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List manipulation

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Title: Lecture 5-B (continuation of 5-A) Subject: Intro to LISP Last modified by: Neli Zlatareva Created Date: 2/18/1998 2:35:46 PM Document presentation format – PowerPoint PPT presentation

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Title: List manipulation

1
List manipulation

Consider student database, where each student is
represented by the following list
(setf student1 '((Paul Bennett) ((hw1 4.3) (hw2
5.0) (hw3 3.5) (hw4 4.8)
(hw5 4.9)) ((test1 9.5) (test2 8.7))
(classw 10.0) (project 18) (final 28)))
Given this representation, to find HW2 grade of
student1
(second (second (second student1))) gt
5.0
To add student major as a second element of
student list
(setf student1 (append (list (first student1))
(list '(CS major)) (rest student1)))
((PAUL BENNETT) (CS MAJOR) ((HW1 4.3) (HW2 5.0)
(HW3 3.5) (HW4 4.8)
(HW5 4.9)) ((TEST1 9.5) (TEST2 8.7)) (CLASSW
10.0) (PROJECT 18) (FINAL 28))
Now to get to student1 HW2 grade, a different
reader procedure must be used
(second (second (third student1)))

2
Better representation for student database
example.

Consider the following association lists for
representing students
((name (Paul Bennett))
((name (Abe Cadman))
(major CS)
(major CS)
(homeworks (4.3 5.0 3.5 4.8 4.9))
(status withdrawn))
(tests (9.5 8.7))
(claswork 10.0)
(project 18)
(final 28))
The following procedure will construct each of
these a-lists
(defun construct-student (name major hw1
optional hw2 hw3 hw4 hw5 test1 test2 classwork
project final)
(if (eql hw1 'withdrawn) (list (list 'name
name) (list 'major major) (list 'status
'withdrawn))
(list (list 'name
name) (list 'major major)
(list 'homeworks
(list hw1 hw2 hw3 hw4 hw5))
(list 'tests (list
test1 test2)) (list 'classwork classwork)
(list 'project
project) (list 'final final))))
To construct student1 and student2, we say

3
Example (cont.)

We can now create getter procedures to access
each one of the elements of
student1. For example, to access the grade for
HW2
(defun get-hw2 (student)
(if (eql (second (assoc 'status student))
'withdrawn)
'withdrawn
(second (second (assoc 'homeworks
student)))))
Once constructor and getter procedures are
defined, the programmer can forget
about details of the representation. If the
representation changes, only affected
procedures must be re-written.
To set-up the student database
(setf students (list
(construct-student '(Paul Bennett)
'CS 4.3 5.0 3.5 4.8 4.9 9.5 8.7 10.0 18 28)
(construct-student '(Abe Cadman) 'CS
'withdrawn)
(construct-student '(Nelson DaCunha)
'CS 4.8 4.0 4.5 3.8 5.0 8.5 9.7 10.0 17 25)
(construct-student '(Susan Melville)
'CS 3.8 5.0 4.7 4.8 5.0 8.3 9.9 10.0 20 24)

4
List transformation returns a list containing
only selected elements of the original list

Example Transform the students list into a list
containing only student names.
(defun names (students)
(if (endp students)
nil
(cons (get-name (first students))
(names (rest students)))))
(defun get-name (student)
(second (assoc 'name student)))
students
(((NAME (PAUL BENNETT)) (MAJOR CS) (HOMEWORKS
(4.3 5.0 3.5 4.8 4.9)) (TESTS (9.5 8.7))
(CLASSWORK 10.0) (PROJECT 18) (FINAL 28))
((NAME (ABE CADMAN)) (MAJOR CS) (STATUS
WITHDRAWN)) ((NAME (NELSON DACUNHA)) (MAJOR
CS) (HOMEWORKS (4.8 4.0 4.5 3.8 5.0))
(TESTS (8.5 9.7)) (CLASSWORK 10.0) (PROJECT
17) (FINAL 25)) ((NAME (SUSAN MELVILLE))
(MAJOR CS) (HOMEWORKS (3.8 5.0 4.7 4.8 5.0))
(TESTS (8.3 9.9)) (CLASSWORK 10.0)
(PROJECT 20) (FINAL 24)) ((NAME (IGOR PEVAC))
(MAJOR CS) (STATUS WITHDRAWN)))
(names students)
((PAUL BENNETT) (ABE CADMAN) (NELSON DACUNHA)
(SUSAN MELVILLE) (IGOR PEVAC))

5
List transformation procedures a general format

When transforming a list into another list, the
resulting list is of the same
length as the original list. The general format
of the transformation procedure
is the following
(defun lttransformation-procgt (list-1)
(if (endp list-1)
NIL
(cons (ltget-desired-element-proc
gt (first list-1))
(lttransformation-procgt
(rest list-1)))))

6
The MAPCAR primitive transforms lists

Mapcar has the following format
(mapcar ltprocedure objectgt ltlist-1gt
...ltlist-ngt ), where
ltprocedure objectgt supplies the name of the
transforming procedure,
ltlist-1gt,..., ltlist-ngt supply lists of elements
to be transformed.
Examples
(mapcar 'zerop '(8 5 0 1 0 5))
(NIL NIL T NIL T NIL)
(mapcar ' '(1 2 3 4 5) '(1 3 5 4 8))
(T NIL NIL T NIL)
(mapcar 'get-name students)
((PAUL BENNETT) (ABE CADMAN) (NELSON DACUNHA)
(SUSAN MELVILLE) (IGOR PEVAC))
(mapcar 'get-hw2 students)
(5.0 WITHDRAWN 4.0 5.0 WITHDRAWN)

7
Filtering undesired elements

Consider the list (5.0 WITHDRAWN 4.0 5.0
WITHDRAWN). To compute
the average grade, we must filter non-numerical
atoms.
(defun clean-grade-list (grade-list)
(cond ((endp grade-list) nil)
((numberp (first grade-list))
(cons (first grade-list)
(clean-grade-list (rest grade-list))))
(t (clean-grade-list (rest
grade-list)))))
(clean-grade-list (mapcar 'get-hw2 students))
(5.0 4.0 5.0)

8
Filtering procedures a general format

Procedures for filtering out elements that do not
satisfy the desired property
have the following general format
(defun ltfiltering-proceduregt (list-1)
(cond ((endp ltlist-1gt) nil)
((lttesting-for-desired-proper
ty-procgt (first list-1))
(cons (first list-1)
(filtering-proceduregt (rest list-1))))
(t (filtering-proceduregt
(rest list-1))))
The resulting list may contain the same or
smaller number of elements
than the original list.

9
The REMOVE-IF and REMOVE-IF-NOT primitives
simplify filtering procedures

Remove-if removes all elements of list, which
satisfy the predicate serving as a filter. Its
general format is the following
(remove-if ltprocedure objectgt ltlistgt)
Remove-if-not removes all elements of list, which
do not satisfy the predicate serving as a filter.
Its general format is the following
(remove-if-not ltprocedure objectgt ltlistgt)
where
ltprocedure objectgt supplies the name of the
filtering procedure,
ltlistgt is the list of elements to be filtered.

10
Examples

To filter all symbols (non-numerical atoms) from
the grades list
(remove-if symbolp (mapcar 'get-hw2
students))
(5.0 4.0 5.0)
Or, also we can say
(remove-if-not numberp (mapcar 'get-hw2
students))
(5.0 4.0 5.0)
Filter zeros from a given list of numbers
(remove-if 'zerop '(2 0 4 6 0 0))
(2 4 6)
Filter non-even elements of a given list of
numbers
(remove-if-not 'evenp '(3 4 5 6 7 8))
(4 6 8)

11
Mapping primitives can take as a procedure object
an already defined function or a lambda expression

Lambda expressions are anonymous functions.
Example compute the square of m
'(lambda (n) ( n n))
ltLISPSCANNED (LAMBDA (N) (DECLARE) ( N N))gt
(setf square '(lambda (n) ( n n)))
ltLISPSCANNED (LAMBDA (N) (DECLARE) ( N N))gt
square
ltLISPSCANNED (LAMBDA (N) (DECLARE) ( N N))gt
(mapcar square '(1 2 3 4 5))
(1 4 9 16 25)
(mapcar '(lambda (n) ( n n)) '(1 2 3 4 5))
(1 4 9 16 25)
Lambda expressions make it possible to create new
functions at run time. Such
run-time functions are called closures.

12
Counting list elements that satisfy a desired
property

Consider the student DB example, and assume that
we want to count students
that have withdrawn from the class. The following
function will do the job
(defun count-w (students)
(cond ((endp students) 0)
((eql (second (assoc 'status (first
students))) 'withdrawn)
( 1 (count-w (rest
students))))
(t (count-w (rest students)))))
The general format of any counting procedure is
(defun ltcounting procgt (list-1)
(cond ((endp list-1) 0)
((lttesting-desired-prop procgt (first
list-1))
( 1 (ltcounting
procgt (rest list-1))))
(t (ltcounting procgt (rest
list-1)))))

13
The COUNT-IF and COUNT-IF-NOT primitives

To count the number of students that have
withdrawn, we can also say
(count-if '(lambda (student)
(eql (second
(assoc 'status student)) 'withdrawn)) students)
2
Or, if we have defined predicate get-w
(defun get-w (student)
(eql (second (assoc 'status student))
'withdrawn))
the equivalent query is
(count-if 'get-w students)
2
To count the number of students that have not
withdrawn
(count-if-not 'get-w students)

14
COUNT-IF counts the number of elements on a list
that satisfy a given property EVERY / SOME test
if every / some element on the list satisfies a
given property

Examples
(count-if 'oddp '(1 2 3 4 5))
3
(every 'oddp '(1 2 3 4 5))
NIL
(some 'oddp '(1 2 3 4 5))
T

15
Searching for an element that satisfies a desired
property

Assume we want to search for a student who have
20 points on the
project. The following functions will do the
job
(defun search-project-20 (students)
(cond ((endp students) nil)
((eql 20 (get-project (first
students)))
(second (assoc 'name
(first students))))
(t (search-project-20 (rest
students)))))
(defun get-project (student)
(second (assoc 'project student)))
(search-project-20 students)
(SUSAN MELVILLE)

16
General format of a searching procedure and its
substitute primitives FIND-IF and FIND-IF-NOT

(defun ltsearching proceduregt (list-1)
(cond ((endp list-1) nil)
((lttesting-desired-prop procgt (first
list-1))
(first list-1))
(t (ltsearching proceduregt (rest
list-1)))))
Find-if and find-if-not are primitives that
search for the first element of a list
satisfying (not satisfying) a desired property.
In the student DB example, to ask
if at least one student has (has not) 20 points
on the project, we can say
(second (assoc 'name (find-if '(lambda
(student)
(eql 20 (get-project
student))) students)) )
(SUSAN MELVILLE)
(second (assoc 'name (find-if-not '(lambda
(student)
(eql 20 (get-project
student))) students)))
(PAUL BENNETT)