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Extended Introduction to Computer Science

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Administration

סגל הקורס:•מרצים: דניאל דויטש, ניר שביט–מתרגלים: ניר אטיאס, סשה אפראצין , איל כהן–עמית ליכטנברג בודקת:–

• Book: Structure and Interpretation of Computer Programs – by Abelson & Sussman

• Web: http://www.cs.tau.ac.il/~scheme

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Course Structure

• Three Elements– Lectures (הרצאות)– Recitations (תרגולים)– Homework (תרגילי בית)

• Final Grade = Homework + MidTerm Exam + Final Exam

חובה להגיש ולקבל ציון עובר עבור לפחות

מהתרגילים!!80%

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Computer Science

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Geometry

GihaEarth

MetraMeasure

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Declarative Knowledge

0 and such that theis 2 yxyyx

“What is true”

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To find an approximation of x:

• Make a guess G• Improve the guess by averaging G and x/G• Keep improving the guess until it is good enough

Imperative Knowledge

“How to”

[Heron of Alexandria]

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X = 2 G = 1

X/G = 2 G = ½ (1+ 2) = 1.5

X/G = 4/3 G = ½ (3/2 + 4/3) = 17/12 = 1.416666

X/G = 24/17 G = ½ (17/12 + 24/17) = 577/408 = 1.4142156

To find an approximation of x:

• Make a guess G• Improve the guess by averaging G and x/G• Keep improving the guess until it is good enough

.2for :Example xx

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“How to” knowledge

Process – series of specific, mechanical steps for deducing information, based on simpler data and set of operations

Procedure – particular way of describing the steps a process will evolve through

Need a language for description:

• vocabulary

• rules for connecting elements – syntax

• rules for assigning meaning to constructs – semantics

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Designing Programs

• Controlling Complexity – Black Box Abstraction– Conventional Interfaces– Meta-linguistic Abstraction

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Scheme

• We will study LISP = LISt Processing– Invented in 1959 by John McCarthy– Scheme is a dialect of LISP – invented by Gerry

Sussman and Guy Steele

Expressions Data or procedures

Syntax Semantics

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The Scheme Interpreter

• The Read/Evaluate/Print Loop– Read an expression

– Compute its value

– Print the result

– Repeat the above

• The Environment score 23

total 25

percentage 92

Name Value

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Designing Programs

• Controlling Complexity

– Problem Decomposition

– Black Box Abstraction • Implementation vs. Interface

– Modularity

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Language Elements

Means of Abstraction

(define score 23) Associates score with 23 in environment table

Syntax Semantics

Means of Combination

(+ 3 17 5) Application of proc to arguments

Result = 25

Primitives 23+*

23

Proc for adding

Proc for multiplying

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Computing in Scheme

==> 23

23

==> (+ 3 17 5)

25

==> (+ 3 (* 5 6) 8 2)

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==> (define score 23)

Name Value

Environment Table

23score

Opening parenthesis

Expression whose value is a procedure

Other expressions

Closing parenthesis

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Computing in Scheme

==> score

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==> (define total 25)

==> (* 100 (/ score total))

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==> (define percentage (* 100 (/ score total))

Name Value

Environment Table

23score

25total

92percentage

==>

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Evaluation of Expressions

To Evaluate a combination: (as opposed to special form)a. Evaluate all of the sub-expressions in some orderb. Apply the procedure that is the value of the leftmost sub-

expression to the arguments (the values of the other sub-expressions)

The value of a numeral: numberThe value of a built-in operator: machine instructions to executeThe value of any name: the associated value in the environment

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Using Evaluation Rules

==> (define score 23)

==> (* (+ 5 6 ) (- score (* 2 3 2 )))

Special Form (second sub-expression is not evaluated)

* + 5 6

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- 23 * 3 22

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121

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Abstraction – Compound Procedures

• How does one describe procedures?

• (lambda (x) (* x x))

To process something multiply it by itself

formal parameters

body

Internal representation

• Special form – creates a “procedure object” and returns it as a “value”

Proc (x) (* x x)

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Lambda

• The use of the word “lambda” is taken from lambda calculus.

– Introduced in the Discrete Math course.– Useful notation for functions.

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Evaluation of An Expression

To Apply a compound procedure: (to a list of arguments) Evaluate the body of the procedure with the formal parameters

replaced by the corresponding actual values

==> ((lambda(x)(* x x)) 5)

Proc(x)(* x x) 5

(* 5 5)

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Evaluation of An Expression

To Apply a compound procedure: (to a list of arguments) Evaluate the body of the procedure with the formal parameters

replaced by the corresponding actual values

To Evaluate a combination: (other than special form)a. Evaluate all of the sub-expressions in any orderb. Apply the procedure that is the value of the leftmost sub-

expression to the arguments (the values of the other sub-expressions)

The value of a numeral: numberThe value of a built-in operator: machine instructions to executeThe value of any name: the associated object in the environment

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Using Abstractions

==> (square 3)

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==> (+ (square 3) (square 4))

==> (define square (lambda(x)(* x x)))

(* 3 3) (* 4 4)

9 16+

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Environment Table

Name Valuesquare Proc (x)(* x x)

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Yet More Abstractions

==> (define f (lambda(a) (sum-of-two-squares (+ a 3) (* a 3))))

==> (sum-of-two-squares 3 4)

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==> (define sum-of-two-squares (lambda(x y)(+ (square x) (square y))))

Try it out…compute (f 3) on your own

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Evaluation of An Expression (reminder)

To Apply a compound procedure: (to a list of arguments) Evaluate the body of the procedure with the formal parameters

substituted by the corresponding actual values

To Evaluate a combination: (other than special form)a. Evaluate all of the sub-expressions in any orderb. Apply the procedure that is the value of the leftmost sub-

expression to the arguments (the values of the other sub-expressions)

The value of a numeral: numberThe value of a built-in operator: machine instructions to executeThe value of any name: the associated object in the environment

The Substitution

model

reductionin lambda

calculus

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Let’s Not Forget The Environment

==> (define x 8)

==> (+ x 1)

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==> (define x 5)

==> (+ x 1)

6The value of (+ x 1) depends on the environment!

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Using the substitution model

(define square (lambda (x) (* x x)))(define average (lambda (x y) (/ (+ x y) 2)))

(average 5 (square 3))(average 5 (* 3 3))(average 5 9) first evaluate operands,

then substitute

(/ (+ 5 9) 2)(/ 14 2) if operator is a primitive procedure, 7 replace by result of operation

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Booleans

Two distinguished values denoted by the constants

#t and #f

The type of these values is boolean

==> (< 2 3)

#t

==> (< 4 3)

#f

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Values and types

Values have types. For example:

In scheme almost every expression has a value

Examples:

1) The value of 23 is 232) The value of + is a primitive procedure for addition3) The value of (lambda (x) (* x x)) is the compound

procedure proc (x) (* x x)

1) The type of 23 is numeral 2) The type of + is a primitive procedure3) The type of proc (x) (* x x) is a compound procedure4) The type of (> x 1) is a boolean (or logical)

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No Value?• In scheme almost every expression has a value

• Why almost?

Example : what is the value of the expression(define x 8)

• In scheme, the value of a define expression is “undefined” . This means “implementation-dependent”

• Dr. Scheme does not return (print) any value for a define expression.

• Other interpreters may act differently.

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More examples

==> (define x 8)

Name Value

Environment Table

8x

==> (define x (* x 2))

==> x

16 16

==> (define x y)

reference to undefined identifier: y

==> (define + -)

>-<#+

==> (+ 2 2)

0

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The IF special form

ERROR2

(if <predicate> <consequent> <alternative>)If the value of <predicate> is #t,

Evaluate <consequent> and return it

Otherwise

Evaluate <alternative> and return it

(if (< 2 3) 2 3) ==> 2

(if (< 2 3) 2 (/ 1 0)) ==>

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IF is a special form

•In a general form, we first evaluate all arguments and then apply the function

•(if <predicate> <consequent> <alternative>) is different:

<predicate> determines whether we evaluate <consequent> or <alternative>.

We evaluate only one of them !

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Syntactic Sugar for naming procedures

(define square (lambda (x) (* x x))

(define (square x) (* x x))

Instead of writing:

We can write:

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(define second )

(second 2 15 3) ==> 15

(second 34 -5 16) ==> -5

Some examples:

(lambda (x) (* 2 x))

(lambda (x y z) y)

Using “syntactic sugar”:

(define (twice x) (* 2 x))

Using “syntactic sugar”:

(define (second x y z) y)

(define twice )

(twice 2) ==> 4

(twice 3) ==> 6

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Summary• Computer science formalizes the computational process• Programming languages are a way to describe this process

• Syntax• Sematics

• Scheme is a programming language whose syntax is structured around compound expressions

• We model the semantics of the scheme via the substitution model, the mathematical equivalent of lambda calculus