Linked list woes (read from file to linked list)

Dev Shed Forums Sponsor:

September 11th, 2012, 03:00 PM

ChokolAwt

The bad and the ugly...

Join Date: Jan 2007

Location: Oz... No??? Neverland then?

Posts: 142

Time spent in forums: 1 Day 6 h 21 m 2 sec
Reputation Power: 0

Linked list woes (read from file to linked list)

well i finished the last homework assignment my teacher assigned thanks to everyones help. I received the warm and fuzzy grade of A+!

So for our next assignment he has us messing around with a "phonebook" of sorts using linked lists.

our input file: "data.txt"

Code:

Smith		Joe		8046688446
Thomas		Fred		1234567890
Gates		Bill		1023456789
Hilton 		Paris		5789146512

im trying to read this in to create a linked list. each node will have first, last and number.

Code:

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

#define MAX_LENGTH	20
#define MAXCHAR	256

struct member_node{
	char first[MAX_LENGTH];
	char last[MAX_LENGTH];
	long int number;
	struct member_node *next;
};


void main(void)
{
	//declaration variables
	//
	FILE *filename; 	//input filename handle
	char *fname = NULL;
	int n = 0; //used for prompting user for file name
	char *first_name;
	char *last_name;
	long int phone_number;
	
	struct member_node *head;// Create entry point to the linked list
	struct member_node *current;// Create pointer for keeping track of the node 
//currently being read, written, deleted, added, ....
	// Add the first elemet to the list
           head = (struct member_node *)malloc(sizeof(struct member_node));
	
	//allocate memory for size of filename (char = 1 bit)
	fname = (char *)malloc(MAXCHAR * sizeof(char));
	first_name = (char *)malloc(MAXCHAR * sizeof(char));
	last_name = (char *)malloc(MAXCHAR * sizeof(char));
	
	//
	//prompt user for file name and continue to do so until a valid file name is given
	while (n != 1)
		{
			printf("Enter file name: ");
			scanf("%s", fname);
			
			//filename is just the handle to that file. have to go through 
                                 //filename for IO operation
			filename = fopen(fname, "r"); 
			
			if (filename == NULL)
				{
					printf("Please enter a valid file name.\n");
				}
			else	
				{
					printf("File was successfully opened.\n");
					n = 1;
				}
		}
	
	printf("before while loop\n");
	while (fscanf(filename, "%s\t\t%s\t\t%ld\n", &first_name, &last_name, &phone_number) != EOF)
		{
			printf("iteration\n");
			strcpy(head -> first, first_name);
			printf("1\n");
			strcpy(head -> last, last_name);
			printf("2\n");
			head -> number = phone_number;
			printf("3\n");
			head -> next = NULL;
			printf("add to list\n");
			// Add next element
			current = head;
			current -> next = (struct member_node *)malloc(sizeof(struct member_node));
			current = current -> next;
			printf("end while\n");
			
		}
	printf("after while loop\n");	
	
	// Print the list
	printf("Member name\tage\n");
	printf("---------------------\n");
	
	// Start at the beginning of the list
	current = head; 
	printf("asdf");
	while(current != NULL)
		{
			printf("%s\t\t%s\t\t%ld\n", current -> first, current -> last, current -> number);
			current = current -> next;
		}
}	/* end main */

Phew... okay. So I'm using the printf's for debugging help since I'm coding in Notepad++. The code compiles fine but crashes when it gets to

Code:

strcpy(head -> first, first_name);

Dereferencing it does nothing either... should i scrap this approach all together and attack it from a different angle? I'm trying to read through the file and instead of just using the buffer in fscanf to find the length (for creating the correct number of nodes) I want to create nodes and fill them as fscanf works its way through the file.

thanks in advance
~Tim

__________________
"Life is not a journey with the intent on arriving at the finish line in a pretty and well preserved body. But rather to skid in broadside, totally worn out, thoroughly used up and loudly proclaiming, "Wow! What a ride!" -Anonymous
Halo! || Diablo 2 LOD Modding || OLGA's BACK!

September 11th, 2012, 03:53 PM

bdb

Contributing User

Join Date: Aug 2012

Posts: 156

Time spent in forums: 1 Week 15 h 48 m 11 sec
Reputation Power: 32

first_name is a pointer to char;
fscanf("%s") wants a pointer to char;

Code:

while (fscanf(filename, "%s\t\t%s\t\t%ld\n", &first_name, ...

you're passing a pointer to pointer.
The result is that first_name itself no longer points to where it should (and crashes on the next statement)

(The same for last_name)

Also, you need to include <string.h> (for strcpy())

September 11th, 2012, 06:26 PM

ChokolAwt

The bad and the ugly...

Join Date: Jan 2007

Location: Oz... No??? Neverland then?

Posts: 142

Time spent in forums: 1 Day 6 h 21 m 2 sec
Reputation Power: 0

ah gotcha. well, i scrapped that last plan and started working backwards from one of my teacher's examples.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAX_LENGTH	20
#define MAXCHAR	256

int main(void)
{
	//declaration variables
	//
	FILE *filename; 	//input filename handle
	char *fname = NULL;
	int n = 0;			//used for prompting user for file name
	int c = 0;
	char *first_name;
	char *last_name;
	char phone_number[10];
	
	struct phonebook *head;// Create entry point to the linked list
	struct phonebook *current;// Create pointer for keeping track of the node currently being read, written, deleted, added, ....
	
	//allocate memory for size of filename (char = 1 bit)
	fname = (char *)malloc(MAXCHAR * sizeof(char));
	
	//
	//prompt user for file name and continue to do so until a valid file name is given
	while (n != 1)
		{
			printf("Enter file name: ");
			scanf("%s", fname);
			
			//filename is just the handle to that file. have to go through filename for IO operation
			filename = fopen(fname, "r"); 
			
			if (filename == NULL)
				{
					printf("Please enter a valid file name.\n");
				}
			else	
				{
					printf("File was successfully opened.\n");
					n = 1;
				}
		}
	
	while(fscanf(filename, "%s %s %s\n", last_name, first_name, phone_number) != EOF)
		{
			head = add_to_head(head, last_name, first_name, phone_number);
			//printf("%s \t %s \t %s", lbuffer, fbuffer, phone);
			//printf("\n");
		}
		
	printf("Last\t\tFirst\t\tNumber\n");
	printf("---------------------\n");
	for(current = head; current != NULL; current = current->next)
		printf("%s\t\t%s\t\t%s\n", current->last_name, current->first_name, current->phone_number);
	
}	/* end main */

also including these functions

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAX_LENGTH 20

struct phonebook
	{
		char fname[MAX_LENGTH];
		char lname[MAX_LENGTH];
		char phone[10];
		struct phonebook *next;
	};

struct phonebook *add_to_head(struct phonebook *list, char *lname, char *fname, char *phone) 
{
	struct phonebook *current;

	if(list == NULL) 
	{
		list = (struct phonebook *)malloc(sizeof(struct phonebook));
		strcpy(list->lname, lname);
		strcpy(list->fname, fname);
		strcpy(list->phone, phone);
		list->next = NULL;
		return(list);
	}
	else 
	{
		current = (struct phonebook *)malloc(sizeof(struct phonebook));
		strcpy(current->lname, lname);
		strcpy(current->fname, fname);
		strcpy(current->phone, phone);
		current->next = list;
		return(current);
	}
}	// end add_to_head

im getting several compiler errors and 1 warning.
warning: '=' 'phonebook *' differs in levels of indirection from int on this line:

Code:

head = add_to_head(head, last_name, first_name, phone_number);

and

Code:

for(current = head; current != NULL; current = current->next)
		printf("%s\t\t%s\t\t%s\n", current->last_name, current->first_name, current->phone_number);

the compiler hates this line too haha. current specifies undefined struct or union 'phonebook?"

September 11th, 2012, 07:31 PM

b49P23TIvg

Contributing User

Join Date: Aug 2011

Posts: 3,354

Time spent in forums: 1 Month 2 Weeks 3 Days 8 h 21 m 53 sec
Reputation Power: 383

I stuck all your code into a single file. You'll need to decide between

lname

and

last_name

and resolve those issues.
I'll resolve them in my copy and maybe post again.

__________________
[code]Code tags[/code] are essential for python code!

September 11th, 2012, 08:41 PM

b49P23TIvg

Contributing User

Join Date: Aug 2011

Posts: 3,354

Time spent in forums: 1 Month 2 Weeks 3 Days 8 h 21 m 53 sec
Reputation Power: 383

Your compiler messages don't work for me. Oh well.

Some difficulties:

Failure to initialize head = NULL in main.

Failure to test memory allocation success.

Failure to explicitly return a value from main.

Potential buffer overrun in [f]scanf("%s",...) because a first name could be three hundred thousand characters long---you could fight this by preprocessing the input file I suppose.

[I think]Your fscanf should test that it returns 3, not EOF. Testing for EOF won't properly trap a partially complete last line of the phone book file.[/I think]

add_to_head is too complicated.
Suppose list were NULL...I think you'll see that you can discard

Code:

	if(list == NULL) 
	{
		list = (struct phonebook *)malloc(sizeof(struct phonebook));
		strcpy(list->lname, lname);
		strcpy(list->fname, fname);
		strcpy(list->phone, phone);
		list->next = NULL;
		return(list);
	}
	else

I made a working version of your program with these changes and maybe some others.

Last edited by b49P23TIvg : September 11th, 2012 at 08:44 PM.

Linked list woes (read from file to linked list)

Developer Shed Advertisers and Affiliates