Thread: a list of structures

i am trying to reproduce something like this standard textbook code-

ptr=malloc(record_count*sizeof(char))
p=ptr;
for (count=0;count<record_count)
*p++=ptr

but what i want is to "save" in memory a list of structures and then reproduce them later, ive got this so far -


RECORD_DATA *stock_rec,*s;

stock_rec=malloc(sizeof(RECORD_DATA));

s=malloc(record_count*sizeof(RECORD_DATA));

stock_rec=s;
while(something)
{
stock_rec= (a function returns a pointer to data struct of type RECORD_DATA )

stock_rec++;
}

i thought this would save the stock_rec one after each other in memory, but it doesn't

all i want to do is save a list of structures but i'm struggling, can anybody help ????? i dont want an array, cos i may not know how big it needs to be

Code:

RECORD_DATA *stock_rec,*s;

// This is not neccisary
// stock_rec=malloc(sizeof(RECORD_DATA));

// Okay, now s is an array of RECORD_DATA
s=malloc(record_count*sizeof(RECORD_DATA));

// stock_rec is the variable we use to step through the array
stock_rec=s;

while(something)
{

// This is the problem, you are just changing the value of stock_rec.  Instead you need to change the value of what it is pointing at.
//stock_rec= (a function returns a pointer to data struct of type 
RECORD_DATA )

// There are two ways to do this...
changeRec (stock_rec); // example below

// orr...
*stock_rec = (a function that returns a RECORD_DATA [not a pointer])

stock_rec++;
}

The first way, passing the pointer to the record in a function, is considered the better way, because you don't want a function to return structures which take a lot of memory.

Code:

void changeRec (RECORD_DATA * p)
{
 p -> info = 0;
 p -> artist = 'M';
 p -> foo = bar;
 // etc...
}

What exactly are you trying to do? Basicly make a dynamic array of structures?

struct mystruct *myArray;
myArray = malloc( sizeof(struct mystruct) * numberOfElements );

myArray[x].somedata = blah;

Or are you trying to make a linked list?

struct mystruct *myList;

myList = newNode( NULL );
myList = newNode( myList );

where 'newNode' is:

Code:

struct mystruct * newNode( struct mystruct * n )
{
    struct mystruct *temp;

    temp = malloc( sizeof( struct mystruct );
    // fill data into temp, or not

    temp->next = n;
    return temp;
}

//edit: Ah, you beat me to it. Dangit.


Quzah.

Many thanx to QuestionC and Quzah for their very illuminating responces.

*stock_rec = (a function that returns a RECORD_DATA [not a pointer])
stock_rec++;

is what i had in my tiny mind, i can see what i was doing wrong now thanx,
i'm also coding my prog with a linked list and an array also for practice, so i'm much the wiser

regards
steve

is this what you meant ???? it works anyway !!

s=malloc(record_count*sizeof(RECORD_DATA));

stock_rec=s;

while(......)
{
p=search_file(part_no);

changeRec(p);stock_rec++ ;

}

//after the processing, regurgitate list -

stock_rec=s; // set stock_rec back to "beginning"
// (last stock_rec set to 999999)

while(strcmp(stock_rec->key,"999999")!=0)
{

printf("key=%6s \n",stock_rec->key);
stock_rec++;
}


void changeRec(RECORD_DATA *p)
{

strcpy(stock_rec->key,p->key);
strcpy(stock_rec->desc,p->desc);
stock_rec->supp_code=p->supp_code;
stock_rec->free_stock=p->free_stock;
stock_rec->min_stock=p->min_stock;
strcpy(stock_rec->movement_date,p-> movement_date);
stock_rec->price=p->price;

}

any comments ????

Well, my rule of thumb is...
if it works, then it's right

But it's still good practice not to use global variables, like you appear to be doing. I suggest changing the changeRec function so that you pass it both, a pointer to p, -and- a pointer to stock_rec (and not use stock_rec as global).

oh oh i always tend to use global variables !!!

i'll try not to in future !!!