CS 152: Programming Language Paradigms March 12 Class Meeting Department of Computer Science San...

CS 152: Programming Language Paradigms

March 12 Class Meeting

Department of Computer ScienceSan Jose State University

Spring 2014Instructor: Ron Mak

www.cs.sjsu.edu/~mak

http://www.cs.sjsu.edu/~mak

http://www.cs.sjsu.edu/~mak

SJSU Dept. of Computer ScienceSpring 2014: March 12

CS 152: Programming Language Paradigms© R. Mak

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Smalltalk IDE

ClassCategories

ClassNames

MessageCategories

MessageSelectors

TextSection



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Screenshot ofthe originalSmalltalk

environment.



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Smalltalk Messages Unary message: A message with no arguments. Keyword message: A message that expect arguments.

The message selector (method name) ends in a colon. Example:

includes is a boolean method that has as an element argumentand returns whether or not the element is in the set.

Does the new set include the element 'Hello' ?

If a message has more than one argument, a keyword and colon must precede each argument. Example:

at:put: is the complete message selector.

Set new includes: 'Hello'

a at: i+1 put: (a at: i).



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Smalltalk Messages, cont’d Binary messages allow you to write arithmetic and

comparison expressions with infix notation.

Examples:3 + 4 "Returns 7"3 < 4 "Returns true"



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Smalltalk Variables Use variables to refer to objects.

Example:

Temporary variables are declared between vertical bars They are not capitalized.

Smalltalk variables have no assigned data type A variable can name any thing.

The assignment operator is := or



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Smalltalk Variables, cont’d Statements are separated by periods. A sequence of messages to the same object are

separated by semicolons: Example:

Smalltalk variables use reference semantics, not value semantics. A variable refers to an object. It does not contain an object.



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Smalltalk Equality and Identity Operators The equality operator is = The object identity operator is ==



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Smalltalk Statements The to:do message creates a loop.

Even control statements are expressed with message passing.

Enclose a code block with square brackets [ ] Similar to a Schema lambda form. A block can contain arguments as block variables.

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Smalltalk Statements, cont’d An ifTrue:ifFalse message expresses

a choice of action.

Print the contents of an array:

array do: [:element | Transcript nextPutAll: element printString; cr]

"Print to the transcript each element followed by a carriage return."



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Review for the Midterm A test of understanding, not memorization.

Open book, open notes, open laptop, open Internet. But not open neighbor! Forbidden to communicate with anyone else during the exam.

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Review for Midterm, cont’d Historical overview

von Neumann architecture Machine and assembly languages FORTRAN COBOL Algol Pascal

Abstractions Data Control Basic Structured Unit



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Review for Midterm, cont’d Paradigms

Functional Logic Object-oriented

von Neumann architecture von Neumann bottleneck

Language syntax Language semantics

Compilers Interpreters



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Review for Midterm, cont’d Good language design

Efficiency Regularity Generality Orthogonality Uniformity Security Extensibility

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Review for Midterm, cont’d Functional programming

Programs as functions Recursion Referential transparency Lack of state First-class objects Higher-order functions

Scheme Atoms and lists Prefix notation Binding List construction and destruction



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Review for Midterm, cont’d Scheme, cont’d

Procedures and predicates Conditional expressions

Recursion flat mutual deep tail recursion

Symbol manipulation Symbolic differentiation

Scope local bindings closure



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Review for Midterm, cont’d Scheme, cont’d

Procedure as a first-class object as an argument as a return value

Metalinguistic power eval apply

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Review for Midterm, cont’d Logic

Deductive reasoning

Prolog Declarative language Objects and relations Fact, rules, and questions Conjuctions Variables

Backtracking place markers

Resolution Unification Cut



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Review for Midterm, cont’d Object-oriented programming

Software reuse and independence Extension Redefinition Abstraction Polymorphism Encapsulation Loose coupling Frameworks

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Review for Midterm, cont’d Smalltalk

Purely object-oriented Objects Messages IDE

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Sample Midterm Question: Scheme Suppose you have a Scheme procedure min-to-head

that takes an argument list of top-level integers and returns a list where the minimum element of the list is moved to the head. If there is a tie, it moves only one of the minimum elements. Example:

(define nums '(4 3 2 1 0 1 8 0 2 9))(min-to-head nums) (0 4 3 2 1 1 8 0 2 9)



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Sample Midterm Question: Scheme, cont’d Write a recursive Scheme procedure sort that uses

procedure min-to-head as a helper to sort an argument list of top-level integers and return a list where the elements are sorted in ascending order. Example:

(sort nums) (0 0 1 1 2 2 3 4 8 9)



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Sample Midterm Question: Scheme, cont’d Base case: The empty list. Procedure sort calls helper procedure min-to-head

to move the smallest element to the head of the list. Call sort recursively on the remainder of the min-to-headed

list in order to move the next smallest element to the second position.

Continue until the base case.

This is a selection sort.

(define sort (lambda (lst) (if (null? lst) '() (let ((mth-lst (min-to-head lst))) (cons (car mth-lst) (sort (cdr mth-lst)))))))



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Sample Midterm Question: Scheme, cont’d Write a recursive Scheme procedure min-to-head.

Base case: The empty list or a list containing only one element. Otherwise, assume that min-to-head was successfully

applied to the cdr of the list. The procedure has moved the minimum value

of the cdr of the list to the second position of the list. Compare the car of the list to the second position.

If necessary, swap the two values to put the lesser value at the head of the list.

(define min-to-head (lambda (lst) (cond ((null? lst) '()) ((null? (cdr lst)) lst) (else (let ((mth-cdr (min-to-head (cdr lst)))) (if (> (car lst) (car mth-cdr)) (append (list (car mth-cdr) (car lst)) (cdr mth-cdr)) lst))))))



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Sample Midterm Question: Scheme, cont’d Assume that min-to-head has correctly worked on the

cdr of the list.

Therefore, the second element of the list is the minimum within the cdr of the list.

Compare the car of the list (the green element) against the red element. If necessary, swap them.

Now the minimum of the entire list is at the head. Recursively call min-to-head on the cdr of the list.



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Sample Midterm Question: Prolog

Given this Prolog database and the query:

Explain the result of the query in terms of resolution, unification, and backtracking.

/*1*/ go(0, 0) :- !./*2*/ go(Control, Control)./*3*/ go(Control, Start) :- Next is Start - 1, go(Control, Next).

/*4*/ nasa(Start) :- go(Control, Start), write(Control), nl, fail./*5*/ launch(Start) :- not(nasa(Start)), write('We have liftoff!').

?- launch(10).



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Sample Midterm Question: Prolog, cont’d

Answer: Resolution: The query launch(10) matches Rule 5. Unification: Rule 5: Variable Start is instantiated

and bound to 10. Rule 5: First subgoal: nasa(10) matches Rule 4.

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?- launch(10).



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Rule 5: First subgoal: nasa(10) matches Rule 4. Variable Start is bound to 10.

Rule 4: First subgoal: Match Fact 2. Variable Control is instantiated and shared with Start which is 10.

Rule 4: First subgoal satisfied. Rule 4: Second subgoal: write(Control) print “10”. Rule 4: Third subgoal: Print a new line. Rule 4: Fourth subgoal: Fail.

?- launch(10).





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Rule 4’s failure causes a backtrack to match Rule 3. Rule 3: First subgoal: Set variable Next to Start – 1 which is 9. Rule 3: Second subgoal: Match Rule 2, set Control to 9.

Rule 4: First subgoal satisfied. Rule 4: Second subgoal: write(Control) print “9”. Continue printing “8”, “7”, etc. down to “0”. When the countdown reaches 0, the cut operation

in Rule 1 stops the countdown from backtracking further.

?- launch(10).





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Rule 4: Fourth subgoal causes the countdown to end in failure. Rule 5: First subgoal has a not,

and therefore that subgoal succeeds. Rule 5: Second subgoal prints “We have liftoff!”

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