8 replies to this topic

[DarkLordoftheMonkeys]

I've managed to create a basic linked list by creating the list items and linking them manually, like this (bear in mind that this is not the whole program, only fragments of it):

#undef NIL

#define NIL 0x00000000


struct line {

	int linenumber;

	char *text;

	struct line *nextline = NIL;

}


struct line line1 = { 1, "text1", NIL };

struct line line2 = { 2, "text2", NIL };

struct line line3 = { 3, "text3", NIL };

line1.nextline = &line2;

line2.nextline = &line3;

That works.

But the problem is I want to be able to create an unlimited number of list items. I want them to be generated automatically, rather than having to individually create them. In order for this to happen, they can not all have names, because obviously I can not give a unique name to all of possibly hundreds of structures.

I have tried linking a structure to an unnamed item, using the following methods:

linex.nextline = &{ num, "", NIL};

*linex.nextline = { num, "", NIL };

linex.nextline = { num, "", NIL };

linex.nextline -> { num, "", NIL };

All of them generate a compiler error.

How do you implement a linked list with an unlimited number of automatically generated items?

[WingedPanther]

malloc is your friend. You will have to create them dynamically using malloc.

[ZekeDragon]

typedef struct _line
{
    int linenumber;
    struct _line* nextline;
    char text[];
} line;

line* add_line(line* prevhead, const char* contents)
{
    int size = strlen(contents) + 1;
    line* retval = malloc(sizeof(line) + (sizeof(char) * size)); // Struct hack, it's beautiful.
    strcpy(retval->text, contents);
    while (prevhead->nextline) prevhead = prevhead->nextline;
    retval->linenumber = prevhead->linenumber + 1;
    retval->nextline = NULL;
    prevhead->nextline = retval;
    return retval;
}

You can use the above function to add each line. See if that fits more along the lines of what you were after. :)

[dcs]

As an aside, defining NIL is a bit of an obfuscation. Just use NULL or 0 like everybody else in C and C++.

[DarkLordoftheMonkeys]

@dcs: I use NIL because that's the name used in computer science and programming theory. I've seen pointers referred to as NIL frequently in texts on Pascal and Modula-2. If NULL is the terminology used in C, I'll use that.

@ZekeDragon: I'm going to try that code, once I figure out what it means. Thanks for the suggestion.

@WingedPanther: I don't really understand what malloc() does and I'm afraid to use it because I have a feeling I might end up overwriting valuable memory. Are there any tutorials on how to use it? Thank you for your help.

[WingedPanther]

malloc is THE function that will enable you to do what you're trying to do. It just grabs new memory and returns a pointer to it. You'll have to learn it to make any progress with this program.

malloc - C++ Reference

[dcs]

DarkLordoftheMonkeys said:

@dcs: I use NIL because that's the name used in computer science and programming theory. I've seen pointers referred to as NIL frequently in texts on Pascal and Modula-2. If NULL is the terminology used in C, I'll use that.

That's exactly what I meant -- it looks like it's from a different language. And yes, NULL or 0 is what is used in C and C++. :)

[DarkLordoftheMonkeys]

I tried malloc() and it works.

The program:

#include <stdio.h>

#include <stdlib.h>


#undef NULL

#define NULL 0x00000000


struct listItem {

	char *value;

	struct listItem *next;

};


int main(){

	struct listItem firstItem = { "1", NULL };

	firstItem.next = (struct listItem *) malloc( sizeof firstItem );

	printf( "%p\n", firstItem.next );

	(*firstItem.next).value = "2";

	printf( "%s\n", (*firstItem.next).value );

	return 0;

}

The output:

0x100120

2

[dcs]

You don't define NULL. It is defined in the standard headers.

Eternally Confuzzled - Linked List Tutorial

[edit]Yikes. You may want to back up to strings a little bit first. You might think you're copying the string "2", but you are instead pointing to a string literal. If you were reading data from a file, you'd have to do things quite differently.