Material to Understand

• The function defun: DEFine FUNction.

     (defun add-ten (n)
       (+ n 10)
     )
  

• The function let that introduces local variables in parallel.

    (let
      (
        (a 10)   ;; a is initialized to 10
        b        ;; b has no initial value
      )
      ... code involving a and b ...
    )
  

• The function let* that introduces local variables sequentially.

    (let*
      (
        (a 10)       ;; a is initialized to 10
        (b (+ a 1))  ;; b is initialized to 11 
      )
      ... code involving a and b ...
    )
  

• Using files with Common Lisp

    1. Open Common Lisp in one window.
    2. Open your favorite editor in another window.
       Suppose you are working on a file called
       lisp.l.  
    3. Make code additions/modifications to lisp.l.
       Save the changes.
    4. At the lisp prompt, type (load "lisp.l").
    5. Test the functions you wrote.  If
       you notice any errors, go to step 3.
    6. At the lisp prompt, type (compile-file "lisp.l")
       to compile your file.  If there are any errors
       or warnings, go to step 3 and fix them.
    7. At the lisp prompt, type (load "lisp").  This
       will load the compiled version of the file
       (called lisp.fas) into the Common Lisp environment.
       The compiled version should run several times faster.
  

Exercises

1. Set a variable named year to 2000.
2. Write a one parameter function named add-one that adds 1 to the number passed in.
3. Test the above function with the call (add-one year). Is year's value 2000 after the call? Explain.
4. (Problem 3-5.) Write a function called palindromize that takes a list as its argument and returns a palindrome that is twice as long as the argument. For example, (palindromize '(I love AI)) should return '(I love AI AI love I).
5. Use let in a function that calculates the two roots of a quadratic equation. For example, (roots 1 5 6) could return '(-2 -3). Develop this function in a file and load it into the CLISP Interpreter. Once the function works, compile it to ensure that there are no warnings. Run the compiled version.
6. Comment the above function appropriately and then have John take a look at it. Here is a model:
   

       
       ;; =================================== 
       ;;            John Paxton 
       ;;              CS 436 
       ;;         September 3, 2004 
       ;; ----------------------------------- 
       ;; Brief overview of this file's contents. 
       ;; =================================== 
   
       ;; ----------------------------------- 
       ;; KEEP-ENDS 
       ;; ----------------------------------- 
       ;; Return a list of the first and last
       ;; top level items. 
       ;; ----------------------------------- 
       ;; PARAMETERS 
       ;;  alist: an arbitrary list 
       ;; ----------------------------------- 
      
       (defun keep-ends ( alist ) 
         (cons (first alist) (last alist)) 
       ) 
   

7. Write a function where the use of let causes the function to perform differently than the use of let*.