return ptr_malloc;
}

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Probably shouldn't do:

Code:

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        ptr_malloc->capacity = 5;

---

if the malloc() failed!

--
Mats

Code:

---

typedef struct
{
        int size;
        int capacity;
        VectorItemT items[0]; /*This must be the last member!
                                              And if your compiler complains,
                                              try changing it to "VectorItemT items[];"*/
}VectorT;

---

Code:

---

VectorT *newVector(int capacity)
{
        VectorT *ptr_malloc = (VectorT*)malloc(sizeof(VectorT) + capacity * sizeof(VectorItemT));
        if (NULL != ptr_malloc) {
            ptr_malloc->size = 0;
            ptr_malloc->capacity =  capacity;
        }
        return ptr_malloc;
}

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Just noticed you changed something inside of the struct, would it be possible to make the array dynamic without making changes inside the struct, just the 2nd part of he code?

Quote:

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Originally Posted by madmax2006

Just noticed you changed something inside of the struct, would it be possible to make the array dynamic without making changes inside the struct, just the 2nd part of he code?

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vart introduced an allocated_capacity member variable, but in my opinion that is redundant: generally, you only need capacity (the number of elements for which space has been allocated) and size (the number of elements in use).

bel5f5's example is wrong: the C standard does not allow fixed size arrays of zero length, so at the very least it requires a compiler extension that is wholly unnecessary in this case. It should be replaced by a pointer instead, as in vart's example. (In this sense you do need to change your original struct definition to change the array to a pointer.)