need assistance with singly linked list - c

Question

I need some help with a singly linked list. I am trying to insert a new node for each word read in from my text file and compares it to the words in a dictionary file.From there, the new node is inserted into a hashtable.I feel that I am close(maybe wistful thinking),but I keep getting a segmentation error everytime I run my program. From the looks of my code, does anyone have any idea on what maybe wrong?
typedef struct Node {
char word[LENGTH+1];
struct Node *Next;
} Node;
hash_table_t *create_hash_table(int size)
hashtable = malloc(sizeof(hash_table_t));
if (hashtable == NULL)
{
return NULL;
}
hashtable->table= malloc(size* sizeof(struct Node *) ) ;
if (hashtable->table== NULL)
{
return NULL;
}
for(int i=0; i<size; i++)
{
hashtable->table[i]=NULL;
hashtable->size =size;
}
return hashtable;
typedef struct hash_table_t{
int size; /* the size of the table */
struct Node **table; /* the table elements */
} hash_table_t;
File *inptr;
Node* TempNode=NULL;
Node* new_node=NULL;
Node* Head=NULL;
char buffer[46];
unsigned int hashval;
int j=0;
int count=0;
int update_counter=0;
inptr= fopen(dictionary, "r");
if (inptr == NULL)
{
printf("Could not open dictionary file");
printf("\n");
return 0;
}
int ch = fgetc(inptr);
for ( ;; )
{
if ( ch == EOF ) //determines how many words are in the file
{
break;
}
if (isalpha(ch) || isdigit(ch) || ispunct(ch))
{
update_counter = 1;
}
if (isspace(ch) && update_counter )
{
count++;
update_counter = 0;
}
}
if (update_counter)
{
count++;
}
sizeOfDictionary=count;
rewind(inptr);
hashtable=create_hash_table(sizeOfDictionary);
while(fread(buffer,sizeof(buffer),1,inptr)!=0)
{
if(Head==NULL)
{
hashval = hash(buffer);
Head = malloc(sizeof(Node));
strcpy(&Head->word[j],buffer);
Head->Next = hashtable->table[hashval];
hashtable->table[hashval]=Head;
Head=Head->Next;
TempNode = Head;
}
else if(Head!=NULL)
{
new_node = malloc(sizeof(Node));
hashval = hash(buffer);
strcpy(&new_node->word[j],buffer);
new_node->Next = hashtable->table[hashval];
hashtable->table[hashval]=new_node;
TempNode=new_node->Next;
}
}
return true;

Answer 1

Let me try to explain what is happening in steps:
Head = malloc(sizeof(Node));
strcpy(&Head->word[j],buffer);
Head
----------
| |
| buffer |
| |
----------
Head->Next = hashtable->table[hashval];
Head=Head->Next;
TempNode = Head;
TempNode
Head
---------- -----------------
| | Next | |
| buffer | -----> | table[hashval] |
| | | |
---------- -----------------
^
This is now
lost forever
new_node = malloc(sizeof(Node));
strcpy(&new_node->word[j],buffer);
new_node->Next = hashtable->table[hashval];
TempNode=new_node->Next;
Head new_node TempNode
---------- ----------------- ---------- -----------------
| | Next | | | | Next | |
| buffer | -----> | table[hashval] | No link | buffer | -----> | table[hashval] |
| | | | here! | | | |
---------- ----------------- ---------- -----------------
^
This is now
lost forever
And so on.
I suspect that the "No link here!" part in my diagram above is causing your segfault. I also do not know how you intend to use your table[hashval], but here goes anyway. This solution just bridges the gaps in your linked list.
Solution:
Replace your while loop with this:
TempNode = Head = NULL;
while(fread(buffer,sizeof(buffer),1,inptr)!=0)
{
if(Head==NULL)
{
hashval = hash(buffer);
Head = malloc(sizeof(Node));
strcpy(&Head->word[j],buffer);
Head->Next = hashtable->table[hashval];
Head->Next->Next = NULL;
TempNode = hashtable->table[hashval] = Head;
}
else
{
new_node = malloc(sizeof(Node));
hashval = hash(buffer);
strcpy(&new_node->word[j],buffer);
new_node->Next = hashtable->table[hashval];
hashtable->table[hashval]=new_node;
new_node->Next->Next = NULL;
TempNode->Next = new_node;
TempNode = new_node;
}
TempNode = TempNode->Next;
}
Visually the differences are:
Head = malloc(sizeof(Node));
strcpy(&Head->word[j],buffer);
Head->Next = hashtable->table[hashval];
Head->Next->Next = NULL;
TempNode = hashtable->table[hashval] = Head;
TempNode
Head
---------- -----------------
| | Next | | Next
| buffer | -----> | table[hashval] | -----> NULL
| | | |
---------- -----------------
TempNode = TempNode->Next
Head TempNode
---------- -----------------
| | Next | | Next
| buffer | -----> | table[hashval] | -----> NULL
| | | |
---------- -----------------
new_node = malloc(sizeof(Node));
strcpy(&new_node->word[j],buffer);
new_node->Next = hashtable->table[hashval];
new_node->Next->Next = NULL;
Head TempNode new_node
---------- ----------------- ---------- -----------------
| | Next | | | | Next | |
| buffer | -----> | table[hashval] | No link | buffer | -----> | table[hashval] | ----> NULL
| | | | here! | | | |
---------- ----------------- ---------- -----------------
TempNode->Next = new_node;
Head TempNode new_node
---------- ----------------- ---------- -----------------
| | Next | | Next | | Next | |
| buffer | -----> | table[hashval] | -----> | buffer | -----> | table[hashval] | ----> NULL
| | | | | | | |
---------- ----------------- ---------- -----------------
TempNode = new_node;
TempNode
Head new_node
---------- ----------------- ---------- -----------------
| | Next | | Next | | Next | |
| buffer | -----> | table[hashval] | -----> | buffer | -----> | table[hashval] | ----> NULL
| | | | | | | |
---------- ----------------- ---------- -----------------
And so on.
Tips:
Free whatever memory you have malloced at the end.
Do not move your Head pointer (unless you need to delete the first element or for some other reason).