Thread: Adding linked list problem

Hi guys. I'm trying to learn how to add two long integers in a linked list.

So 1->2->3->NULL and 4->5->6->NULL should give me 5->7->9->NULL

I know how to approach the problem but the problem is, the thing I'm trying to do isn't working and the compiler gives me an error.

So far, this is a very very rough draft of my code and I'm basically going to change it later but for the purposes of testing, this should be enough.

So my method of creating the linked list is this:

Code:

void create_list(struct list **ptr,char *input) {
	int i;
	struct list *tmp;
	
	for(i = 0; i < strlen(input); i++){
		if(i == 0){
			(*ptr) = malloc(sizeof(struct list));
			(*ptr)->data = input[i]-'0';
			(*ptr)->next = NULL;
			(*ptr)->prev = NULL;
		}
		else {
			tmp = malloc(sizeof(struct list));
			tmp->data = input[i]-'0';
			tmp->next = NULL;
			tmp->prev = *ptr;
			(*ptr)->next = tmp;
			*ptr = tmp;
		}
	}
	//printf("%d", temp->prev->data);
}

I know it works because as you can see, I included a printf just for testing and if I enter values like 1234 in the command prompt, I get the value 3 when printf works so that's fine.

So what I have for my add method is this:

Code:

void add(struct list *first, struct list *second) {

int carry = 0;

while(*first != NULL && *second != NULL) {


}
}

Yes, I'm aware the add method is virtually empty but my problem is simple. Why is the while loop giving me an error?

What I'm trying to do is traverse the list and I included a while loop to check if both lists are not null and then I'm going to get the data from both list's tail and add them together. But I just have to figure out the while loop first.

I thought I already was.

Like I said, when I enter 12345, and the printf evalutes, it returns 4 because temp->prev->data = 4. So I know I'm doing it backwards.

Besides, that's not my problem. I just need to figure out why the while loop gives me an error.


Maybe I cannot actually compare a linked list with != ? But then how would I say loop it until the linked lists are null?

You are trying to compare dreferenced pointers to a pointer... NULL = void *0... lose the asterisks in your while loop and see what happens.

Okay so I have

Code:

while(first!=NULL && second!=NULL) and it works fine now.

I still don't get it. Can you explain it in simple terms?


And can you also check if I'm doing the algorithm right?
What I'm trying to do is get the last data of first so first->data and then add it to second->data
So it will be first->data + second ->data.
Will that work? And will the while loop go the first/second->prev->data or do I have to implement it myself?