Understanding LISP Primitives: FIRST, REST, CAR, and CDR

1. FIRST (or CAR) and REST (or CDR) take lists apart. Consider the list (First day of the semester). * (first '(First day of the semester)) FIRST * (rest '(First day of the semester)) (DAY OF THE SEMESTER) A First Rest (B C) LISP primitives on sequences (A B C)

2. * (first ( )) NIL * (rest ( )) NIL * (first '((a b) (c d)) (A B) * (rest '((a b) (c d)) ((C D)) * (car '(a . b)) A * (cdr '(a . b)) B Examples

3. * (first (rest '(a b c))) ;second returns B ;the 2nd element * (first '(rest (a b c))) ;third returns REST ;the 3rd element,... * (first (rest (a b c))) ;tenth returns ? ;the 10th element >>> Error:Undefined function: A while evaluating: (A B C) (#<COMPILED-FUNCTION 3:E196> ...) Debugger 1> The QUOTE mark ' stops evaluation

4. * (first (first (rest '(rest ((a b)(c d)(e f)))))) (A B) * (first '(((a b) (c d) (e f)))) ((A B) (C D) (E F)) Given (apple (orange)((pear))(((kiwi))))), write a sequence of FIRST and REST which returns PEAR. * (first (first (first (rest (rest '(apple (orange) ((pear)) (((kiwi))))))))))))) PEAR * (rest '(apple (orange) ((pear)) (((kiwi))))))))) ((ORANGE) ((PEAR)) (((KIWI)))) * (rest '((orange) ((pear)) (((kiwi))))) (((PEAR)) (((KIWI)))) * (first (first (first '(((pear)) (((kiwi))))))) PEAR Examples

5. The process of reserving a place in memory to store a value is called binding. The process of storing a value for a symbol is called assignment. The process of recovering a value from memory is called evaluation. SETF / SETQ assign values to symbols * (setf ab-list '(a b)) (a b) * ab-list (a b) * (setf ab-list '(a b) cd-list '(c d)) (c d) ; accepts multiple symbol-value pairs, ; but returns only the last assignment The SETF / SETQ primitives

6. * (setf fact1 '(CS462 is a fun course)) (CS462 IS A FUN COURSE) * fact1 (CS462 IS A FUN COURSE) CS462 is a fun course SETF alters the contents of the cons cell storing the symbol. fact1

7. * (first fact1) CS462 * fact1 (CS462 IS A FUN COURSE) * (setf (first fact1) 'CS463) fact1 CS463 is a fun course * fact1 (CS463 IS A FUN COURSE)

8. * (cons 'a '(b c)) (A B C) FIRST A (A B C) CONS (A B C) REST (B C) CONS, APPEND and LIST primitives construct lists * (append '(a b c) '(x y z)) ;combines elements (A B C X Y Z) * (list '(a b c) '(x y z)) ;combines lists ((A B C) (X Y Z))

9. *(append 'list1 'list2) ; does not accept atoms as arguments ERROR * (list 'list1 ' (x y z)) ; arguments can be both atoms and lists (LIST1 (X Y Z)) CONS, APPEND and LIST do not alter symbol values.

10. * (rest '(a b c d)) (B C D) ; list without its first element * (nthcdr 2 '(a b c d)) (C D) ; list without ‘n’ first elements. * (nthcdr 4 '(a b c d)) NIL * (butlast '(a b c d) 2) (A B) ; list without ‘n’ last elements. * (butlast '(a b c d)) (A B C) ; list without its last element. * (last '(a b c d)) (D) ; list of just the last element. * (last '((a b) (c d) (e f))) ((E F)) REST, NTHCDR, BUTLAST and LAST shorten lists

11. Add D at the end of the list (A B C) * (append '(a b c) (list 'd)) (A B C D) Add D at the beginning of the list (A B C) * (cons 'd '(a b c)) (D A B C) Add D as a second element in the list (A B C) * (append (list (first '(a b c))) (list 'd) (nthcdr 1 '(a b c))) (A D B C) Create a list of D and the last element of (A B C) * (setf new-list (list 'd (first (last '(a b c)))) ) (D C) * new-list (D C) More examples

12. * (length '(a b ((c d) (e f)))) 3 * (length (append '(a b ((c d) (e f))) '(x y z))) 6 * (reverse '(a b ((c d) (e f)))) (((C D) (E F)) B A) * (reverse (append '(a b ((c d) (e f))) '(x y z))) (Z Y X ((C D) (E F)) B A) LENGTH counts the number of top-level elements, REVERSE reverses the order of top-level elements

13. Build a list (a d) out of the list (a b c d). I way: Use the CONS primitive * (cons (first '(a b c d)) (last '(a b c d))) (A D) II way: Create a new procedure both-ends * (both-ends '(a b c d)) (A D) To create such a procedure, LISP does the following: 1. Reserves place in memory for the argument of both-ends. 2. Evaluates the argument and stores its value in the reserved place. 3. Evaluates the form (cons (first '(a b c d))(last '(a b c d))) comprising the body of the procedure both-ends, and returns the result. 4. The space reserved for the argument’s value is freed. User-defined procedures: the DEFUN primitive

14. (defun <procedure name> (<parameter list>) <form 1> <form 2> … <form n>) Example: * (defun both-ends (whole-list) (cons (first whole-list) (last whole-list))) BOTH-ENDS General form of the DEFUN primitive DEFUN does not evaluate its arguments, it only establishes the procedure definition.

15. * (setf whole-list '(a b c d)) (A B C D) * whole-list (A B C D) * (both-ends whole-list) (A D) * whole-list (A B C D) ; the value was not affected by both-ends which used the same atom as argument. Example (cont.)

16. Parameters in procedures are local variables. They are bound to argument value only inside the procedure. Values of global variables are set with the SETF primitive. Example: * (defun both-ends-global ( ) (setf whole-list (cons (first whole-list)(last whole-list)))) BOTH-ENDS-GLOBAL * whole-list (A B C D) * (both-ends-global) (A D) * whole-list (A D) LISP distinguishes between local (lexical) and special (global) variables

17. * (defun both-ends-two-parameters (x y) (cons (first x) (last y))) BOTH-ENDS-TWO-PARAMETERS * (setf x '(a b) y '(c d)) (C D) * (both-ends-two-parameters x y) (A D) Procedures may have any number of parameters

18. * (defun both-end-with-side-effect (x y) (setf side-effect-1 '(This is a side effect)) (setf side-effect-2 '(Another side effect)) (cons (first x) (last y))) BOTH-END-WITH-SIDE-EFFECT * side-effect-1 Unbound symbol: SIDE-EFFECT-1 * side-effect-2 Unbound symbol: SIDE-EFFECT-2 * (both-end-with-side-effect x y) (A D) * side-effect-1 (THIS IS A SIDE EFFECT) * side-effect-2 (ANOTHER SIDE EFFECT) Procedures may produce side effects