C: Help understanding pointers - c

Question

I have been building this hash table function that is given an array it gets a word from and a pointer to an array that it fills with my linked list struct. It compiles correctly but I get a segmentation fault at - *hashTable[hashVal] = *newNode; .
void hashTableCreate(char *array, list *hashTable[]) {
while(arrayPos < getArrayLength(array)) {
char *temp = getWord(array);
int hashVal = hashingFunc(temp);
if((*hashTable[hashVal]).word == temp ) {
(*hashTable[hashVal]).count = (*hashTable[hashVal]).count+1;
}
else {
list *newNode = malloc(sizeof(list));
strcpy(newNode->word,temp);
newNode->count = 1;
*hashTable[hashVal] = *newNode;
}
}
}
It's called like this:
void timeStructures(char *newArray) {
list *hashTable[3000];
hashTableCreate(newArray, hashTable);
}
I know this is to do with the way I am using my pointers but I can't figure it out. Any help would be appreciated.

Answer 1

There are several problems on this code:
You seem you are using extra asterisks when assigning newNode to hashTable.
You can use ++ operator to increment count, and use -> to access a member of a pointer to struct.
getWord is unknown to me, but seems impossible it can return a pointer to a string contained inside an element of hashTable, which means the next if will always be false (newNode's word is a copied string, not a pointer to array's word), as it compares memory addresses instead of strings.
You never free temp neither hashTable elements, which seems another misconception of what you are doing here.
As noted by #kaylum , you should initialize hashTable so you avoid comparing against unassigned array elements, neither modifying count on them.
hashTable has to be passed as a pointer to hashTableCreate, or this will work on a copied version of it:
Try this:
void hashTableCreate(char *array, list *(*hashTable[])) {
while(arrayPos < getArrayLength(array)) {
char *temp = getWord(array);
int hashVal = hashingFunc(temp);
if((*hashTable)[hashVal] != NULL &&
strcmp((*hashTable)[hashVal]->word, temp) == 0 ) {
(*hashTable)[hashVal]->count++;
}
else {
list *newNode = malloc(sizeof(list));
strcpy(newNode->word,temp);
newNode->count = 1;
(*hashTable)[hashVal] = newNode;
}
free(temp);
}
}
void timeStructures(char *newArray) {
list *hashTable[3000];
int i;
// Initialize hashTable pointers to NULL
for(i = 0; i < 3000; i++) {
hashTable[i] = NULL;
}
hashTableCreate(newArray, &hashTable);
// Free hashTable elements, malloc'ed at hashTableCreate
for(i = 0; i < 3000; i++) {
if(hashTable[i] != NULL) {
free(hashTable[i]);
}
}
}
Note 1: you never check if hashVal is higher than 3000, which might be fine if you know what you are doing, aka, you know hashingFunc will never return 3000 or higher. If you do not, you'll need to care about growing hashTable appropriately.
Note 2: executing getArrayLength on each loop might be a performance problem for large lists. You probably prefer to create a variable to contain its value and execute it only once.
Note 3: It might be some easier if you declare hashTable as a plain list of elements, not a list of pointers to elements, like list hashTable[3000];, but I don't know exactly what you're looking for. Also, it will always waste 3000 list elements of memory, so might be a performance problem.