Programming Languages Meeting 14 December 9/10, 2014.

Programming Languages

Meeting 14December 9/10, 2014

CATS

Planning

• Final Exam, next week, Tuesday, December 16, 2:30 – 5:00 pm, MSC 290

• Or Wednesday, December 17, 6:15 – 8:45 pm, MSC G86

Topics for Final

• Syntax and syntax specification: EBNF– Nonterminal, terminal, production, start symbol– Grammar of expressions• Inferred precedence and association

– Syntax trees• Abstract syntax

Topics (1.1)

• Semantics and semantic specification– Env, Loc, Store, Value, Memory– Program functions– Backus notation for functional programming

Topics (2)

• Semantics examples– REF, a language of pointers– Multiple assignment statements

• Control structures– Sequence – While– If-then– For

Topics (3)

• Parameter passing– Value– Result– Value-result– Reference– Name

Topics (4)

• Functional programming– Lists – Atoms– Primitive functions– Functionals– User-defined functions– Predicates

Topics (4.1)

• Developing from primitives and functionals– Use of recursion

• Catching error conditions– Proper error messages

• Measures of lists– Length– Depth

Topics (5)

• Scripting languages– AWK– Perl

• Regular expressions

AWK Questions

AWK Programs

{for (w=1;w<=NF;w++) count[$w]++}END {for (w in count) print count[w],w}

/^$/ {count++}END {print count}

/<..*>/

Paradigms

• Programming paradigm: pattern of problem-solving thought that underlies a particular genre of programs and languages.

• Four main programming paradigms:– Imperative– Object-oriented– Functional– Logic (declarative)

Imperative

• Follows the classic von Neumann-Eckert model:– Program and data are indistinguishable in memory– Program: a sequence of commands– State: values of all variables when program runs– Large programs use procedural abstraction

• Examples of imperative languages: – Cobol, Fortran, C, Ada, Perl– Historically: PL1, Algol, Pascal

Object-oriented

• Collection of objects (instantiated from classes) that interact by passing messages that transform the state.

• Need to know:– Ways of sending messages– Inheritance– Polymorphism

• Examples of OO languages:– Smalltalk, Modula, Java, C++, C#, Python

Functional

• Models a computation with a collection of functions acting on a set of objects, usually lists.– Input: domain of the function– Output: codomain of the function

• Functional languages are characterized by:– Functional composition– Recursion

• Examples of functional languages:– Lisp, Scheme, ML, Haskell

Logical

• Logic programming declares what outcome the program should accomplish, rather than how it should be accomplished.

• Programs are:– Sets of constraints on a problem– Achieve all possible solutions– Nondeterministic, in some cases

• Example of a logic programming language: – Prolog

Special Languages

• Symbolic computation– Macsyma, Mathematica, Maple, Reduce

• Mathematical– APL, Basic, Cayley

• Music– ChucK

• Markup– HTML, MusicXML, LaTeX

• Scripting– AWK, Perl, JavaScript, PHP

• and hundreds more for every special circumstance

PERL

• Practical Extraction and Support Language• A glue language under Unix• Written by Larry Wall• See www.perl.com

PERL

• Scripting language• More powerful than AWK• More flexible than shell scripts• Syntax derived from C• Large language• Later versions contain classes• Many irregularities in design• Many alternatives for syntax

PERL

Beginning assumptions and notations• Statements are terminated by ; (semicolon)• Comments start with # and occupy the rest of

the line.– NO two-line comments

• Parentheses around arguments to built-in functions are optional

PERL

• First line of program is an interpreter directive, written as a special type of comment, giving the full path to the Perl interpreter

#!/usr/local/bin/perl -w

Example 1

#!/usr/local/bin/perl –wprint(“Hello, world!\n”);

Example 1

#!/usr/local/bin/perl –wprint(“Hello, world!\n”);

Inserts new line character

PERL

• Scalars– Integer, real, string• Identifier must start with $

• Arrays– Indexed by integers• Identifier must start with @

– Indexed by strings• Identifier must start with %

Example 2

#!/usr/local/bin/perl –w

print(“What is your name? ”);$name = <STDIN>;chomp($name);print “Hello, $name!\n”;

Example 2

#!/usr/local/bin/perl –w

print(“What is your name? ”);$name = <STDIN>;chomp($name);print “Hello, $name!\n”;

Critical space

Example 2

#!/usr/local/bin/perl –w

print(“What is your name? ”);$name = <STDIN>; #Reads one line chomp($name); #Cuts off EOLprint “Hello, $name!\n”;

PERL

• Control structuresif-then-elsifforeach $element (%list) { … }

Example 3

#!/usr/local/bin/perl –w

print “What is your name? ”;$name = <STDIN>;chomp($name);if ($name eq “Nico”)

{print “Hello, $name! Glad you’re here.\n”}

elsif ($name eq “Daphne”){print “Going outside, $name?\n”}

else{print “Hello, $name.\n”};

PERL

• Dynamically typed– Type inferred from operations– Compare

2 < 10 (true)2 lt 10 (false)2 < “10” (true)“2” lt 10 (false)

• Numbers, strings, regular expressions• Operators are not overloaded

Arrays of Strings

• Can be initialized with a list of strings @words = (“camel”,”llama”,”alpaca”);• Or use the keystroke saving function qw

@words=qw(camel llama alpaca);• Note that an individual element is a string so that the

string identifier syntax must be used $words[0] has value camel

Array Bounds

• Out of bounds arrays– Impossible with associative arrays: strings are not

presumed to have order (in the sense of a next function)

– Impossible with integer indexed arrays@a = (2,3,5,7);$a[7] = 17;

We’ll check the Env and Store

Scalar Operators

• Numeric: + - * / ** %• Numeric comparison: < <= == >= > !=• String: . (for concat) x (for multiple concat)• String comparison: lt le eq ge gt ne• Assignment:

= (carries value of expression)binary assignment: += *= .= etcincrement: ++$i $i++

(increment then assign vs. assign then increment)

Parameter Passing

• Parameter passing– Formal parameters not included– Values of arguments to a function come in the

array @_– Example, the call range(1,74,3) puts 1 in $_[0], 74

in $_[1], and 3 in $_[2]– Thus, a function can have a varying number of

arguments.

Parameter Passing (2)

• If parameter values are passed to a procedure (subroutine in PERL language) in an array local to the procedure, how are values returned to the calling program?

• Options– – –