在此散列例程中： 1.）我能够添加字符串。 2.）我能够查看我添加的字符串。 3.）当我尝试添加一个重复的字符串时，它会抛出一个错误，表示已存在。 4.）但是，当我尝试删除哈希表中已存在的相同字符串时， 然后lookup_routine调用哈希函数来获取索引。此时，它会向已添加的哈希索引抛出不同的哈希索引。因此，我的删除例程失败了吗？
我能够理解为什么对于相同的字符串，哈希函数每次计算不同的索引（而相同的逻辑在视图哈希表例程中工作）？有人能帮助我吗？
$ ./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:
==============================================================
我的例程粘贴在下面：
#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;
}
答案 0 :(得分：9)
你的哈希函数运行超出了字符串的结尾，就像这样。
for(; *str != '\0'; str++)
{
hashval += *str;
}