Insert Function using strcmp()

Question

I'm trying to make an insert function, but I don't know if I'm allowed to used a char string and a node in the argument.

Does this work? Why not?

    void insert(char* str, node* head) {
      if (head == NULL) {
        node* new_node = malloc(sizeof(struct node));
        assert(new_node != NULL);
        new_node->value = strdup(str);
    }
    while (head != NULL) {
      if (strcmp(str, head->value)) {
        node *new_node = malloc(sizeof(struc node));
        assert(new_node != NULL);
        new_node->link = head;
        new_node->value - strdup(str);
      }
    }
    node *prev = head;
    head = head->link;

Answer 1

You havet to return the new head of the list by return value. If you insert one node you have to allocate memory for one node. Don't forget the initialize member prev of the first node and the member link of the last node with NULL:

node* insert(char* str, node* head)
{
    node* new_node = malloc(sizeof(struct node));
    assert(new_node != NULL);
    new_node->value = strdup(str);
    new_node->link = NULL;          // successor of new node is NULL

    if ( head == NULL )
    {
        new_node->pev = NULL;       // prdecessor of first node is NULL
        head = new_node;   
        return new_node;            // head was NULL, return new head
    }

    node *lastNode = head;          // go to last node of list
    while ( head->link != NULL )
        lastNode = lastNode->link;  // step one forward

    lastNode->link = new_node;      // successor of last node is new node
    new_node->prev = lastNode;      // predecesor of new node is last node
    return head;
}

-

node *head = NULL;
head = insert( "abc", head );
head = insert( "def", head ); 

An other solution would be to use an in and output paramter for your paramter head in function insert:

void insert(char* str, node** head)
                        // ^^ in and output parameter
{
  node* new_node = malloc(sizeof(struct node));
  assert(new_node != NULL);
  new_node->value = strdup(str);
  new_node->link = NULL;          // successor of new node is NULL

  node* prev = NULL;
  node* act = *head;
  while ( act != NULL )           // go to last node in list
  {
      prev = act;
      act = act->link;            // step one forward
  }

  new_node->prev = prev;     // predecessor of new node is last node or NULL
  if ( prev == NULL )
      *head = new_node;      // new node is the first node in list,
                             //   write the new node back to head
  else
      prev->link = new_node; // successor of last node is new node
}

-

node *head = NULL;
insert( "abc", &head );
insert( "def", &head );