From: "Graham Reeds" <grahamr AT dtn DOT ntl DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: Link-List Problems
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Date: Mon, 23 Oct 2000 04:30:41 +0100
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

I'm now working on my linked list code again, and I've hit a brick wall.

The code works insofar as it doesn't produce any warnings or errors.  The
basics are:

- first iteration through: genarray() gets called to make 1000 elements in a
linked list.
- as each node is created its contents are echoed to the screen.
- as each node is added to the list, its contents are echoed to the screen
(again).
- after the list is created, it is run through to make sure it is working
right.

Unfortunately this is where the problems start.  The list doesn't work -
there is no list.  For some reason there is no elements or links or anything
there. For some inexplicable reason it is kaput.  I have ran through it
several times, on several pieces of paper, and I cannot get it to work.

It either produces 0.0 0.00 for it's info or 1.9 2.69 and crashes with a
GPF.

Any ideas (sorry in advance for the amount of code)?

/* Function - AddNode()
** adds a node to the end of list
** if no nodes are present initialises list */
void            addnode(record_t *record)
  {
  if (recordhead == NULL)       /* empty list */
    {
    recordtail = recordhead = record;
    printf("\t\t\tno head\n" \
           "\t\t\trecord->x[0] = %5.1f\n" \
           "\t\t\trecord->avgz = %5.2f\n",
           record->x[0],
           record->averagez);
    }
  else                          /* 1 or more nodes */
    {
    recordtail->next = record;
    record->prev = recordtail;
    record->next = NULL;
    recordtail = record;
    printf("\t\t\trecord->x[0] = %5.1f\n" \
           "\t\t\trecord->avgz = %5.2f\n",
           record->x[0],
           record->averagez);
    }

  return;
  }

/* Function (Private) - GenArray()
** generates a nonsorted linked-list of a given size */
void            genarray(int numelements)
  {
  int           i;
  record_t     *record;

  printf("*Creating Array\n");
  for (i=0; i<numelements; i++)
    {
    printf("\tcreating node no.:\t%i\n", i);
    if (!(record = createnode()))    /* create node - just a malloc()*/
      { printf("Error: Couldn't create node.\n\n");  exit(1); }
    record->x[0] = i;    /* just for debugging purposes... */
    record->averagez = (float)(random() % 100000);
    printf("\t\tnode %i record->x[0] = %5.1f\n" \
           "\t\tnode %i record->avgz = %5.2f\n",
           i, record->x[0],
           i, record->averagez);
    addnode(record);            /* add node */
      if (!(i % 25))
    getch();
    }
  printf("*Finished Creating Array\n");
  return;
  }

/* Function - Main()
** RELEVANT PARTS ONLY
*/
int             main()
  {
  int           i, j, st, et;
  FILE         *fp;
  record_t     *recordptr;

  fp = fopen("res_list.txt", "wt");

  /* LOOP START */
  for (i=0; i<1; i++)
    {
    printf("\n%s Sort:\n", sortmethods[i]);
    fprintf(fp, "\n%s Sort:\n", sortmethods[i]);
    sortfunction = (void *)((char *)sortfunctions[i]);

    /* start sorting */
    for (j=0; j<6; j++)
      {
      /* generate array */
      genarray(arrayelements[j]);

      /* DISPLAY LIST */
      printf("*Displaying List\n");
      recordptr = recordhead;
      i = 0;
      while (recordptr->next != NULL)
        {
        printf("\t%i:\t%5.1f\t%5.2f\n", i, recordptr->x[0],
recordptr->averagez);
        if (!(i % 25))
          getch();
        recordptr = recordptr->next;
        i ++;
        }
  ...rest of code...

I've ran through it on paper several times - that's why there is all those
printf() statements so I can see what is going on, and what is not working.

The full source including makefile can be found at
http://homepage.dtn.ntl.com/grahamr/sortll.zip

Thanks for your time.
--

Stay Lucky, Graham "Mournblade" Reeds.
ICQ No.:  30514803
http://homepage.dtn.ntl.com/grahamr