LISP primitives on sequences

1
LISP primitives on sequences

• 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 B C)
2
Examples

• (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

3
The QUOTE mark ' stops evaluation

• (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
• gtgtgt ErrorUndefined function A
• while evaluating (A B C)
• (ltCOMPILED-FUNCTION 3E196gt ...)
• Debugger 1gt

4
Examples

• (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

5
The SETF / SETQ primitives

• 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

6
SETF alters the contents of the cons cell storing
the symbol.

• (setf fact1 '(CS462 is a fun course))
• (CS462 IS A FUN COURSE)
• fact1
• (CS462 IS A FUN COURSE)

fact1
7

• (first fact1)
• CS462
• fact1
• (CS462 IS A FUN COURSE)
• (setf (first fact1) 'CS463)

fact1 (CS463 IS A FUN COURSE)
8
CONS, APPEND and LIST primitives construct lists

• (cons 'a '(b c))
• (A B C)

(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, NTHCDR, BUTLAST and LAST shorten lists

• (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))

11
More examples

• 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)

12
LENGTH counts the number of top-level elements,
REVERSE reverses the order of top-level elements

• (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)

13
User-defined procedures the DEFUN primitive

• 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 arguments value is
  freed.

14
General form of the DEFUN primitive

• (defun ltprocedure namegt (ltparameter listgt)
• ltform 1gt ltform 2gt
• ltform ngt)
• Example
• (defun both-ends (whole-list)
• (cons (first whole-list)
• (last whole-list)))
• BOTH-ENDS

DEFUN does not evaluate its arguments, it only
establishes the procedure definition.
15
Example (cont.)

• (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.

16
LISP distinguishes between local (lexical) and
special (global) variables

• 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)

17
Procedures may have any number of parameters

• (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)

18
Procedures may produce side effects

• (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)