Material to Understand

• The list '(1 2 3) is represented internally by Common Lisp as follows. Each box contains a data item and a pointer to the next box.

      +--------+   +--------+    +---------+
      | 1 | -->|   | 2 | -->|    | 3 | nil |
      +--------+   +--------+    +---------+
  

• The list '(1 (2) 3) is represented internally by Common Lisp as follows. The first cell in the second box points down to the box containing 2 and nil

      +--------+   +--------+    +---------+
      | 1 | -->|   |   | -->|    | 3 | nil |
      +--------+   +--------+    +---------+
  
                   +--------+               
                   | 2 |nil |               
                   +--------+                
  

Exercises

1. Draw a picture of how Common Lisp would represent '(1 (2 3) ((4 5) 6)).
2. Draw a picture of how Common Lisp would represent '(defun add-one ( n ) (+ n 1)).
3. Describe one advantage of Lisp representing data and code in the same manner.
4. Draw a picture showing the effects of the following.

      (setf list1 '(2 3))
      (setf list2 (cons 1 list1))
   

5. Draw a picture showing the effects of the following.

      (setf list1 '(1 2))
      (setf list2 '(3))
      (setf list3 (append list1 list2))
   

6. Explain why it is preferable to use cons over append.
7. Why is it dangerous to do the following:

      (setf alist '(1 2 3))
      (setf (second alist) 4)
   

8. Why is eq more efficient than equal?