Help with linked list

This is a discussion on Help with linked list within the C Programming forums, part of the General Programming Boards category; Hi! I have problem with the "ReadList" and "WriteList" function. Code: # include <stdio.h> # include <stdlib.h> # include <ctype.h> ...

  1. #1
    Code Warrior
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    Unhappy Help with linked list

    Hi!

    I have problem with the "ReadList" and "WriteList" function.

    Code:
    # include <stdio.h>
    # include <stdlib.h>
    # include <ctype.h>
    # include <conio.h>
    
    # define FALSE 0
    
    typedef struct 
    {  
    	int Value;
    	struct List *link;
    } List;
    
    // other functions
    void clrscr (void);
    void Menu (int *Choose);
    // functions for the list
    List *AddElement (List *start, int Data);
    void ReadList (List *start, char File01[]);
    void WriteList (List *start, char File01[]);
    void PrintList (List *start);
    
    int main()
    {
    	int Choose, Data;
    	List *start = NULL;
                   
    	start = ReadList(start, "data.txt");
    	do
    	{
    		Menu(&Choose);
    		switch (Choose)
    		{
    			case 1:
    				printf("\nEnter number: ");
    				scanf("%d", &Data);
    				start = AddElement(start, Data); 
    				break;
    			case 2:
    				if (start != NULL)
    				{
    					PrintList(start);
    				}
    				else
    				{
    					printf("\nList is empty.");
    				}
    				getch();
    				break;
    			case 3:
    				break;
    			default:
    				printf("\nWrong selection. Try again.");
    				getch();
    				break;
    		}
    	} while (Choose != 3);
                    WriteList (start, "data.txt");
    	return 0;	
    }
    
    void Menu (int *Choose) 
    {
        unsigned char Char;
    
        do 
    	{
    		clrscr();
    		printf("List");
    		printf("\n\n1 - Add element into sorted list");
    		printf("\n2 - Print elements on the screen");
    		printf("\n3 - Exit");
    		printf("\n\nChoose: ");
    		Char = getchar();
    		if ((isdigit (Char) == FALSE) && (Char != '\n')) 
    		{
    			printf ("\nYou must enter number."); getch();
    		}
        } while (isdigit (Char) == FALSE);
        *Choose = (int) Char - '0'; 
    }
    
    void clrscr()
    {
    	system("cls");
    }
    
    List *AddElement (List *start, int Data)
    {
    	List *pom, *New;
    	
    	New = (List *) malloc(sizeof(List));
    	New->Value = Data;
    	if ( (start == NULL) || (New->Value <= start->Value) ) 
    	{
    		(List *)New->link = start;
    		start = New;
    	}
    	else
    	{
    		pom = start;	
    		while ( (pom->link != NULL) && (New->Value > ((List *)pom->link)->Value) )
    		{
    			pom = (List *)pom->link;
    		}
    		New->link = pom->link;
    		(List *)pom->link = New;
    		
    	}
    	return start;
    }
    
    void PrintList (List *start)
    {
    	while (start != NULL)
    	{
    		printf("%d\t", start->Value);
    		start = (List *)start->link;
    	}
    }
    Thnx in advance .
    Last edited by GaPe; 02-08-2002 at 03:02 AM.
    Current projects:
    1) User Interface Development Kit (C++)
    2) HTML SDK (C++)
    3) Classes (C++)
    4) INI Editor (Delphi)

  2. #2
    Code Warrior
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    Uf, I totally forgot to copy/paste these two functions:

    Code:
    List *ReadList (List *start, char File01[])
    {
    	FILE *fin;
    	List *New;
    
    	fin = fopen(File01, "rb");
    	while ( !feof(fin) )
    	{
    		New = (List *) malloc (sizeof(List));
    		fread(New, sizeof(List), 1, fin);
    		(List *)New->link = start;
    		start = New;
    	}
    	fclose(fin);
    	return start;
    }
    
    void WriteList (List *start, char File01[])
    {
    	FILE *fout;
    
    	fout = fopen(File01, "wb+");
    	while (start != NULL)
    	{
    		fwrite(start, sizeof(List), 1, fout);
    		start = (List *)start->link;
    	}
    	fclose(fout);
    }
    The problem is that when I exit my program, the WriteList function always adds 0 to the "data.txt" file. What is wrong?
    Current projects:
    1) User Interface Development Kit (C++)
    2) HTML SDK (C++)
    3) Classes (C++)
    4) INI Editor (Delphi)

  3. #3
    Registered User
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    typedef struct
    {
    int Value;
    struct List *link;
    } List;

    The 0 is the value of link for the last element.... the value after the last element is NULL, and NULL is 0. So... the last value in your data.txt is going to be 0.
    Callou collei we'll code the way
    Of prime numbers and pings!

  4. #4
    Code Warrior
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    But why the program doesn't print out the 0 on the screen when I use function "PrintList"?
    Current projects:
    1) User Interface Development Kit (C++)
    2) HTML SDK (C++)
    3) Classes (C++)
    4) INI Editor (Delphi)

  5. #5
    Banned Troll_King's Avatar
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    char File01[100], *File01Ptr;

    File01Ptr = File01;
    File01Ptr = "data.txt";
    Didn't get to the linked list part of your code because after reading the first few lines I noticed this illegal code.

    Should be:
    strcpy(File01Ptr,"data.txt");

    because you can't assign a whole string safely to a single character such as:
    File01Ptr = "data.txt";

    Better yet to just use:
    strcpy(File01,"data.txt");

    I'm wondering why you called the char pointer File01Ptr, have you used file pointers before?

    FILE *File01Ptr;

  6. #6
    Code Warrior
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    Yes I used file pointers before and this code is right. I have a string and a pointer to that string (File01[100] and *File01Ptr). In my case a file pointer is "fin" and "fout".
    Current projects:
    1) User Interface Development Kit (C++)
    2) HTML SDK (C++)
    3) Classes (C++)
    4) INI Editor (Delphi)

  7. #7
    Banned Troll_King's Avatar
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    The char pointer points to a single character, it should be used with strcpy if you want to assign a string to that position. I have been doing C for 3 years. I know this for a fact. I can write linked lists in my sleep.

  8. #8
    Code Warrior
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    If I write "printf("%s", File01Ptr);" I get "data.txt" which is totally right.

    If you write linked lists in your sleep than, please, help me out and tell me why do I get those 0 in my file.
    Current projects:
    1) User Interface Development Kit (C++)
    2) HTML SDK (C++)
    3) Classes (C++)
    4) INI Editor (Delphi)

  9. #9
    Banned Troll_King's Avatar
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    No, I am aware that some compilers will allow you to assign a string to a pointer to char. I am familiar with how it works, but it is poor code and can cause problems depending on how the OS manages memory.

    If you change it than I'll help you with the list. There is no point in helping someone with partly retarded code in their program.

  10. #10
    Banned Troll_King's Avatar
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    //call
    start = ReadList(start, File01Ptr);

    //prototype
    void ReadList (List *start, char File01[]);

    Same situation, the code will compile but it is confused, specify the pointer for what it is.

    void ReadList (List *,char *);

    Now I'll keep going.

  11. #11
    Banned Troll_King's Avatar
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    You are working with a binary file. Send it to me so I don't have to create it myself.

  12. #12
    Code Warrior
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    Ok I changed the code.
    Current projects:
    1) User Interface Development Kit (C++)
    2) HTML SDK (C++)
    3) Classes (C++)
    4) INI Editor (Delphi)

  13. #13
    Banned Troll_King's Avatar
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    while ( !feof(fin) )
    {

    }

    Take this out of the ReadList function and put it in main like so:

    //Open the file
    int count =0;
    while(! feof(fin))
    {
    ReadList(fin,start);
    count ++;
    }


    Than in ReadList do this:

    List *New, *Next;

    declare another pointer in the readlist function where you create the list. You did the allocation correctly but than you need to build the list like this for example.

  14. #14
    Code Warrior
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    Here's the file and change it's extension to .txt.
    Attached Files Attached Files
    Current projects:
    1) User Interface Development Kit (C++)
    2) HTML SDK (C++)
    3) Classes (C++)
    4) INI Editor (Delphi)

  15. #15
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    Code:
    void OpenFile(FILE **fptr, char path[], char mode[]);
    
    int main()
    {
    	int Choose, Data;
    	List *start = NULL;
    	FILE *fin;
    
    	OpenFile(&fin,"data.txt","rb");
    
    	int count = 0;
    	while ( !feof(fin))
    	{
    		start = ReadList(fin,start);
    		count ++;
    	}
    	//...
    	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);
    	}
    }
    This is cleaner. Okay now for the ReadList function.
    Last edited by Troll_King; 02-08-2002 at 03:22 AM.

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