Transcript Lecture 5

Procedures with optional parameters which do
not require matching arguments
Example: consider the exponent function which may take one argument, m, in
which case it returns the square of m, or two arguments, m and n, in which
case it returns n power of m.
* (defun exponent (m &optional n)
optional parameter.
(if n
; if the value for an optional parameter is
(if (zerop n)
; not provided, it is assumed NIL.
1
(* m (exponent m (- n 1))))
(* m m)))
EXPONENT
* (exponent 2)
4
* (exponent 2 3)
; here n is an
Optional parameters may be given default
values, if we do not want their default value to
be NIL.
Example: the exponent procedure
* (defun exponent (m &optional (n 2))
(if (zerop n)
1
(* m (exponent m (- n 1)))))
EXPONENT
* (exponent 2)
4
* (exponent 2 3)
8
Optional parameters eliminate the need for
auxiliary procedures, because the first call uses
the default value, while all other recursive calls
ignore it
* (trace exponent)
(EXPONENT)
* (exponent 4)
| 1 Entering: EXPONENT, argument-list: (4)
|
2 Entering: EXPONENT, argument-list: (4 1)
|
| 3 Entering: EXPONENT, argument-list: (4 0)
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| 3 Exiting: EXPONENT, value 1
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2 Exiting: EXPONENT, value 4
| 1 Exiting: EXPONENT, value 16
16
Example
Consider math-quiz, a function which posts n math problems to the user
(who can set n as she wants) tutoring her in arithmetic (+, -, * or / is to be
set by the user, as well as the range of numbers to be exercised).
(defun math-quiz (&optional (op '+) (range 100) (n 3))
(dotimes (i n)
(exercise (random range) op (random range))))
(defun exercise (num1 op num2)
(format t "~%The expression is ")
(format t "~a ~a ~a" num1 op num2)
(format t "~% ... and the answer is ...")
(print (eval (list op num1 num2))))
* (math-quiz)
;all parameters are optional, no argument is required.
The expression is 85 + 97
... and the answer is ...
182
The expression is 25 + 76
... and the answer is ...
101
The expression is 47 + 60
... and the answer is ...
107
Example (cont.)
Optional parameters are position-dependent. To provide a non-default
value for n, we must specify values for all optional parameters that come
before n (even if they are their default values).
* (math-quiz '+ 100 5)
; ”+” and “100” are the default values for the first two
arguments
The expression is 61 + 46 ; they must be provided if we want to change the default value
... and the answer is ...
; of the last argument.
107
The expression is 3 + 69
... and the answer is ...
72
The expression is 34 + 76
... and the answer is ...
110
The expression is 61 + 49
... and the answer is ...
110
The expression is 3 + 78
... and the answer is ...
81
Keyword parameters.
Keyword parameters are similar to optional parameters, except that they are
position-independent because bindings are determined by keywords not by
order.
Example: the math-quiz function.
(defun math-quiz2 (&key (op '+) (range 100) (n 3))
(dotimes (i n)
(exercise (random range) op (random range))))
(defun exercise (num1 op num2)
(format t "~%The expression is ")
(format t "~a ~a ~a" num1 op num2)
(format t "~% ... and the answer is ...")
(print (eval (list op num1 num2))))
* (math-quiz2 :n 2)
The expression is 44 + 40
... and the answer is ...
84
The expression is 84 + 43
... and the answer is ...
127
The rest parameter.
This takes as its value the list of all arguments that have been unaccounted for.
Example: consider function sum-two which takes only two arguments and adds them.
(defun sum-two (number &rest numbers)
(sum-two-aux number numbers))
(defun sum-two-aux (sum list-of-numbers)
(if (endp list-of-numbers)
sum
(sum-two-aux (+ sum (first list-of-numbers))
(rest list-of-numbers))))
* (trace sum-two sum-two-aux)
(SUM-TWO-AUX SUM-TWO)
* (sum-two 2 4 6 3 7)
| 1 Entering: SUM-TWO, argument-list: (2 4 6 3 7)
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2 Entering: SUM-TWO-AUX, argument-list: (2 (4 6 3 7))
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| 3 Entering: SUM-TWO-AUX, argument-list: (6 (6 3 7))
|
|
4 Entering: SUM-TWO-AUX, argument-list: (12 (3 7))
|
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| 5 Entering: SUM-TWO-AUX, argument-list: (15 (7))
|
|
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6 Entering: SUM-TWO-AUX, argument-list: (22 NIL)
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|
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6 Exiting: SUM-TWO-AUX, value 22
|
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| 5 Exiting: SUM-TWO-AUX, value 22
.....
22
The aux parameter
This is not matched to any argument, because it is intended to define auxiliary
local variables similar to let*.
Example: the both-ends procedure.
* (setf whole-list '(a b c d)) ; the value of whole-list must be provided before
(A B C D)
; function definition, otherwise an error will occur
* (defun both-ends-new (whole-list &aux (first-el (first whole-list))
(last-list (last whole-list)))
(cons first-el last-list))
* (trace both-ends-new)
(BOTH-ENDS-NEW)
* (both-ends-new whole-list)
| 1 Entering: BOTH-ENDS-NEW, argument-list: ((A B C D))
| 1 Exiting: BOTH-ENDS-NEW, value (A D)
(A D)
Various parameters can be combined in the following order: optional
parameters come first after regular parameters, next are sole rest
parameters followed by key parameters, followed by aux parameters.
Representing structures in Lisp
We can create user-defined data types to represent any data type in Lisp.
These are called structure types, and they come with automatically created
access procedures. defstruct is the primitive that creates new structure types.
(defstruct <structure name>
(<field name 1> <default value 1>)
(<field name 2> <default value 2>)
...
(<field name n> <default value n>))
Example:
(defstruct person
(sex nil)
(personality 'nice))
Lisp structures (cont.)
Structures are implemented as vectors, where the type (structure name) is the
zero element, field 1 is the first element, … field n is the n-th element. This means
that structures are more efficient than lists, because each element can be
accessed in a single step.
Notes: 1. ) defstruct defines a new data type.
2.) defstruct does not create instances of that data type, but automatically
creates a data-constructor procedure, data-reader procedures (a
separate one for each field), data type predicate, as well as
generalizes the setf primitive to handle the new data type.
Example:
(setf person-instance-1 (make-person)) ; make-person with no parameters creates a
;new instance of type person with fields filled with default values in defstruct
(setf person-instance-2 (make-person :sex 'female))
;providing a new value for keyword :sex.
; default value changes by
Lisp structures (cont.)
To read data from created instances:
* (person-sex person-instance-2)
FEMALE
* (person-personality person-instance-1)
NICE
Because data is accessed by an automatically generated reader procedure,
it is said to be procedurally indexed.
To change the values of existing fields:
* (setf (person-sex person-instance-1) 'female)
* (person-sex person-instance-1)
FEMALE
Lisp structures (cont.)
defstruct also creates a data type predicate:
* (person-p person-instance-1)
T
* (person-p '(a b c))
NIL
Example: Define a data type rock that contains fields for color, size and
worth. Assume that the default color is gray, default size – pebble, default
worth – nothing.
(defstruct rock
(color 'gray)
(size 'pebble)
(worth 'nothing))
(setf high-hopes-rock (make-rock :color 'gold :worth 'high))
Lisp structures (cont.)
A big advantage of structure types is that one structure can include fields of
another structure, thus forming a representational hierarchy.
Example: Consider structure employee
employee
salesperson
hacker
…
* (defstruct employee
(length-of-service 0)
(payment 'salary))
EMPLOYEE
* (defstruct (hacker (:include employee))
(preferred-language 'lisp))
HACKER
* (setf employee-example (make-employee))
#S(EMPLOYEE :LENGTH-OF-SERVICE 0 :PAYMENT SALARY)
* (setf hacker-example (make-hacker))
#S(HACKER :LENGTH-OF-SERVICE 0 :PAYMENT SALARY :PREFERRED-LANGUAGE LISP)
* (employee-length-of-service employee-example)
0
* (employee-length-of-service hacker-example)
0
One structure can include another with one or more of the fields repeated in both. In such
case, the default value in more specialized structure shadows the default value in more
general structure.
* (setf employee-example (make-employee))
#S(EMPLOYEE :LENGTH-OF-SERVICE 0 :PAYMENT SALARY)
* (defstruct (salesperson (:include employee (payment 'commission)))
(preferred-car 'mercedes))
SALESPERSON
* (setf salesperson-example (make-salesperson))
#S(SALESPERSON :LENGTH-OF-SERVICE 0 :PAYMENT COMMISSION :PREFERRED-CAR MERCEDES)
* (employee-payment hacker-example)
SALARY
* (employee-payment salesperson-example)
COMMISSION
To print the contents of an instance of a structure:
* (describe hacker-example)
#S(HACKER :LENGTH-OF-SERVICE 0 :PAYMENT SALARY :PREFERRED-LANGUAGE LISP) is a named
structure of type HACKER.
It has as an included structure EMPLOYEE.
Its slot names and values are:
LENGTH-OF-SERVICE - 0
PAYMENT - SALARY
PREFERRED-LANGUAGE - LISP
To print the structure itself, however, a special printing procedure must be defined and
included in the structure’s definition.
Example:
* (defstruct (employee2 (:print-function print-employee2))
(name 'anna)
(ss# 'unknown)
(length-of-service 5)
(payment 'salary))
EMPLOYEE2
* (defun print-employee2 (structure &rest ignore)
(format t "structure for ~a with ss# ~a"
(employee2-name structure)
(employee2-ss# structure)))
PRINT-EMPLOYEE2
* (setf employee-anna (make-employee2))
structure for ANNA with ss# UNKNOWN
* (describe employee-anna)
structure for ANNA with ss# UNKNOWN is a named structure of type EMPLOYEE2.
Its slot names and values are:
NAME - ANNA
SS# - UNKNOWN
LENGTH-OF-SERVICE - 5
PAYMENT - SALARY
Representing tables as association lists: the
ASSOC and RASSOC primitives.
Consider a two-column table, where the first column contains properties of
a given object, and the second column contains the values of these
properties. Such object descriptions can be represented by expressions
called association lists (or a-lists). These have two different formats:
Format 1: ((key-1 value-1) (key-2 value-2) .... (key-n value-n))
Format 2: ((key-1 . value-1) (key-2 . value-2) .... (key-n . value-n))
The assoc primitive searches a-lists by key. Its format is the following:
(assoc <key> <a-list>)
The rassoc primitive searches a-lists by value, but it works only on a-lists of
dotted pairs. Its format is the following:
(rassoc <value> <a-list>)
Example: a table of days of the week and their
average temperatures.
* (setf week-7-1 '((Mon 28) (Tue 32) (Wed 37)
(Th 31) (Fri 33) (Sat 26) (Sun 29)))
((MON 28) (TUE 32) (WED 37) (TH 31) (FRI
33) (SAT 26) (SUN 29))
* (setf week-7-2 '((Mon . 28) (Tue . 32) (Wed .
37) (Th . 31) (Fri . 33) (Sat . 26) (Sun . 29)))
((MON . 28) (TUE . 32) (WED . 37) (TH . 31)
(FRI . 33) (SAT . 26) (SUN . 29))
* (assoc 'fri week-7-1)
(FRI 33)
* (assoc 'fri week-7-2)
(FRI . 33)
* (second (assoc 'fri week-7-1))
33
* (rest (assoc 'fri week-7-2))
33
* (rassoc 26 week-7-1)
NIL
* (rassoc 26 week-7-2)
(SAT . 26)
* (first (rassoc 26 week-7-2))
SAT
Another example on a-list of dotted pairs:
* (setf state-table '((al . alabama) (az . arizona)))
((AL . ALABAMA) (AZ . ARIZONA))
* (assoc 'al state-table)
(AL . ALABAMA)
* (rest (assoc 'al state-table))
ALABAMA
* (assoc 'ct state-table)
NIL
* (rassoc 'alabama state-table)
(AL . ALABAMA)
* (first (rassoc 'alabama state-table))
AL
Representing tables as property lists: the GET
primitive.
There are two types of values that can be assigned to symbols:
– Ordinary values. Example: (setf number 5)
– Property values. These are placed together in a list, called property list
(or p-list) and are “attached” to the symbol (i.e. can be accessed only
through the symbol itself). Example: consider symbol day-1 with the
following properties: avg-temp, sun-rise, and sun-set:
* (setf (get 'day-1 'avg-temp) 37)
used in combination
37
; to get to and set a value for a property
* (setf (get 'day-1 'sun-rise) '6h45m)
6H45M
* (setf (get 'day-1 'sun-set) '17h10m)
17H10M
* (get 'day-1 'sun-rise)
a value has been assigned to
6H45M
;setf and get are
; of day-1
; after
;
The DESCRIBE and REMPROP primitives
The describe primitive can be used to see the contents of the p-list:
* (describe 'day-1)
DAY-1 is an internal symbol in package USER.
Its value is unbound.
Its function definition is unbound.
Its property list contains:
Property: SUN-SET, Value: 17H10M
Property: SUN-RISE, Value: 6H45M
Property: AVG-TEMP, Value: 37
The remprop primitive removes a property from the p-list:
* (remprop 'day-1 'avg-temp)
T
* (describe 'day-1)
DAY-1 is an internal symbol in package USER.
Its value is unbound.
Its function definition is unbound.
Its property list contains:
Property: SUN-SET, Value: 17H10M
Property: SUN-RISE, Value: 6H45M
Representing tables as arrays
Arrays are represented in the same way as in JAVA, and their indexes start
with 0.To declare an array, a combination of SETF and MAKE-ARRAY
primitives is used as follows:
* (setf array-1 (make-array 10))
; the first array element is in position 0, the last in
#(0 0 0 0 0 0 0 0 0 0)
; position 9
* (setf array-2 (make-array '(5 2))) ; the first array element is in position 0, 0
#2A((0 0) (0 0) (0 0) (0 0) (0 0))
* (setf array-3 (make-array '(2 5 3)))
#3A(((0 0 0) (0 0 0) (0 0 0) (0 0 0) (0 0 0)) ((0 0 0) (0 0 0) (0 0 0) (0 0 0) (0 0 0)))
To initialize an array at the same time:
* (setf array-1 (make-array 10 :initial-contents '(1 2 3 4 5 6 7 8 9 10)))
#(1 2 3 4 5 6 7 8 9 10)
* (setf array-2 (make-array '(5 2) :initial-contents '((1 2) (3 4) (5 6) (7 8) (9 10))))
#2A((1 2) (3 4) (5 6) (7 8) (9 10))
The size of array dimension can be determined by the ARRAY-DIMENSION
primitive:
* (array-dimension array-2 0)
5
Retrieving and changing array elements
To retrieve an item from an array, we use the AREF primitive as follows:
* array-1
#(1 2 3 4 5 6 7 8 9 10)
* (aref array-1 5)
; retrieves the 6-th item
6
* array-2
#2A((1 2) (3 4) (5 6) (7 8) (9 10))
* (aref array-2 2 1)
; retrieves the item in the 3-rd raw, 2-nd column
6
To change the value stored in a specified position, we use a combination of
SETF and AREF primitives:
* (setf (aref array-1 5) 33)
33
* array-1
#(1 2 3 4 5 33 7 8 9 10)
* (setf (aref array-2 2 1) 66)
66
* array-2
#2A((1 2) (3 4) (5 66) (7 8) (9 10))
Hash tables: yet another way to represent tables
Primitives on hash tables:
* (setf table (make-hash-table))
#<HASH-TABLE 0/37 9B:19F6>
* (setf (gethash 'al table) 'alabama)
ALABAMA
* (gethash 'al table)
ALABAMA
T
* (gethash 'ct table)
NIL
NIL
* (remhash 'al table)
T
* (gethash 'al table)
NIL
NIL
* (clrhash table)
#<HASH-TABLE 0/37 9B:19F6>
; initializes the table
; modifies the table
; retrieves values
; removes a key-value pair from the table
; removes all key-value pairs from the table