问题描述:

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 function 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<stdio.h>

#include<string.h>

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;

}

网友答案:

Your hash function runs past the end of the string, just do e.g.

for(; *str != '\0'; str++)
{
 hashval += *str;

}
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