Thread: Having a hard time with linked lists.

I am new to C and I am having a hard time with linked lists (preferred Java lists much more).

Anyway, this function is supposed to insert a string into the linked list if it doesnt already exist. If the string does exist in the list, increase the value by 1.

When I run my code, I insert aaa 2 times and print the list to get...
STRING VALUE
aaa 2

But inserting aaa 2 times and bbb once gives...
STRING VALUE
bbb 3

I have spent an hour just rearranging code and trying different things but I am having a hard time. I got a lot of the function from my textbook.

Code:

int insertIt(char* string, ListNodePtr *sPtr){
	ListNodePtr newPtr;     /*pointer to new node*/
	ListNodePtr previousPtr;    /*pointer to previous node in list*/
	ListNodePtr currentPtr;     /*pointer to current node*/
	newPtr = malloc(sizeof(ListNode));  /*creates node*/


	if(newPtr != NULL) {            /*is space available*/
		newPtr->symbol = string;    /*place string in node*/
		newPtr->value = 1;          /*place value in node*/
		newPtr->nextPtr = NULL;     /*node does not link to another node*/
		previousPtr = NULL;
		currentPtr = *sPtr;


		while(currentPtr != NULL) {
			if(currentPtr->symbol == string) {   /*If symbol exists...*/
				currentPtr->value = (currentPtr->value+1);   /*increase value by 1*/
				break;
			}
			else
				previousPtr = currentPtr;
				currentPtr = currentPtr->nextPtr;
		}
			/*create new node at beginning of list*/
			if(currentPtr == NULL) {
				newPtr->nextPtr = *sPtr;
				*sPtr = newPtr;
			}
			else if(currentPtr->symbol != string)
			{
				newPtr->nextPtr = *sPtr;
				*sPtr = newPtr;
			}
	}
	else
		printf("%s not inserted.\n", string); fflush(stdout);
	return 0;
}/*end function insertIt*/

I really have no idea what I am doing here.

So how do I use free()?
Do I just put free(string); at the end of the function?

Nope... free() releases allocated memory. You are allocating memory twice for each node in your list... once for the node itself and once for the char array content... When deleting nodes or tearing down the list you are are going to have to walk the entire list, freeing first the string then the node ... for every node in the list.

For every call to malloc() you must have a matching call to free()...

I don't have enough of your code to show you how to do it, but it's not that difficult... I'm sure your books cover it anyway.

There are quite a few tutorials about linked lists on the web... a little Google action might be in order

You might also look up the free() function in your C Library Documetation ...