Hi, I have a C hashing routine which is behaving strangely?

Posted by aks on Stack Overflow See other posts from Stack Overflow or by aks
Published on 2010-05-18T07:55:50Z Indexed on 2010/05/18 8:00 UTC
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Hi,

In this hashing routine: 1.) I am able to add strings. 2.) I am able to view my added strings. 3.) When i try to add a duplicate string, it throws me an error saying already present. 4.) But, when i try to delete the same string which is already present in hash table, then the lookup_routine calls hash function to get an index. At this time, it throws a different hash index to the one it was already added. Hence, my delete routine is failing?

I am able to understand the reason why for same string, hash fucntion calculates a different index each time (whereas the same logic works in view hash table routine)? Can someone help me?

This is the Output, i am getting:

$ ./a

Press 1 to add an element to the hashtable

Press 2 to delete an element from the hashtable

Press 3 to search the hashtable

Press 4 to view the hashtable

Press 5 to exit

Please enter your choice: 1

Please enter the string :gaura

enters in add_string

DEBUG purpose in hash function:

str passed = gaura Hashval returned in hash func= 1 hashval = 1 enters in lookup_string

str in lookup_string = gaura DEBUG purpose in hash function:

str passed = gaura Hashval returned in hash func= 1 hashval = 1

DEBUG ERROR :element not found in lookup string

DEBUG Purpose

NULL

Inserting...

DEBUG1 : enters here

hashval = 1 String added successfully

Press 1 to add an element to the hashtable

Press 2 to delete an element from the hashtable

Press 3 to search the hashtable

Press 4 to view the hashtable

Press 5 to exit

Please enter your choice: 1

Please enter the string :ayu

enters in add_string

DEBUG purpose in hash function:

str passed = ayu Hashval returned in hash func= 1 hashval = 1 enters in lookup_string

str in lookup_string = ayu DEBUG purpose in hash function:

str passed = ayu Hashval returned in hash func= 1 hashval = 1

returns NULL in lookup_string

DEBUG Purpose

NULL

Inserting...

DEBUG2 : enters here

hashval = 1 String added successfully

Press 1 to add an element to the hashtable

Press 2 to delete an element from the hashtable

Press 3 to search the hashtable

Press 4 to view the hashtable

Press 5 to exit

Please enter your choice: 1

Please enter the string :gaurava

enters in add_string

DEBUG purpose in hash function:

str passed = gaurava Hashval returned in hash func= 7 hashval = 7 enters in lookup_string

str in lookup_string = gaurava DEBUG purpose in hash function:

str passed = gaurava Hashval returned in hash func= 7 hashval = 7

DEBUG ERROR :element not found in lookup string

DEBUG Purpose

NULL

Inserting...

DEBUG1 : enters here

hashval = 7 String added successfully

Press 1 to add an element to the hashtable

Press 2 to delete an element from the hashtable

Press 3 to search the hashtable

Press 4 to view the hashtable

Press 5 to exit

Please enter your choice: 4 Index : i = 1 String = gaura ayu Index : i = 7 String = gaurava

Press 1 to add an element to the hashtable

Press 2 to delete an element from the hashtable

Press 3 to search the hashtable

Press 4 to view the hashtable

Press 5 to exit

Please enter your choice: 2

Please enter the string you want to delete :gaura

String entered = gaura enters in delete_string

DEBUG purpose in hash function:

str passed = gaura Hashval returned in hash func= 0 hashval = 0 enters in lookup_string

str in lookup_string = gaura DEBUG purpose in hash function:

str passed = gaura Hashval returned in hash func= 0 hashval = 0

DEBUG ERROR :element not found in lookup string

DEBUG Purpose

Item not present. So, cannot be deleted

Item found in list: Deletion failed

Press 1 to add an element to the hashtable

Press 2 to delete an element from the hashtable

Press 3 to search the hashtable

Press 4 to view the hashtable

Press 5 to exit

Please enter your choice:

My routine is pasted below:

include

include

struct list { char *string; struct list *next; };

struct hash_table { int size; /* the size of the table */ struct list *table; / the table elements */ };

struct hash_table * hashtable = NULL;

struct hash_table *create_hash_table(int size) { struct hash_table *new_table; int i;

if (size<1) return NULL; /* invalid size for table */

/* Attempt to allocate memory for the table structure */
if ((new_table = malloc(sizeof(struct hash_table))) == NULL) {
    return NULL;
}

/* Attempt to allocate memory for the table itself */
if ((new_table->table = malloc(sizeof(struct list *) * size)) == NULL) {
    return NULL;
}

/* Initialize the elements of the table */
for(i=0; i<size; i++) 
 new_table->table[i] = '\0';

/* Set the table's size */
new_table->size = size;

return new_table;

}

unsigned int hash(struct hash_table *hashtable, char *str) { printf("\n DEBUG purpose in hash function:\n"); printf("\n str passed = %s", str);

unsigned int hashval = 0;
int i = 0;

for(; *str != '\0'; str++)
{
 hashval += str[i];
 i++;
}
hashval = hashval % 10;
printf("\n Hashval returned in hash func= %d", hashval);
return hashval;

}

struct list *lookup_string(struct hash_table *hashtable, char *str) { printf("\n enters in lookup_string \n"); printf("\n str in lookup_string = %s",str);

struct list * new_list;
unsigned int hashval = hash(hashtable, str);

printf("\n hashval = %d \n", hashval);

if(hashtable->table[hashval] == NULL)
{
  printf("\n DEBUG ERROR :element not found in lookup string \n");
  return NULL;
}

/* Go to the correct list based on the hash value and see if str is
 * in the list.  If it is, return return a pointer to the list element.
 * If it isn't, the item isn't in the table, so return NULL.
 */


for(new_list = hashtable->table[hashval]; new_list != NULL;new_list = new_list->next)
{
    if (strcmp(str, new_list->string) == 0)
      return new_list;
}
printf("\n returns NULL in lookup_string \n");
return NULL;

}

int add_string(struct hash_table *hashtable, char *str) { printf("\n enters in add_string \n");

struct list *new_list;
struct list *current_list;
unsigned int hashval = hash(hashtable, str);
printf("\n hashval = %d", hashval);

/* Attempt to allocate memory for list */
if ((new_list = malloc(sizeof(struct list))) == NULL)
{
 printf("\n enters here \n");
 return 1;
}

/* Does item already exist? */
current_list = lookup_string(hashtable, str);

if (current_list == NULL)
{
 printf("\n DEBUG Purpose \n");
 printf("\n NULL \n");
}

/* item already exists, don't insert it again. */
if (current_list != NULL)
{
 printf("\n Item already present...\n");
 return 2;
}

/* Insert into list */
printf("\n Inserting...\n");

new_list->string = strdup(str);
new_list->next = NULL;
//new_list->next = hashtable->table[hashval];
if(hashtable->table[hashval] == NULL)
{
  printf("\n DEBUG1 : enters here \n");
  printf("\n hashval = %d", hashval);
  hashtable->table[hashval] = new_list;
}
else
{
  printf("\n DEBUG2 : enters here \n");
  printf("\n hashval = %d", hashval);
  struct list * temp_list = hashtable->table[hashval];
  while(temp_list->next!=NULL)
   temp_list = temp_list->next;

  temp_list->next = new_list;
  // hashtable->table[hashval] = new_list;
}

return 0;

}

int delete_string(struct hash_table *hashtable, char *str) { printf("\n enters in delete_string \n");

struct list *new_list;
struct list *current_list;
unsigned int hashval = hash(hashtable, str);
printf("\n hashval = %d", hashval);

/* Does item already exist? */
current_list = lookup_string(hashtable, str);

if (current_list == NULL)
{
 printf("\n DEBUG Purpose \n");
 printf("\n Item not present. So, cannot be deleted \n");
 return 1;
}

/* item exists, delete it. */
if (current_list != NULL)
{
struct list * temp = hashtable->table[hashval];
if(strcmp(temp->string,str) == 0)
{
  if(temp->next == NULL)
  {
    hashtable->table[hashval] = NULL;
    free(temp);
  }
  else
  {
    hashtable->table[hashval] = temp->next;
    free(temp);
  }
}
else
{
  struct list * temp1; 
  while(temp->next != NULL)
  {
    temp1 = temp; 

    if(strcmp(temp->string, str) == 0)
    {
      break;
    }
    else
    {
      temp = temp->next;
    }
  }
  if(temp->next == NULL)
  {
    temp1->next = NULL;
    free(temp);
  }
  else
  {
    temp1->next = temp->next;
    free(temp);
  }
}
}
return 0;

}

void free_table(struct hash_table *hashtable) { int i; struct list *new_list, *temp_list;

if (hashtable==NULL) return;

/* Free the memory for every item in the table, including the 
 * strings themselves.
 */
for(i=0; i<hashtable->size; i++) {
    new_list = hashtable->table[i];
    while(new_list!=NULL) {
        temp_list = new_list;
        new_list = new_list->next;
        free(temp_list->string);
        free(temp_list);
    }
}

/* Free the table itself */
free(hashtable->table);
free(hashtable);

}

void view_hashtable(struct hash_table * hashtable) { int i = 0; if(hashtable == NULL) return;

for(i =0; i < hashtable->size; i++)
{
    if((hashtable->table[i] == 0) || (strcmp(hashtable->table[i]->string, "*") == 0)) 
    {
        continue;
    }
    printf(" Index : i = %d\t String = %s",i, hashtable->table[i]->string);
    struct list * temp = hashtable->table[i]->next;
    while(temp != NULL)
    {
      printf("\t %s",temp->string);
      temp = temp->next;
    }

    printf("\n");
}

}

int main() { hashtable = create_hash_table(10); if(hashtable == NULL) { printf("\n Memory allocation failure during creation of hash table \n"); return 0; }

int flag = 1;

while(flag)
{
    int choice;

    printf("\n Press 1 to add an element to the hashtable\n");
    printf("\n Press 2 to delete an element from the hashtable\n");
    printf("\n Press 3 to search the hashtable\n");           printf("\n Press 4 to view  the hashtable\n");              printf("\n Press 5 to exit \n");
    printf("\n Please enter your choice: ");

    scanf("%d",&choice);

    if(choice == 5)
     flag = 0;

    else if(choice == 1)
    {
        char str[20];
        printf("\n Please enter the string :");
        scanf("%s",&str);
        int i;
        i = add_string(hashtable,str);

        if(i == 1)
        {
          printf("\n Memory allocation failure:Choice 1 \n");
          return 0;
        }
        else if(i == 2)
        {
          printf("\n String already prsent in hash table : Couldnot add it again\n");
          return 0;
        }
        else
        {
            printf("\n String added successfully \n");

        }
    }   


    else if(choice == 2)
    {
        int i;
        struct list * temp_list;
        char str[20];
        printf("\n Please enter the string you want to delete :");
        scanf("%s",&str);

        printf("\n String entered = %s", str);

        i = delete_string(hashtable,str);

        if(i == 0)
        {
          printf("\n Item found in list: Deletion success \n");
        }
        else
          printf("\n Item found in list: Deletion failed \n");
    }


    else if(choice == 3)
    {
        struct list * temp_list;
        char str[20];
        printf("\n Please enter the string :");
        scanf("%s",&str);
        temp_list = lookup_string(hashtable,str);

        if(!temp_list)
        {
          printf("\n Item not found in list: Deletion failed \n");
          return 0;
        }
        printf("\n Item found: Present in Hash Table \n");
    }


    else if(choice == 4)
    {
        view_hashtable(hashtable);
    }
    else if(choice == 5)
    {
        printf("\n Exiting ....");
        return 0;
    }
    else
     printf("\n Invalid choice:");
};

free_table(hashtable);
return 0;

}

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