Imperative programming
TDDC74 Programming:
Abstraction and Modelling
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Repetitionsföreläsning 2
Tilldelning - Imperativ programmering
Objektorientering
Procedurobjekt
Omgivningsdiagram
Pekare i cons-celler
Anders Haraldsson
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Computation as statements that change a program
state
Imperative programs are a sequence of commands
for the computer to perform
A functional program evaluates expression. An
expression has a value.
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set!
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Iterative version of factorial
set! is used for changing the value bound to a name
Syntax: (set! <variable> <value>)
Examples:
(set! flag #f)
(set! x (+ x 1))
(set! fac-value (fac 10))
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Bank-account example with an object oriented
view.
Object-oriented programming
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Objects
–
–
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(define (factorial n)
(let ((res 1))
(define (loop)
(if (= n 0)
res
(begin
(set! res (* n res))
(set! n (- n 1))
(loop))))
(loop)))
Attributes/properties
Behaviour/Methods
Class
Inheritance
Instance
Message passing
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(define (make-account balance)
(define (withdraw amount)
(if (>= balance amount)
(begin (set! balance (- balance amount))
(display balance))
(display “Insufficient funds”)))
(define (deposit amount)
(set! balance (+ balance amount)) balance)
(define (dispatch m)
(cond ((eq? m ‘withdraw) withdraw)
((eq? m ‘deposit) deposit)
(else (error “Unknown request” m))))
dispatch)
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Bank-account example
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(define my-account (make-account 1000))
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((my-account ‘withdraw) 100)
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((my-account ‘deposit) 300)
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DUGGA 3
Paketering med den objektorienterade modellen.
Förstå inkapsling, dispatch-funktionen.
Åtkomst av lokala data och procedurer.
Förstå kod. Göra tillägg eller ändringar i denna.
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How to Explain What Progams Do
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Definition: Binding
In order to explain what programs do, we need to
make the states explicit
We use a so called environment model of
computation
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A binding refers to association of a value to a name,
for example,
x:10, the value of x is 10
f:<procedure>, the value of f is a procedure
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Definition: Binding and frame
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Definition: The Global Environment
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A binding refers to association of a value to a name,
for example,
x:10, the value of x is 10
f:<procedure>, the value of f is a procedure
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A frame is simply a set of bindings
The set could be empty
Graphical representation:
A so called global environment (GE) is distinguished
from other environments and contains the global
state.
GE contains only one frame; this frame is the final
frame of all other environemts as well
Graphical representation:
GE
x: 10
y: 20
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Lots of Environments
Definition: Procedure
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GE
pi: 3.1415
A procedure is an object that contains two parts:
–
–
E1
x: 42
y: #f
a: 33
y: 20
x: 10
y: 20
E4
x: 30
z: 20
a: 10
b: 1729
E5
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one part containing the parameters and the body of the
procedure and
another part containing the environment in which the
procedure was created.
Graphical representation:
E2
E3
E6
Env
w: 127
y: 20
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Para:
Body:
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The Creation of Environment Diagrams
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define
Initially, we have
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GE
(define <name> <value>)
creates a new binding in the current (or first) frame
of the current environment
CE
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define, lambda, set! and procedure application
change the diagram
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The Creation of Environment Diagrams
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lambda
(define y 69) ; in environment GE
GE
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y: 69
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CE
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(lambda (par1 par2 ...)
body)
creates a procedure object in the current
environment
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The Creation of Environment Diagrams
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set! and evaluating a variable
(lambda () (…)) ; in GE
GE
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y: 69
CE
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Para:
Body:
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(set! <name> <new-value>)
changes the old value of <name> in the current
environment to <new-value>. The most recent
binding in the current environment is affected.
To find the value of a variable follows the same
strategy. Start to look after the variable in the current
environment and follow the frames to find the most
recent binding of the variabel
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The Creation of Environment Diagrams
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Procedure Application
(set! y 42) ; in GE
GE
When applying a procedure object to arguments,
z create a new environment by creating a new frame
in which we bind the parameters to the given
arguments
z connect this frame to the environment of the
procedure and let CE point to this new environment
z evaluate the body of the procedure in CE
y: 42
CE
Para:
Body:
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DUGGA 3
Evaluation of let
Uppgift 2
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Kunna förklara begreppen och vad som händer i
omgivningsdiagrammet.
Translate let to its lambda-equivalent and evaluate
as usual
(define foo
(let ((state value))
(lambda (arg) body)))
(define foo
((lambda (state)
(lambda (arg) body))
value)
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let*, letrec and others are treated similarly
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Making Flips
Making Flips
(define (make-flip)
(let ((state 0))
(lambda ()
(begin (set! state (- 1 state))
state))))
(define (make-flip)
(let ((state 0))
(lambda ()
(begin (set! state (- 1 state))
state))))
är samma som
(define (make-flip)
((lambda (state)
(lambda ()
(begin (set! state (- 1 state))
state)))
0))
(define flip (make-flip))
(define blip (make-flip))
(flip)
(blip)
(flip)
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Environment Diagram: Define function
Environment Diagram: create instance
> (define (make-flip)
….. )
> (define flip (make-flip))
GE
make-flip:
GE
flip:
CE
make-flip:
CE
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Para:()
Body:((lambda(state)
...)
0)
Para:()
Body:((lambda(state)
(lambda ()
...))
0)
Para: (state)
Body: (lambda()
...)
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Environment Diagram: create instance
State:0
Para: ()
Body: (begin ...)
Environment Diagram: evaluate instance
> (define blip (make-flip))
> (flip)
GE
make-flip:
flip:
blip:
GE
make-flip:
CE
flip:
blip:
CE
State:0
Para:()
Para: (state)
Body:((lambda(state)
Body: (lambda()
...)
...)
0)
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Para: ()
Body: (begin ...)
State:0
Para: (state)
Body: (lambda()
...)
State:1
Para:()
Para: (state)
Body:((lambda(state)
Body: (lambda()
...)
...)
0)
Para: ()
Body: (begin ...)
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Para: ()
Body: (begin ...)
State:0
Para: (state)
Body: (lambda()
...)
Para: ()
Body: (begin ...)
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DUGGA 3
Evaluate next intance
> (blip)
Uppgift 3
GE
make-flip:
flip:
blip:
Kunna rita omgivningsdiagram där procedurobjekt
ingår.
CE
State:1
Para:()
Para: (state)
Body:((lambda(state)
Body: (lambda()
...)
...)
0)
State:1
Para: (state)
Body: (lambda()
...)
Para: ()
Body: (begin ...)
Para: ()
Body: (begin ...)
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Mutable (compound) data
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Ändring av car resp cdr-delen i conscellen
To modify list structures:
Changes the car and cdr pointers
(define p ’(a b c))
p
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en ()
c
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en ()
c
(set-car! p ’x)
(set-car! cons-cell new-value)
(set-cdr! cons-cell new-value)
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p
p => (x b c)
(set-cdr! p (cddr p))
p
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e1
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en ()
c
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p => (x c)
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DUGGA 3
Sätta - samman listor – kopiering
Uppgift 1
start
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ny
Kunna rita cons-celler och utföra destruktiva ändringar
med set-car! och set-cdr!
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slut
()
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()
(define start ’(a b))
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e2
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(define (append L1 L2)
(if (null? L1)
(cons (car L1)
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(define ny (append start
slut))
start => (a b)
L2
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(define slut ’(x y))
(append (cdr L1) L2))))
slut => (x y)
ny => (a b x y)
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Sätta - samman listor – pekarändring
Mutable lists – difficult to see consequences
start
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e1
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(define x ‘(a b c))
(define y ‘(d e))
z (set-car! x y)
z (set-cdr! y ‘f)
z (define z (cons (cdr x) (cdr x)))
z z -> ?
z = ((b c) b c)
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ny
slut
e1
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e2
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()
(define start ’(a b))
(define slut ’(x y))
(define (append! L1 L2)
(if (null? L1)
start => (a b x y)
L2
(begin (set-cdr! (last L1) L2)
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(define ny (append! start
slut))
L1)))
slut => (x y)
ny => (a b x y)
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Identity and sharing
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Number of cons-cells
eq? cannot be applied to lists (to see if two lists are
identicel by contents), however
eq? can test if x and y are the same object
(equal pointers)
(list ’a ’b) = (cons ’a (cons ’b ’()))
(define p (let ((cons-cell (cons ’a ’())))
(cons cons-cell (cons cons-cell ’()))))
p => ((a) (a))
equal? checks if two lists have the same elements
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Queue primitives
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()
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()
(eq? (car p) (car (cdr p))) = ?
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()
()
(eq? (car p) (car (cdr p))) = ?
Queue
make-queue: -> queue
empty-queue?: queue -> bool
front-queue: queue -> item
insert-queue!: item x queue -> queue
delete-queue!: queue -> queue
print-queue: queue ->
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(define bank-queue (make-queue))
(insert-queue! ’kalle bank-queue)
(insert-queue! ’stina bank-queue)
(front-queue bank-queue) => kalle
(delete-queue! bank-queue)
(insert-queue! ’lisa bank-queue)
(insert-queue! ’eva bank-queue)
(front-queue bank-queue) => stina
(delete-queue! bank-queue)
(print-queue bank-queue)
lisa
eva
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Creating an Abstract datatype (ADT)
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Queue representation
Constructors
Selectors
Mutators (!-procedures)
Recognizors
Printers
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To allow fast access we need two pointers
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–
The front of the queue, i.e. the first pair
The rear of the queue, i.e. the last pair
Now that we have introduced set!, set-car!, and setcdr!, the picture is complete.
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Front of queue
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c
()
Rear of queue
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Queue representation
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Queue implementation
Empty queue
e()1
(define (insert-queue! Item queue)
(let ((new-pair (cons item '())))
(cond ((empty-queue? queue)
(set-car! queue new-pair)
(set-cdr! queue new-pair)
queue)
(else (set-cdr! (cdr queue) new-pair)
(set-cdr! queue new-pair)
queue))))
()
One element queue
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()
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DUGGA 3
Queue with object oriented packaging
Uppgift 4
(define (make-queue)
(let ((queue (cons ‘() ‘()))
(define (empty?)
(null? (car queue)))
(define (front)
(car (car queue)))
…
…
(define (dispatch m)
(cond ((eq? m ‘empty) empty?)
((eq? m ‘insert) (lambda (item) (insert! item)))
((eq? m ‘front) front)
((eq? m ‘delete) delete!)
((eq? m ‘print) print)
(else (error “Unknown request” m))))
dispatch)
Skapa andra datastrukturer (än sekventiella listor) där
man får en effektivare representation om man ändrar
pekare.
En kö. Konstant tid att hitta början resp slutet av kön.
Implementeras lämpligen som en abstrakt datatyp
(ADT).
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Queue-head
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