Insertion Agents:

The following code inserts a given element either before or after a chosen element in the list. These agents utilize the protected agents get_before_element_address (this returns the address of the SL_CELL which points to the SL_CELL containing the chosen element); get_element_address (this returns the address of the SL_CELL containing the chosen element); and get_after_element_address (this returns the address of the SL_CELL which is pointed to by the SL_CELL containing the chosen element).

    // ******************************************** //
    // insert a new element after a given element   //
    // ******************************************** //
void CHAR_SLLIST::insert_after(CHAR after_element,CHAR new_element)
{
  if(!(head==NULL) ){
    //use private search agents for class
    CHAR_SL_CELL *temp = get_after_element_address(after_element); 
    CHAR_SL_CELL *temp2 = get_element_address(after_element);
    if(temp==NULL && temp2!=NULL){
      //after_element is at the end of list.
      CHAR_SL_CELL *cell = new CHAR_SL_CELL("int");
      cell->element = new_element;
      cell->next = NULL;
      temp2->next = cell;
      }
    if(temp!=NULL && temp2!=NULL){
      //temp is address of cell after cell with after_element
      //temp2 is address of cell containing after_element
      CHAR_SL_CELL *cell = new CHAR_SL_CELL("int");
      cell->element = new_element;
      cell->next = temp;   //cell->next points to before_element cell
      temp2->next = cell;  //temp points to cell containing new_element  
      }
    if(temp2==NULL){
      //after_element is not in the list, so do not insert
      cout << after_element << " is not in the list, so do not insert."
           << endl;
      }
    }
   else{
     cout << "List is empty, so do not insert." << endl;
     }
}

    // ******************************************** //
    // insert a new element before a given element  //
    // ******************************************** // 
void CHAR_SLLIST::insert_before(CHAR before_element,CHAR new_element)
{
  if(!(head==NULL) ){
    //use private search agents for class
    CHAR_SL_CELL *temp = get_before_element_address(before_element); 
    CHAR_SL_CELL *temp2 = get_element_address(before_element);
    if(temp==NULL && temp2!=NULL){
      //before_element is at head of list.
      CHAR_SL_CELL *cell = new CHAR_SL_CELL("int");
      cell->element = new_element;
      cell->next = head;
      head = cell;
      }
    if(temp!=NULL && temp2!=NULL){
      //temp is address of cell prior to cell with before_element
      //temp2 is address of cell containing before_element
      CHAR_SL_CELL *cell = new CHAR_SL_CELL("int");
      cell->element = new_element;
      cell->next = temp2;   //cell->next points to before_element cell
      temp->next = cell;    //temp points to cell containing new_element  
      }
    if(temp2==NULL){
      //before_element is not in the list, so do not insert
      cout << before_element << " is not in the list, so do not insert."
           << endl;
      }
    }
   else{
     cout << "List is empty, so do not insert." << endl;
     }
}