Creating Linked List of Locations in C

Question

FIX: I wasn't saving the locations back into world so I was just leaking the information. Credit to Skeeto.

while(!feof(fp)){
    loc = readLocation(fp);
    join(loc,world);
    }

should actually be

while(!feof(fp)){
    loc = readLocation(fp);
    world = join(loc,world);
    }

EDIT: As mentioned in the comments, yes I am a student, but I'm not looking for someone to do my work for me. I'm simply trying to locate possible logic errors because if I could fill this list properly, I can finish my project very easily. This is just a small portion of a very immersive project, helping with this will only allow me to continue the project, not complete it and turn it in. I only provided so much detail because 1) I've never posted here before so didn't know any better, and 2) wanted the reader to understand the workings of this in order to aid them in assisting me. Also, with any concerns as to skype, if that ended up being where successful help was given, I would provide the fix above this 'edit' as well as crediting the stackoverflow user for helping.

TLDR: Yes I'm a student, No im not trying to have someone do my project. This is a very small portion and will only allow me to continue, not complete. If help was given via skype I would update this post with the fix as well as credit the helper.

Hello and thank you for any help in advance.

I am trying to create a linked list that holds objects of type Location *. I have Location defined as

typedef struct location{
char *name;
char *longer;
char *shorter;
char *north;
char *south;
char *east;
char *west;
char *logic;
int visited;
char *items[20];
} Location;

Furthermore I can succesfully read in all the values for the location and display all attributes so that is not an issue.

In the 'engine' of my game (the main), i attempt to read all the locations into a list as seen in the following (I'm certain readLocation works correctly because I threw a print statement into the loop printing the name of the locations using the loc variable)

world = 0;
FILE *fp = fopen("world.in","r");
char *garb = readToken(fp);
free(garb); //garbage token at begging of world.in just to check file exists
int count = 0; //used later, ignore for now
while(!feof(fp)){
    loc = readLocation(fp);
    join(loc,world);
    }

world is global variable declared as Node * and initialized to 0 (I think i need to do that but am not sure)

In olist.h I create the node structure as

typedef struct node
{
Location *place;
struct node *next;
} Node;

and in olist.c this is how i construct the Node as well as join the nodes

//place is the attribute of the Node that holds the location and next points to the next Node in the list
Node *newNode(Location *loc,Node *next)
    {
    Node *n = malloc(sizeof(Node));
    if (n == 0)
        {
        fprintf(stderr,"newNode: out of memory!\n");
        exit(1);
        }
    n->place = loc;
    n->next = next;
    return n;
    }

//s is the location i wish to join to the list and rest is list I'm joining to
Node *join(void *s,Node *rest)
    {
    return newNode(s,rest);
    }

Unfortunately, after successfully reading in all locations, world is still an empty list. Thanks for any help and I will be happy to provide further information via this forum or skype: F3V3Rz_MoDz (its a very old name)

Answer 1

If I'm not mistaken, your problem is in the following line of code :

n->next = next;

The head of your linked list is world, but your newNode() function keeps sending world to the back of the list (n->next = next places next at the back). What you want here is to append at the end of your linked list.

Here is an example of code that you can use to do this :

Node *lastNode = next;
while (lastNode->next != NULL)
    lastNode = lastNode->next;

lastNode->next = n;

Basically, you iterate through your linked list until you get to the end, then you append the newly created node.

EDIT : The problem you were having is that the world variable is in the end of your linked list. Every time you call join() you are pushing world at the back of the list. Here's a representation of your list :

Before join() :

world -> null

After join() :

newnode -> world -> null

Therefore, everytime you try to iterate through the list, world does not see the newly created nodes that are before him.

My solution does the following :

world -> newnode -> null

Which basically keeps your world variable in front. So you don't have to do world = join(loc, world)