Thread: Help with linked list

Okay, I tested it and it works. I'll post the whole code in a short while. Now I have to go back through the whole damn thing and readjust it. See the whole problem centered around feof reading the end of file. This solves the problem because it tells you how many records are in the file. That way you can design your loop to read the correct number of records and avoid reading end of file. So in about 45 minutes I'll post my linked list solution, but look at this, look up ftell and let me know if you get it.

Code:

	
  //seek to the end of the file
  fseek(fin,sizeof(List),SEEK_END);
  //size represents number of records in file
  long size = ( ftell(fin) / sizeof(List) ) - 1;

Divide by size of the list because the ftell returns how many records there are of the specified size, so if it return 5 records than it will be 5 * size of records. And in a structure that could mean 5 * n, where n would be the size of the structure, therefore to get the true number of records you have to divide by the size of the structure. Also the minus one is due to the fact that SEEK_END goes right to the end of file, so the minus 1 takes away the end of file records from the count.

I almost have the example finished, infact I did have it completed but the ADT was a stack, where I think you will want a queue instead, than I ran into another snag but I'm almost out of it. The code will look fine.

binary files are a real ***** because of the end of file. I had forgotten how much of a ***** it would be, a few more minutes. Do you understand how to get the record count after reading this?

Here is a linked list. For some reason I had to make it a stack implimentation. I was having trouble creating a queue using the bianry file and random access. Notice that I changed the name of the datafile. Change it if your file has a different extension. I wonder about that binary file. Where did you get if from? I think that if you need a queue than Salem will have to help or else someone else. Anway this works as a Linked List Stack.

Code:

# include <stdio.h>
# include <stdlib.h>
# include <ctype.h>
# include <conio.h>


typedef struct node_s
{  
	int Value;
	struct node_s *link;
}List;

List * AddNode(FILE *,List *,int count);
void OpenFile(FILE **fptr, char path[], char mode[]);
void DisplayList(List*,int);

int main()
{
	List *first = NULL;
	FILE *fin = NULL;

	//start at the first record
	long rec_num = 0;

	OpenFile(&fin,"c:\\data.txt","rb");

	//seek to the end of the file
	fseek(fin,sizeof(List),SEEK_END);
	//size represents number of records in file
	long size = ( ftell(fin) / sizeof(List) ) - 1;

	for(rec_num = 0; rec_num < size; rec_num++)
	{
		first = AddNode(fin,first,rec_num);

	}

	DisplayList(first,rec_num);

	return 0;
}

void OpenFile(FILE **fptr, char path[], char mode[])
{
	*fptr = fopen(path,mode);
	if (*fptr == NULL) 
	{
		fprintf(stderr,"Error: File a:\\name?? is missing!\n");
		exit(1);
	}
}

List* AddNode (FILE *fin,List *first, int count)
{
	List *next = NULL;

	next = (List *) malloc (sizeof(List));
	next->link = NULL;

	fseek(fin,sizeof(List) * count,SEEK_SET);
	fread(next, sizeof(List), 1, fin);
		
	next->link = first;
	first = next;
	return first;

}

void DisplayList(List *start,int rec_num)
{
	int i;
	for(i=0; i < rec_num; i++)
	{
		printf("%d\n",start->Value);
		start = start->link;
	}
}

Diiference between a queue and a stack is the order that the records are stored in the list.

BTW I have to write a Cobol test in 4 hrs so I don't want to spend any more time on this right now, but I though of something more that I wanted to add. Since with ftell you can discover how many records there are it would be easy enough to dynamically allocate an array of structures and use one call to fread to load that array.

//Get size of datafile
//dynamically allocate array of structures
fread(array_of_structures, sizeof(List), size, fin);

a. We know how to get the file size already

b. allocate an array of structures of 'size'

List *array = (List*) malloc (sizeof (List) * size);

c. Read stuctures into array of structures in one statement

fread(array, sizeof(List), size, fin);


Basically you are looking at something like that, than you can index the array for example:

for(int i = 0; i < size; i++)
printf("%d", array[i].Value);

Something to that affect. I passed my Cobol test this afternoon but now I have to strive for a couple hours sleep before I go to work. I'm tired as hell. Totally overextended. Tommorow though I'll test this code if I have time.

A queue is not a problem to implement, but the code you gave here look okay from peeking at it. The actual difficulty is for some reason using 'random access' and fread would produce a situation where the fread call would read the end-of-file, and feof or ferror would not prevent it from reading end-of-file, therefore the fseek ... SEEK_END and ftell was used to know ahead of time how many records there were. Now I know in my sleep how to write a queue, but couldn't do it because EITHER the datafile was corrupt or else there was some difficulty with the end-of-file causeing disaster. I'm too burdened to look into it now, have too much work but I would indeed like to know the solution. Use the datafile provided and any code, make the list a queue. This would help. Maybe it is not difficult but for some reason this morning I was having trouble. I have written far to many ADT's using text files that the binary file was presenting problems that I'm not accustomed too. For sure there are several solutions.