Beginner C Pointer Problem - c

Question

I am a new C user have problems with pointers.
The function add_word(*word_to_add) should take a c string and add it to the appropriate word_node linked list. However, subsequent additions seem to be overwriting the word element of all nodes in the hash table. I think this is happening because I am setting each nodes word element to be a pointer to word_to_add rather than copying the value of word_to_add.
#define HASH_TABLE_SIZE 10
static char **word_list;
struct word_node {
char* word;
struct word_node *next;
};
static struct word_node *word_hash_table[HASH_TABLE_SIZE];
static int hash(char *word) {
int ascii_char;
int key;
key = 0;
while (*word != '\0') {
ascii_char = tolower(*word);
key += ascii_char;
word++;
}
key %= HASH_TABLE_SIZE;
return key;
}
void ws_add_word(char *word_to_add)
{
int word_position;
word_position = hash(word_to_add);
if (word_hash_table[word_position] == NULL)
{
struct word_node* p;
p = malloc(sizeof(struct word_node));;
p->word = word_to_add;
p->next = NULL;
word_hash_table[word_position] = p;
++num_words;
} else {
struct word_node* p;
p = word_hash_table[word_position];
while (p->next != NULL)
{
p = p->next;
}
struct word_node* q;
q = malloc(sizeof(struct word_node));
q->word = word_to_add;
q->next = NULL;
p->next = q;
++num_words;
}

Answer 1

I think your code is ok as is. If you are having problems, it may be in how you are calling the function. For example, here is a quick & dirty sample:
#include <stdio.h>
#include <stdlib.h>
int num_words;
/* paste sample code here */
#define HASH_TABLE_SIZE 10
// rest of code
void ws_add_word(char *word_to_add)
{
// function code
}
/* end of sample code */
int main (void)
{
char a[] = "Hello";
char b[] = "Helkp"; // chosen to have the same hash result
ws_add_word((char *)&a);
ws_add_word((char *)&b);
}
I compiled with gcc -g and ran through gdb. Doing so and stepping through ws_add_word and examining the contents of word_hash_table seems to do what you want. If it still doesn't work, you should give an example of how you are calling ws_add_word.
Also, if you simply change p->word = word_to_add with strcpy(p->word,word_to_add), you will probably get a seg fault because p->word has not been set to anything meaningful yet. You need to p->word = (char *)malloc(N) where N is big enough, and then strcpy, if that's what you really want to do. Whether you want to or not depends on whether the memory location pointed to by char *word_to_add will be valid the next time you need to use it.

Answer 2

You cannot just equate two strings or character pointers like:
char* p1, p2;
p1=p2; // THIS IS INVALID
In your code you have done this mistake two times:--
p->word = word_to_add;
and
q->word = word_to_add;
This says that you are making p->word point to word_to_add which is a pointer.It will just start referring to the same object but not copy the string which you expect.
It won't copy the contents of word_to_add to q->word, which you are assuming to be done.
You need to copy the strings using any string copy function like: strcpy, strncpy or memcpy.
replace p->word = word_to_add with
strcpy(p->word,word_to_add);
and similarly
replace q->word = word_to_add with
strcpy(q->word,word_to_add);
--
Kr. Alok

Answer 3

Looks good to me, except for line 10 in ws_add_word.
Change
q->word = word_to_add;
to
p->word = word_to_add;

Answer 4

The node's char pointer 'word' seems to point to the original string rather than holding a copy of its own. This is fine as long as the original string doesn't change. For example:
//Assume word_to_add is "String 1"
p->word = word_to_add;
//Now change word_to_add to "String 2"
//After this p->word would be "String 2"
I don't know if that was the intended behavior. You might want to allocate memory for every 'word' pointer in the node structure and copy the word_to_add string.