CSC324 – Introduction

Afsaneh Fazly1

January 7, 2013

1Thanks to A.Tafliovich, S.McIlraith, E.Joanis, S.Stevenson, G.Penn, D.Horton

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A few administrative notes

You must carefully read the course information sheet, posted onthe course webpage:

www.cs.toronto.edu/~afsaneh/csc324.html

• Communication media: course forum.

• Readings: recommended textbooks, lecture and tutorial notes,and online material.

• Course grading scheme (assignments, midterm, final exam).

• Policy on plagiarism, lateness, and emergencies.

• Course prerequisites: please drop if you do not have them.

• If you intend to take the course, attend the first two weeks.

• Drop on ROSI as soon as you decide you do not want to takethe course, so students on waiting list can enroll.

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Resources

Office Hours:Monday 3–4pm BA 4261Wednesday 1–2pm BA 4261

please drop by even if it is just to introduce yourself!

DCS Help Centre: a study space with help from experienced TAs.web.cs.toronto.edu/program/ugrad/ug_helpcentre.htm

UofT Writing Centre:http://www.writing.utoronto.ca

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Why are you here?

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Why are you here?

• Because I am keen to learn about all the cool conceptsunderlying the design and implementation of differentprogramming languages, so I can · · ·

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Course Objectives

• choose the right language for a given task.

• design a new language.

• learn new languages quickly.

• find out unused features of languages I already know.

• increase my vocabulary of programming constructs.

• be a better programmer.

And of course:

• find a good job!

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Course Objectives

Computer programming is an art · · · Programmers whosubconsciously view themselves as artists will enjoy whatthey do and will do it better.

— Donald E. Knuth

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What is Covered?

Principles of programming languages:

• issues in designining data type systems.

• issues in disigning procedures.

Programming paradigms:

• functional programming (exemplified by Scheme and ML)

• logic programming (exemplified by Prolog)

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Today

Course title: Programming languages.

Levels of programming languages.

Translation.

Programming paradigms.

A few historical notes.

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On Programming

The main idea is to treat a program as a piece ofliterature, addressed to human beings rather than to acomputer.

— Donald E. Knuth

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On Language (as a general concept)

a tool for communication.

a formal symbolic system that (arbitrarily) associates a collectionof forms with their meanings.

• Components:• specification of the forms, a.k.a., Syntax.• specification of the meanings, a.k.a., Semantics.

• Kinds:• natural languages (e.g., English) for human–human

communication.• artificial languages, including programming languages (e.g.,

Python) for human–machine communication.

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On Programming Languages

“an artificial language designed to communicate instructions to amachine.” [Wikipedia]

“a set of conventions for communicating an algorithm.” [Horowitz]

• Components• formal syntax.• formal semantics.

(we will see more of these later)

• Target of communication• computer/machine.• other programmers.

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On Programming Languages

Let us change our traditional attitude to the constructionof programs. Instead of imagining that our main task isto instruct a computer what to to, let us concentraterather on explaining to human beings what we want acomputer to do.

— Donald E. Knuth

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Levels of Programming Languages

• Machine languages.

• Assembly languages.

• High-level languages.

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Levels: Machine Languages

move content of mem location 08125 to register 7

add content of mem location 08125 to register 7

shift content of register 3 left one bit

skip next instruction if register 1 is zero

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Levels: Machine Languages

move content of mem location 08125 to register 7

add content of mem location 08125 to register 7

shift content of register 3 left one bit

skip next instruction if register 1 is zero

• Instructions are encoded as numbers.

• No variables; operands are memory addresses or registernumbers.

• Programming requires deep understanding of the machinearchitecture.

• Programs are not portable because instructions and theirencoding are machine-specific.

• Programs are extremely hard to write, debug, and read.

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Levels: Assembly Languages

1 sub 3 5 ;sub 3 from reg 5

2 skp 5 ;skip if reg 5 is 0

3 jmp 10 ;jump to line 10

4 <some instrns>

10 <other instrns>

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Levels: Assembly Languages

1 sub 3 5 ;sub 3 from reg 5

2 skp 5 ;skip if reg 5 is 0

3 jmp 10 ;jump to line 10

4 <some instrns>

10 <other instrns>

• Operations and operands have symbolic names.

• Can use macros as shorthand for common sequences of code.

• An assembler translates the program into machine code.

• Still machine dependent.

• Almost as hard to write as machine code.

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Levels: High-level Languages

if (x == 3) 1 sub 3 5 ;sub 3 from reg 5

<some instrns> 2 skp 5 ;skip if reg 5 is 0

else 3 jmp 10 ;jump to line 10

<other instrns> 4 <some instrns>

10 <other instrns>

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Levels: High-level Languages

if (x == 3) 1 sub 3 5 ;sub 3 from reg 5

<some instrns> 2 skp 5 ;skip if reg 5 is 0

else 3 jmp 10 ;jump to line 10

<other instrns> 4 <some instrns>

10 <other instrns>

• Include higher level constructs.

• Programs are much easier to write, debug, and read.

• Programs are now machine independent.

• Programs may still be “language-implementation dependent”.

• Usually support type checking and other checks that helpdetect bugs (we will see more of this later).

• Examples: Fortran, C, Scheme, Java, Python, . . .

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Quiz (1)

How many different programming languages have been developedin the past 60 years?

• thousands?

according to Wikipeida

• hundreds?

223 according to TIOBE index [www.tiobe.com]

Why the big difference?

• Depends on the definition of programming language, e.g.,TIOBE does not include XML and HTML.

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Quiz (1)

How many different programming languages have been developedin the past 60 years?

• thousands? according to Wikipeida

• hundreds? 223 according to TIOBE index [www.tiobe.com]

Why the big difference?

• Depends on the definition of programming language, e.g.,TIOBE does not include XML and HTML.

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Quiz (1)

How many different programming languages have been developedin the past 60 years?

• thousands? according to Wikipeida

• hundreds? 223 according to TIOBE index [www.tiobe.com]

Why the big difference?

• Depends on the definition of programming language, e.g.,TIOBE does not include XML and HTML.

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Quiz (1)

How many different programming languages have been developedin the past 60 years?

• thousands? according to Wikipeida

• hundreds? 223 according to TIOBE index [www.tiobe.com]

Why the big difference?

• Depends on the definition of programming language, e.g.,TIOBE does not include XML and HTML.

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Quiz (2)

Which languages are the “most popular” today?

• C, Java, Objective-C, C++, C#[ based on TIOBE ratings, calculated by counting hits of the most

popular search engines ]

• top 10 in list are imperative, why?

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Quiz (2)

Which languages are the “most popular” today?

• C, Java, Objective-C, C++, C#[ based on TIOBE ratings, calculated by counting hits of the most

popular search engines ]

• top 10 in list are imperative, why?

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Summary

Want computer programs to:

• be performed by a machine.

• be understandable (i.e., easy to read and write) by humanprogrammers.

Need to:

• write code in a high-level language.

• write another program to translate high-level code tomachine instructions.

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Summary

Want computer programs to:

• be performed by a machine.

• be understandable (i.e., easy to read and write) by humanprogrammers.

Need to:

• write code in a high-level language.

• write another program to translate high-level code tomachine instructions.

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Translation

The process of converting a program written in a high-levellanguage into machine language. Two general methods:

source code

target code

compiler

input output

Compilation

source code

input output

Interpretation

interpreter

• Compilation: the whole program is translated beforeexecution.

• Interpretation: translate and execute, one statement at a time.

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Translation

Compilation:

• Can execute translated program many times because theentire translation is produced.

• Program execution is much faster because the translator canperform optimization.

• Harder to provide useful source-level feedback whendebugging because executing the target code.

Interpretation:

• Must re-translate for every execution.

• Program execution is 10 to 100 times slower!.

• Easier to provide source-level feedback when debugging.

• Flexibility supports rapid prototyping.

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Translation

Compilation:

• Can execute translated program many times because theentire translation is produced.

• Program execution is much faster because the translator canperform optimization.

• Harder to provide useful source-level feedback whendebugging because executing the target code.

Interpretation:

• Must re-translate for every execution.

• Program execution is 10 to 100 times slower!.

• Easier to provide source-level feedback when debugging.

• Flexibility supports rapid prototyping.

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Pseudo-compilation: Hybird

source code

compiler

intermed. code

interpreter outputinput

• A compiler translates the whole program before execution, butonly into intermediate code.

• An interpreter translates and executes the intermediate codeone statement at a time.

• The intermediate code can be executed on any machine thathas an interpreter for the intermediate code.

Java uses this hybrid strategy: Intermediate code called bytecode.

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Determinants of Programming Language (PL) Design

Computer architecture: the von Neumann architecture.

Application/domain for which a language is originally designed.

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The von Neumann Architecture

the fetch-execute cycle

[image from faculty.cs.wwu.edu]

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Application Affecting PL Design

Historical examples:

• Fortran: first high-level PL, invented for scientific andmathematical computation in the 1950s.

• Lisp: invented to provide features for processing symbolic datain lists, needed for research in artificial intelligence.

• COBOL: designed for business computing in the 1950s.

• C: originally designed in the 1960s for developing systemsoftware (e.g., the Unix OS).

Note: PL designs have in turn affected practices in programdevelopment.

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Programming Paradigms

Imperative (e.g., Fortran, Algol, C, Pascal)

• program statements are commands.Example: “Add 17 to x”

• key operations are assignment and iteration.

• closely fit the von Neumann architecture.

Functional (e.g., Lisp, Scheme, ML, Haskell)

• program statements are noun phrases describing values ofexpressions.Example: “reverse of a list is last element followed by thereverse of the rest of the list”

• key operations are expression evaluation by applying afunction.

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Programming Paradigms

Logic-based (e.g., Prolog)

• program statements describe facts and rules.Example: “fact: Doug is Tom’s father.

rule: If x is y’s father and y is z’s father, then x is z’s grandfather.”

• key operation is unification.

Object-oriented (e.g., Simula, C++, Java, Python)

• program statements describe communication amongobjects.

• key operations are message passing and inheritance.

• can be imperative (e.g., Java) or functional (e.g., Scala).

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Declarative vs. Imperative Programming

Declarative programming:

• consist of declarations, specifying what to compute.

• variables are logical variables, not nicknames for memorylocations or machine registers.

• logic-based (and to some extent functional) languages supportthis programming style.

Imperative programming:

• consist of assignments and control-flow statements, specifyinghow to perform a computation.

• most programming today is imperative.

Note: many “hows” can be subsumed by a single “what”.

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Why are Imperative Languages more Popular?

• Inertia: lots of code out there to maintain.

• Lack of competent programmers: a lot of programmersfinished their education a long time ago.

• Lack of good Programming Environments: there’s beenprogress, largely due to open-source collaboration.

• Efficiency: the biggest problem, but there’s been progress.

Things are changing:

• Twitter and Linkedin use Scala: a hybrid of functional andobject-oriented language.To learn more, see http://www.scala-lang.org/

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What Makes a Good Language

Language should be easy to learn:

• Simple syntax.

• Simple and appropriate semantics.

• Small number of primitive concepts.• Orthogonality: concepts combine systematically, with no

exceptions.• No ambiguity: Meaning of a construct does not depend on

context.• Low synonymy: Not many different ways to perform a certain

task.

Programs should be easy to read and write:

• Code should reflect intuition of the programmer.

• Programs should be portable.

... and more technical principles that we will see later.

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What Makes a Good Language

Language should be easy to learn:

• Simple syntax.

• Simple and appropriate semantics.

• Small number of primitive concepts.• Orthogonality: concepts combine systematically, with no

exceptions.• No ambiguity: Meaning of a construct does not depend on

context.• Low synonymy: Not many different ways to perform a certain

task.

Programs should be easy to read and write:

• Code should reflect intuition of the programmer.

• Programs should be portable.

... and more technical principles that we will see later.

34

What Makes a Good Language

Language should be easy to learn:

• Simple syntax.

• Simple and appropriate semantics.• Small number of primitive concepts.• Orthogonality: concepts combine systematically, with no

exceptions.• No ambiguity: Meaning of a construct does not depend on

context.• Low synonymy: Not many different ways to perform a certain

task.

Programs should be easy to read and write:

• Code should reflect intuition of the programmer.

• Programs should be portable.

... and more technical principles that we will see later.

34

What Makes a Good Language

Language should be easy to learn:

• Simple syntax.

• Simple and appropriate semantics.• Small number of primitive concepts.• Orthogonality: concepts combine systematically, with no

exceptions.• No ambiguity: Meaning of a construct does not depend on

context.• Low synonymy: Not many different ways to perform a certain

task.

Programs should be easy to read and write:

• Code should reflect intuition of the programmer.

• Programs should be portable.

... and more technical principles that we will see later.

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Quiz (3)

Which factors contribute to the popularity of a programminglanguage?

• how easy it is to read and write programs in the language.

• how efficiently programs written in the language run.

• luck!

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Quiz (3)

Which factors contribute to the popularity of a programminglanguage?

• how easy it is to read and write programs in the language.

• how efficiently programs written in the language run.

• luck!

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