Ad:

C Discussion Thread
Unsolved
Views: 551

Jul 2nd, 2009

File problem in link list

Expand Post »

hello.. its me again and Im having problems with files again..this time, its with link list..

I can't get my program to display the contents it reads from a file.
Please help..
I want to solve this myself but I have no time, my exam will start in 1 hour..

c Syntax (Toggle Plain Text)
void save_file(LIST L)
{
	 FILE *fp;
	 char filename[30];
	 LIST p;
 
	 printf("What is the name of the file? ");
	 flushall();
	 gets(filename);
 
	 if((fp = fopen(filename, "wb")) == NULL)
	 {
	printf("Cannot write file");
 
	 }
	 for(p = L; p != NULL; p=p->next)
	fwrite(&p, sizeof(celltype), 1, fp);
 
	 printf("File Saved");
	 fclose(fp);
	 getch();
	 clrscr();
}
void load_file(LIST *A)
{
	 FILE *fp;
	 char filename[30];
	 size_t c;
	 LIST p, temp;
 
	 printf("What is the filename? ");
	 flushall();
	 gets(filename);
	 if((fp = fopen(filename, "rb")) ==NULL)
	 {
	printf("File does not exist");
 
	 }
	 for(p = *A; p != NULL; p = p->next)
	 {
		fread(&p, sizeof(celltype), 1, fp);
	 }
 
	 printf("File Loaded");
	 getch();
	 clrscr();
}
void save_file(LIST L)
{
	 FILE *fp;
	 char filename[30];
	 LIST p;

	 printf("What is the name of the file? ");
	 flushall();
	 gets(filename);

	 if((fp = fopen(filename, "wb")) == NULL)
	 {
	printf("Cannot write file");

	 }
	 for(p = L; p != NULL; p=p->next)
	fwrite(&p, sizeof(celltype), 1, fp);

	 printf("File Saved");
	 fclose(fp);
	 getch();
	 clrscr();
}
void load_file(LIST *A)
{
	 FILE *fp;
	 char filename[30];
	 size_t c;
	 LIST p, temp;

	 printf("What is the filename? ");
	 flushall();
	 gets(filename);
	 if((fp = fopen(filename, "rb")) ==NULL)
	 {
	printf("File does not exist");

	 }
	 for(p = *A; p != NULL; p = p->next)
	 {
		fread(&p, sizeof(celltype), 1, fp);
	 }

	 printf("File Loaded");
	 getch();
	 clrscr();
}

Last edited by Whilliam; Jul 2nd, 2009 at 8:29 pm.

Similar Threads

a problem with linked list in C++
Not repeated elements in link list (help) in C++
Help... Link list problem in C++
link list with copy and delete elements in C++
insert file data to linked list? in C++

Whilliam

Reputation Points: 19

Solved Threads: 0

Junior Poster

Offline

110 posts
since Oct 2008

Permalink

Jul 2nd, 2009

Re: File problem in link list

I hope your implementation of list is something like :
typedef struct node * LIST;
In the save_file function:

Quote ...

for(p = L; p != NULL; p=p->next)
fwrite(&p, sizeof(celltype), 1, fp);

You need to specify the address of the character array which holds the value.Say your structure is as

c Syntax (Toggle Plain Text)
struct node{
char *store_this;
};
struct node{
char *store_this;
};

And after all the proper memory assignments for the node and the character array you should use it as:

c Syntax (Toggle Plain Text)
for(p = L; p != NULL; p=p->next)
	fwrite(&(p->store_this), sizeof(celltype), 1, fp);
for(p = L; p != NULL; p=p->next)
	fwrite(&(p->store_this), sizeof(celltype), 1, fp);

And the statements LIST *A and p=*A; are absolute nonsence because if p is of type node* then p=A is enough and if p is of type node then p=*A becomes meaningless.

And if you have implemented the list as something like:

c Syntax (Toggle Plain Text)

typedef struct node LIST;

Then the statements p=p->link; loose value. Its kind of an confusion with your program.

Last edited by csurfer; Jul 2nd, 2009 at 8:44 pm.

csurfer

Reputation Points: 485

Solved Threads: 88

Posting Pro

Offline

564 posts
since Jan 2009

Permalink

Jul 2nd, 2009

Re: File problem in link list

Exactly as csurfer said. You're trying to traverse your linked list but you're also overwriting each node from the file read thus invalidating your pointer!

Exit your functions upon errors. Don't keep trying to load or save data when you have no file handle!

wildgoose

Reputation Points: 546

Solved Threads: 99

Practically a Posting Shark

Offline

891 posts
since Jun 2009

Permalink

Jul 2nd, 2009

Re: File problem in link list

sob.. its still not working..

here's my main function and structure:

c Syntax (Toggle Plain Text)
typedef struct
{
     char LN[16];
     char FN[24];
     char MI;
}name;
 
typedef struct
{
     unsigned long ID;
     name n;
     char course[8];
     int year;
}studrec;
 
typedef struct cell
{
	  studrec s;
	  struct cell *next;
}celltype, *LIST
 
void main(void)
{
		LIST L, A;
		char ans;
		int quit = 0;
		studrec n;
 
		L = NULL;
      A = NULL;
 
		clrscr();
 
		do
		{
	  printf("Press\nN for Input\nI for Insert\nS to save file\nL to load a file\nV to view\nQ to quit\n");
	  ans = getch();
	  clrscr();
 
	  if(ans == 'N' || ans == 'n')
	  {
			 input(&n);
			 clrscr();
	  }
	  else if(ans == 'I' || ans == 'i')
			 insert(&L, n);
	  else if(ans == 'S' || ans == 's')
			 save_file(L);
	  else if(ans == 'L' || ans == 'l')
	  {
			 load_file(&A);
			 view(A);
	  }
	  else if(ans == 'V' || ans == 'v')
	  {
			 view(L);
			 getch();
			 clrscr();
	  }
	  else if(ans == 'Q' || ans == 'q')
          quit = 1;
		}while(quit == 0);
}
typedef struct
{
     char LN[16];
     char FN[24];
     char MI;
}name;

typedef struct
{
     unsigned long ID;
     name n;
     char course[8];
     int year;
}studrec;

typedef struct cell
{
	  studrec s;
	  struct cell *next;
}celltype, *LIST

void main(void)
{
		LIST L, A;
		char ans;
		int quit = 0;
		studrec n;

		L = NULL;
      A = NULL;

		clrscr();

		do
		{
	  printf("Press\nN for Input\nI for Insert\nS to save file\nL to load a file\nV to view\nQ to quit\n");
	  ans = getch();
	  clrscr();

	  if(ans == 'N' || ans == 'n')
	  {
			 input(&n);
			 clrscr();
	  }
	  else if(ans == 'I' || ans == 'i')
			 insert(&L, n);
	  else if(ans == 'S' || ans == 's')
			 save_file(L);
	  else if(ans == 'L' || ans == 'l')
	  {
			 load_file(&A);
			 view(A);
	  }
	  else if(ans == 'V' || ans == 'v')
	  {
			 view(L);
			 getch();
			 clrscr();
	  }
	  else if(ans == 'Q' || ans == 'q')
          quit = 1;
		}while(quit == 0);
}

Whilliam

Reputation Points: 19

Solved Threads: 0

Junior Poster

Offline

110 posts
since Oct 2008

Permalink

Jul 3rd, 2009

Re: File problem in link list

How many records in a data file?

C Syntax (Toggle Plain Text)
 for(p = *A; p != NULL; p = p->next)
	 {
		fread(&p, sizeof(celltype), 1, fp);
	 }
 for(p = *A; p != NULL; p = p->next)
	 {
		fread(&p, sizeof(celltype), 1, fp);
	 }

You have to test the eof().

Can fread() allocate a node?
?

Last edited by adatapost; Jul 3rd, 2009 at 1:12 am.

adatapost

Moderator

Reputation Points: 2136

Solved Threads: 1228

Posting Genius

Offline

6,527 posts
since Oct 2008

Permalink

Jul 3rd, 2009

Re: File problem in link list

Hey you want us to do all the work ???

Try on your own man...Put in some effort...Go on reading each line like a compiler does and ya if you are using Boreland C then may be F7 will help you trace program flow line by line.So try to debug it.

Some of the errors which you can easily find :

1>No semicolon after the struct cell.

2>You have declared A and L as LIST A,L; and LIST is itself struct node * type how can you pass pointer to this pointer...? As in p = *A;
@adatapost :

Let him correct these primitive mistakes first adatapost memory allocation and other things are off limits if he cannot trace these simple mistakes.Basically he is not putting in the effort required!!!

Last edited by csurfer; Jul 3rd, 2009 at 1:30 am.

csurfer

Reputation Points: 485

Solved Threads: 88

Posting Pro

Offline

564 posts
since Jan 2009

This thread is more than three months old

No one has posted to this discussion for at least three months. Please let old threads die and do not reply to them unless you feel you have something new and valuable to contribute that absolutely must be added to make the discussion complete. Otherwise, please start a new thread in this forum instead.

Previous Thread in C Forum Timeline: file problem

Next Thread in C Forum Timeline: Using isdigit

DaniWeb Message

Cancel Changes

User Name
Password