在链表中插入一个元素

Question

I have now known why the linked list was not updated from the previous questions. It turns out that I have to iterate the coordinates of x but that's not important in this question of mine.

When I insert an element in the linked list the elements before where I want to insert a value disappears. For example I have elements that would print out "helo" and I want to insert another 'l' after e, the output would be "(space)ello. Here is my insertion code and structure:

struct node {
struct node *previous;
int c;
int x;
int y;
struct node *next;
}*head;

void checker(int ch, int xpos, int ypos)
{
    int flag=0;
    struct node *temp,*temp1,*var,*insert_node;
    var=(struct node *)malloc(sizeof(struct node));
    temp=(struct node *)malloc(sizeof(struct node));
    insert_node=(struct node*)malloc(sizeof(struct node));
    temp=head;
    while(temp!=NULL)
    {
        if(temp->x==xpos && temp->y==ypos)
        {
            insert_node->c=ch;
            insert_node->x=xpos;
            insert_node->y=ypos;
            insert_node->next=NULL;
            temp1=temp;
                while(temp1!=NULL)
                {
                    if(temp1->y==ypos)
                    temp1->x++;
                    temp1=temp1->next;
                }
                var->next=insert_node;
                insert_node->next=temp;
                head=var;

            flag=1;
            break;
        }
            var=temp;
            temp=temp->next;
    }
    if(flag==0)
        characters(ch,xpos,ypos);
}

It seems like var has only one element inside instead of two, it takes "h" from helo forgranted.

Answer 1

You throw away the list from the original head up to the point you find the matching x and y in the list when you assign head = var . Sit down and draw a couple of pictures to convince yourself that is wrong.

To insert a new node before the matching node in the list: keep track of the current node in the list and the previous node visited. Then when you are ready to insert a new node in front of current_node, do this:

insert_node->next = current_node;
if (previous_node == NULL)
    head = insert_node;
else
    previous_node->next = insert_node;

In your code, temp plays the role of current_node (the one you are examining). You don't have a pointer to the previous node, so declare one. Set current_node to head, previous_node to NULL, then start running the list, and when you find the node in the list you want to put insert_node in front of, use the code above. Note the special case when you want to insert at the front of the list. I leave it as an exercise to figure out what to do if you want to insert the new node after current_node .

Answer 2

        var->next=insert_node;
        insert_node->next=temp;

should be:

        insert_node->next=temp->next;
        temp->next=insert_node;