66-2210-01 Programming in Lisp

1
66-2210-01 Programming in Lisp

• Data Abstraction and Mapping

2
Data Representation

• Record course information
• Basic definition
• (setf course-1 '(66221001 course number
• (Programming in Lisp) course name
• (Alok Mehta) instructor
• (Computer Science))) department
• What is the name of course-1?
• gt (second course-1)
• (Programming in Lisp)
• Poor practice - Depends on the ordering of the
  list
• What if order changes, or, you need to add
  something to the list
• (setf course-1 '(66221001
• 1 Credit hours
• (Programming in Lisp) Course name
• (Alok Mehta) Instructor
• (Computer Science))) Department
• All programs that access name of course-1 have to
  change
• gt (third course-1)

3
Association Lists

• How do you know 1 represents credit hours?
• Better way to represent data Association Lists
• (setf course-1 '(
• (course-number 66221001)
• (credit-hours 1)
• (name (Programming in Lisp))
• (instructor (Alok Mehta))
• (department (Computer Science))))
• What is the name of course-1?
• gt (second (third course-1))
• (PROGRAMMING IN LISP)
• gt (assoc 'name course-1)
• (NAME (PROGRAMMING IN LISP))
• gt (second (assoc 'name course-1)
• (PROGRAMMING IN LISP)

4
Simple Database

• A database as a list of records
• (setf course-database '(
• ((course-number 66221001)
• (credit-hours 1)
• (name (Programming in Lisp))
• (instructor (Alok Mehta))
• (department (Computer Science)))
• ((course-number 66220001)
• (credit-hours 1)
• (name (Programming in C))
• (instructor (Louis Ziantz)))
• ))
• What is the name?
• (second (assoc 'name (first course-database)))

5
Access Functions

• Write functions to access low-level details
• (defun get-course-name (course)
• (second (assoc 'name course)))
• Dont have to remember how data is stored
• Can change storage details easily
• Access to data is achieved at a higher level
• Easier to read and understand
• More maintainable code
• Write functions to construct and manipulate data
• (defun make-course (key course-number
  credit-hours
• name instructor department)
• (list (list 'course-number course-number)
• (list 'credit-hours credit-hours)
• (list 'name name)
• (list 'instructor instructor)
• (list 'department department)
• ))

6
Calling Access Functions

• gt (setf course-1 (make-course
• name '(Programming in Java)
• instructor (Alok Mehta)))
• ((COURSE-NUMBER NIL)
• (CREDIT-HOURS NIL)
• (NAME (PROGRAMMING IN JAVA))
• (INSTRUCTOR (ALOK MEHTA))
• (DEPARTMENT NIL))
• gt (get-course-name course-1)
• (PROGRAMMING IN JAVA)

7
A simple database example

• (setf courses (list
• (make-course course-number '66220001
• name '(Programming in C)
• instructor '(Louis Ziantz))
• (make-course course-number '66221001
• name '(Programming in Lisp)
• instructor '(Alok Mehta))
• (make-course course-number '66222001
• name '(Programming in Java)
• instructor '(Alok Mehta))
• (make-course course-number '66223001
• name '(Programming in Perl)
• instructor '(Louis Ziantz))
• ))

8
Which courses are offered?

• Example Usage
• gt (get-course-numbers courses)
• (66220001 66221001 66222001 66223001)
• (defun get-course-number (course)
• (second (assoc 'course-number course)))
• (defun get-course-numbers (course-list)
• (if (endp course-list)
• NIL
• (cons (get-course-number (first
  course-list))
• (get-course-numbers (rest
  course-list)))))
• Implements a Transformation
• SQL Equivalent
• SELECT course_number
• FROM courses

9
Which are taught by Alok?

• List the courses taught by Alok Mehta
• gt (get-courses-taught-by-alok courses)
• (((COURSE-NUMBER 66221001) ) ((COURSE-NUMBER
  66222001) ...) ...)
• (defun get-course-instructor (course)
• (second (assoc 'instructor course)))
• (defun taught-by-alok-p (course)
• (if (equal '(Alok Mehta) (get-course-instructor
  course))
• t
• NIL))
• (defun get-courses-taught-by-alok (course-list)
• (cond
• ((endp course-list) NIL)
• ((taught-by-alok-p (first course-list))
• (cons (first course-list)
• (get-courses-taught-by-alok (rest
  course-list))))
• (t (get-courses-taught-by-alok (rest
  course-list)))))

10
How many are taught by Alok

• List the courses taught by Alok Mehta Filter
• gt (get-courses-taught-by-alok courses)
• Implements Filtering
• SQL SELECT FROM COURSES
• WHERE instructor Alok Mehta
• How many courses are taught by Alok? Count
• gt (length (get-courses-taught-by-alok courses))
• 2
• SQL SELECT COUNT() FROM COURSES
• WHERE instructor Alok Mehta
• What is the first course taught by Alok? Find
• gt (first (get-courses-taught-by-alok courses))
• ((COURSE-NUMBER 66221001)
• (CREDIT-HOURS NIL)
• (NAME (PROGRAMMING IN LISP))
• (INSTRUCTOR (ALOK MEHTA))
• (DEPARTMENT NIL))

11
More efficient way to Count

• A more efficient way to count
• Dont have to get all courses first!
• (defun count-courses-taught-by-alok (course-list)
• (cond ((endp course-list) 0)
• ((taught-by-alok-p (first course-list))
• ( 1 (count-courses-taught-by-alok
• (rest course-list))))
• (t (count-courses-taught-by-alok
• (rest course-list)))))
• A more efficient way to find first course
• (defun find-first-course-taught-by-alok
  (course-list)
• (cond ((endp course-list) nil)
• ((taught-by-alok-p (first course-list))
• (first course-list))
• (t (find-first-course-taught-by-alok
• (rest course-list)))))

12
Cliches

• Many procedures have a common template
• (defun ltlist-transformergt (input-list)
• (if (endp input-list)
• nil
• (cons (ltelement-transformergt (first
  input-list))
• (ltlist-transformergt (rest
  input-list)))))
• Example
• List-transformer get-course-numbers
• Element-transformer get-course-number
• (defun get-course-numbers (course-list)
• (if (endp course-list)
• NIL
• (cons (get-course-number (first
  course-list))
• (get-course-numbers (rest
  course-list)))))
• This template is frequently used
• Lisp has a primitive (MAPCAR) to make this easier

13
Mapcar

• MAPCAR
• (mapcar ltproceduregt ltargumentgt)
• Applies ltproceduregt to each element of ltargumentgt
• Example
• gt (mapcar 'oddp '(1 2 3))
• (T NIL T)
• gt (mapcar 'get-course-number courses)
• (66220001 66221001 66222001 66223001)
• gt (mapcar ' '(1 2 3) '(3 2 1))
• (NIL T NIL)

14
Remove-If, Remove-If-Not

• Remove-If, Remove-If-Not
• gt (remove-if-not 'taught-by-alok-p courses)
• Removes all courses for which taught-by-alok-p
  returns NIL
• This will return all courses taught by Alok
• gt (remove-if 'taught-by-alok-p courses)
• Removes all courses for which taught-by-alok-p
  returns non-NIL
• This will return all courses NOT taught by Alok
• Count-If, Count-If-Not
• gt (count-if 'taught-by-alok-p courses)
• Counts the number of elements for which
  taught-by-alok-p returns non-NIL
• Find-If, Find-If-Not
• gt (find-if 'taught-by-alok-p courses)
• Returns the FIRST element of courses for which
  taught-by-alok-p returns non-NIL

15
Funcall

• Funcall
• Template
• (funcall 'ltproceduregt ltarg1gt ltargNgt)
• Calls the specified procedure with the given
  argments
• Equivalent to
• (ltproceduregt ltarg1gt ltargNgt)
• Example
• gt (funcall ' 3 2 7) Same as ( 3 2 7)
• 12
• Example 2
• gt (defun eval-infix (arg1 operator arg2)
• (funcall operator arg1 arg2))
• gt (eval-infix 3 ' 2)
• Useful if you need to pass in a procedure name,
  to be applied at a future time

16
Apply

• Apply
• Similar to Funcall
• (typically) takes two arguments,
• procedure object
• list of arguments to be passed to procedure
  object
• (funcall ' 3 2 7) Same as ( 3 2 7)
• (apply ' '(3 2 7)) Same as ( 3 2 7)
• Apply can actually take additional arguments
• Extra arguments are combined into a single list
• Following are equivalent
• (apply ' '(1 2 3 4 5 6))
• (apply ' 1 2 3 '(4 5 6))
• Argument fed to is (append (list 1 2 3) (4 5
  6))

17
Lambda Procedures

• Useful if you need an anonymous procedure
• Dont want to give it a name
• A named procedure
• (defun taught-by-alok-p (course)
• (if (equal '(Alok Mehta) (get-course-instructor
  course))
• t
• NIL))
• An equivalent un-named procedure
• (lambda (course)
• (if (equal '(Alok Mehta) (get-course-instructor
  course))
• t
• NIL))
• Example usage How many courses are taught by
  Alok?
• (count-if '(lambda (course)
• (if (equal '(Alok Mehta)
• (get-course-instructor course))
• t
• NIL))
• courses)

18
Lambda procedures

• Advantage
• Dont have to make up names for procedures
• Definition of procedure is close to where it is
  used
• (defun get-courses-taught-by-alok (course-list)
• (remove-if-not
• '(lambda (x)
• (equal '(Alok Mehta) (get-course-instructor
  x)))
• course-list))

19
Summary

• Use readers, constructors, and writers
• To hide data details
• Transform, Filter, Count, Find are common
• Can be defined recursively
• Built-in functions in Lisp make things easier
• Mapcar, Remove-If(Not), Count-If(Not),
  Find-If(Not)
• Funcall, Apply
• Lambda

20
Iteration

• DOTIMES (Review)
• (dotimes (ltcountergt ltupper-boundgt ltfinal-resultgt)
• ltbodygt)
• Example
• (dotimes (i 5) (print i)) prints 0 1 2 3 4
• DOLIST
• (dolist (ltelementgt ltlist-of-elementsgt
  ltfinal-resultgt)
• ltbodygt)
• Example
• (dolist (elem '(a b c d)) (print elem)) prints
  a b c d

21
Example of DOLIST

• Given a list of ages of people, how many adults?
• List of ages
• gt (setf ages '(3 4 17 21 22 34 2 7))
• Adult defined as gt 21 years old
• gt (defun adultp (age) (gt age 21))
• Using Count-if
• (defun count-adult (ages) (count-if 'adultp
  ages))
• Using dolist
• (defun count-adult (ages aux (nadult 0))
• (dolist (age ages nadult)
• (if (adultp age) (setf nadult ( 1
  nadult)))))

22
Example (cont.)

• Get the ages of the first two adults
• (defun first-two-adults (ages aux (nadult 0)
  (adults nil))
• (dolist (age ages)
• (if (adultp age)
• (progn (setf nadult ( nadult 1))
• (push age adults)
• (if ( nadult 2) (return
  adults))))))
• Notes
• PROGN (and PROG1) are like C/C Blocks
• gt (prog1 (setf a 'x) (setf b 'y) (setf c 'z))
• X
• gt (progn (setf a 'x) (setf b 'y) (setf c 'z))
• Z
• RETURN exits the DOLIST block
• Note does not necessarily return from the
  procedure!
• Takes an optional return value

23
DO

• DO is more general than DOLIST or DOTIMES
• Example
• (defun do-expt (m n) Return MN
• (do ((result 1) Bind variable Result
  to 1
• (exponent n)) Bind variable Exponent
  to N
• ((zerop exponent) result) test and
  return value
• (setf result ( m result)) Body
• (setf exponent (- exponent 1)) Body
• Equivalent C/C definition
• int do_expt (int m, int n)
• int result, exponent
• for (result1,exponentn (exponent ! 0) )
• result m result
• exponent exponent - 1
• return result

24
DO Template

• Full DO Template (There is also a DO)
• (DO ( (ltp1gt lti1gt ltu1gt)
• (ltp2gt lti2gt ltu2gt)
• (ltpNgt ltiNgt ltuNgt) )
• ( ltterm-testgt
• lta1gt lta2gt ltaNgt
• ltresultgt )
• ltbodygt )
• Rough equivalent in C/C
• for ( ltp1gtlti1gt, ltp2gtlti2gt,,ltpNgtltiNgt //Note
  Lispparallel
• !(ltterm-testgt) // C/C has a
  continuation-test
• ltp1gtltu1gt, ltp2gtltu2gt,,ltpNgtltuNgt)
• ltbodygt
• lta1gt lta2gt ltaNgt
• ltresultgt // Note (DO) in Lisp evaluates to
  ltresultgt
• // Note ltp1gt,ltp2gt,,ltpNgt are now restored to
  original values

25
Do-expt, Loop

• Here is another (equivalent) definition of
  do-expt
• (defun do-expt (m n)
• (do ((result 1 ( m result))
• (exponent n (- exponent 1)))
• ((zerop exponent) result)
• Note that there is no body!
• ))
• Loop
• An infinite loop, terminated only by a (return)
• (loop
• (print '(Say uncle))
• (if (equal (read) 'uncle) (return)))