Problem in C - How to erase a array in a database struct

Question

include files:

stdio.h
string.h
ctype.h
genlib.h
simpio.h
strlib.h

The database struct looks like this:

typedef struct{

    catT *cats;
    int currentMaxSize;
    int currentNumberOfCats;
    int nextId;

} *DBT;

and the "cats" struct looks like this:

typedef struct {

    int id;
    string name;
    char gender;
    int birthYear;
    int numberOfColours;
    string colours[MAX_COLOURS];

} catT;

If we say I got 3 cats in the DBT database and want to erase one of them, how can i code it? I want to have a function to erase a cat!

yankee · Accepted Answer · 2011-02-04 09:53:44Z

1

Well, the size of catT is fixed. Which is good.

If you want to erase the last cat in the list, then it's easy. Just use realloc() so make your DBT smaller. (new size will be 2*sizeof(catT)).

If you want to remove a cat that is not the last cat in the list, then change that. If you don't care about sorting then just override the cat which you want to delete with the last cat in the list (do this using memcpy()). Then you can remove the last cat in the list.

answered Feb 4, 2011 at 9:53

yankee

41.3k17 gold badges113 silver badges169 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Daniel Gehriger · Accepted Answer · 2011-02-04 10:01:39Z

1

void freeCat(int atIndex, DBT db)
{
    if (atIndex < db->currentNumberOfCats)
    {
        if (atIndex < db->currentNumberOfCats - 1)
        {            
            memmove(db->cats + atIndex, 
                    db->cats + atIndex + 1, 
                    db->currentNumberOfCats - atIndex - 1);
        }
        db->currentNumberOfCats--;
    }
}

But this is expensive. If you are going to repeatedly erase cats, use a linked list or consider allocating catT on the heap (catT **cats), so you only have to move the pointers around (don't forget to free the cat then).

edited Feb 4, 2011 at 10:01

answered Feb 4, 2011 at 9:54

Daniel Gehriger

7,4782 gold badges37 silver badges57 bronze badges

4 Comments

FredrichP Over a year ago

Daniel Gehriger, this is nearly what i'm looking for, the only poblem is that it is the last cat which is removed and the cat i want to remove just gets the ID of the cat after!

Daniel Gehriger Over a year ago

Not sure I understand... can you rephrase?

FredrichP Over a year ago

the thing is that the cat (with index I want to remove) just gets a higher ID, let say I want to remove the cat on index 0. That cats ID will increase with one, and the last cat will not be written (because db->currentNumberOfCats--)... The thing is that this functon do not erase a cat, just inrease the cats ID with 1?

Daniel Gehriger Over a year ago

@FredrichP: I think there is a mix up between cat ids and the indices into the array. Erasing a cat does not change the id member of any of the cats. And I assumed that currentNumberOfCats is unrelated to the id, it's just number of cats (their id could be anything). If, however, the id has to match the array index, then either get rid of that member entirely, or renumber after moving.

Oralet · Accepted Answer · 2011-02-04 09:53:36Z

0

I assume you will allocate memory for 3 cats with *cats of DBT.
To erase one of the cats, you need to free the memory for the cat you want to erase.
I can't write the code since I don't know the variables and functions you use but I hope this gives you some idea.

answered Feb 4, 2011 at 9:53

Oralet

3471 gold badge2 silver badges12 bronze badges

Comments

FredrichP · Accepted Answer · 2011-02-04 10:41:41Z

0

void freeCat(int atIndex, DBT db) { int i;

if (atIndex < db->currentNumberOfCats)
{         
    if (atIndex < db->currentNumberOfCats - 1)
    {
        for(i = atIndex; i < db->currentNumberOfCats; i++){
            db->cats[i] = db->cats[i+1];
        }
    }         
    db->currentNumberOfCats--;
}

}

I did this function and it works perfect! Thanks everyone and @Daniel Gehriger for the start of the function, this helped me very much!

answered Feb 4, 2011 at 10:41

FredrichP

214 bronze badges

Collectives™ on Stack Overflow

Problem in C - How to erase a array in a database struct

4 Answers 4

Comments

4 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

4 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related