CS1 Introduction to Computation

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CS1Introduction to Computation
http//www.cs.caltech.edu/courses/cs1

• Day 1 October 1, 2007
• Automated Processing of Information

Add cards signed at end of class.
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Outline

• 15 Administrative details
• 10 Pep talk
• 10 What class is about / philosophy
• 15 Review mathematical notation
• translate to Scheme

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Course Admin
4
People
Mike Vanier (mvanier_at_cs) Donnie Pinkston
(donnie_at_cs)
(lectures)
Joseph Schaeffer Donnie Pinkston
(recitations)
David Dicato Nick Fortino Brent
Goldman Alexander Hudson Ilya Loksha Keegan
McAllister
Catherine Chou Cheng William Hong Radhika
Marathe Hamik Mukelyan Kim Scott Tamas Szalay
(lab)
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Website

• http//www.cs.caltech.edu/courses/cs1
• (also http//www.cs.caltech.edu/cs1)
• ? Administrative Handout
• CS1man
• Other Good LinksTM

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Organization

• Lectures 2x50min/week
• Introduce new material
• Tie ideas to mathematics
• Big Idea page
• Punctuate with interludes (midpoint)
• Attendance reqd ? Daily Quiz
• Recitations 1x50min/week
• Ask questions and work examples
• Smaller groups (target 25)
• Staffed lab 4d x 8 hrs
• TAs, assigned times

• Comprehensive Text
• SICP
• (Abelson/Sussman)
• Required Reading
• Weekly assignments
• Allow rework (1 wk)
• Online, assignment 1 posted
• Exams
• midterm, final
• Point system

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Your Part

• Read the text
• Go to lectures and sections
• Ask your TA for help
• Learn from your mistakes
• (rework gives you a chance to do so)
• Tell us what makes things hard
• Fill out feedback, talk to ombuds, TAs,
  instructors

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Point System

• Daily Quiz (0.0 - 0.5) x 20 10
• Homework (0 - 3) x 9 27
• Midterm
  6
• Final
  18
• Possible
  61
• Pass
  40

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Grading homework

• Set has multiple parts (A through F, e.g.)
• Score for entire set is minimum of the section
  scores
• Each part receives integer in 0, 3

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Time on Assignments

• Taking excessive time on a problem is a symptom
  that you missed an important concept
• Its possible to do things the wrong way (missing
  what were teaching you)
• but it probably will take you a lot more time
• Time hint on assignments
• suggested upper bound
• feedback to you ? you should see a TA
• Dont spend hours alone on a problem!

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Rework Philosophy

• Programs that are 1 wrong are wrong
• as far as the computer is concerned
• Good match ? pass/fail nature
• Emphasizes mastery

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Optional Seminar

• Everything You Need to Know About Linux, the CS
  Cluster, and CS 1 Software In 60 Minutes or Less
• Useful if youre new to Linux or need/want a
  refresher
• 1000pm Thursday Jorgensen 074
• (basement)
• South-side basement door will be unlocked

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Lab

• No formal lab sections first week
• TAs in lab T/W/Th/F for week 1
• 2 PM to midnight
• not 6 PM 8 PM, T/Th 2-4, F 8-midnight
• Lab is in Jorgensen 154
• combo 3-5-2

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Your Action Items

• Hand in (today)
• daily quiz, section time prefs, feedback
• Sign up for CS account (via the web)
• also results in card key access
• Get text book (and/or read on web)
• read thru section 1.1.5
• Visit web site
• read admin handout
• find lab 1
• start getting familiar with CS system

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Pep Talk
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Before I begin...

• I'd like to read a quote from Kip Thorne
• R.P.Feynman Professor of Theoretical Physics

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Kip Thorne's advice to you

• "To undergraduates I would say, youre living in
  an artificial environment where the pressure is
  intense, your peers are brilliant, and you see
  yourselves competing with them. You should try to
  put that competition away make it not affect
  you, and focus instead on simply learning and
  enjoying science."

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Kip Thorne's advice to you

• "This is a totally artificial environment. Youll
  never be in this situation again. When you leave
  Caltech, you will find yourself surrounded by
  more ordinary mortals. Now, thats a weird kind
  of advice, but I have seen too many talented
  undergraduates get discouraged and drop out,
  either actually or spiritually."

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Kip Thorne's advice to you

• "My main advice, then, is to focus on enjoying
  science. Make your learning of science something
  that reaches out beyond your studies and into the
  world more broadly. "

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Kip Thorne's advice to you

• "Keep up with whats going on outside of your own
  areas of study. Maintain your intellectual
  curiosity and learn thereby how to function like
  a real scientist rather than just focusing on the
  narrow, particular areas that are involved in
  your coursework."

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Alrighty then...
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I'm sure you're all asking...

• What is this course really all about?
• Learning to program?
• Learning computer science?
• Learning an employable skill?
• Learning an enjoyable skill?
• YES!

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Setting the mood...

• I etch a pattern of geometric shapes onto a
  stone. To the uninitiated, the shapes look
  mysterious and complex, but I know that when
  arranged correctly they will give the stone a
  special power, enabling it to respond to
  incantations in a language no human being has
  ever spoken. I will ask the stone questions in
  this language, and it will answer by showing me a
  vision a world created by my spell, a world
  imagined within the pattern on the stone.

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• A few hundred years ago in my native New
  England, an accurate description of my occupation
  would have gotten me burned at the stake. Yet my
  work involves no witchcraft I design and program
  computers. The stone is a wafer of silicon, and
  the incantations are software. The patterns
  etched on the chip and the programs that instruct
  the computer may look complicated and mysterious,
  but they are generated according to a few basic
  principles that are easily explained.

Danny Hillis The Pattern On the Stone
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Programming fun

• Frederick P. Brooks, Jr.The Mythical Man-Month
• joy of making things
• things that are useful to other people
• things that are complex and intricate
• joy of always learning
• delight of working in such a tractable medium
• purely abstract "thought stuff"
• yet its effects are real

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Programming fun

• Frederick P. Brooks, Jr.The Mythical Man-Month
• "Programming then is fun because it gratifies
  creative longings built deep within us and
  delights sensibilities we have in common with all
  men and women."

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Alan Perlis

• From the introduction to our textbook
• "We're responsible for stretching computers,
  setting them off in new directions, and keeping
  fun in the house."

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Alan Perlis

• From the introduction to our textbook
• "What's in your hands, I think and hope, is
  intelligence the ability to see the machine as
  more than when you were first led up to it, that
  you can make it more."

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"Double happiness"

• "People who program have double happiness.
  First of all they experience the sheer beauty of
  elegant mathematical patterns that surround
  elegant computational procedures. Then they
  receive a practical payoff when their theories
  make it possible to get other jobs done more
  quickly and more economically.''
• Donald Knuth

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Why learn computer science?

• Computer science/programming has
• great theory
• tons of practical applications
• financial rewards
• and it's fun
• And if that isn't enough...
• Computer programming is an essential tool in
  every branch of science

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Fantasia (1940)
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Is it magic?

• What computers can do may seem almost magical
  e.g.
• simulate global climate
• play chess better than humans
• prove mathematical theorems
• connect people to the internet

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Clarkes Third Law

• Any sufficiently advanced technology is
  indistinguishable from magic.
• Profiles of the Future
• Arthur C. Clarke

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Goal Magic ? Technology

• We want to fundamentally understand computers and
  information processing
• so it doesnt appear as unfathomable magic
• but as a technology we have control over

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Information Processing

• is an important component of our physical world
• has very interesting laws, characteristics, and
  behaviors
• of its own
• and in close relation to matter
• something we can and must understand to master
  our world

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CS 1 is...

• an introduction to computer programming
• big ideas and concepts
• much unfamiliar material
• emphasis on deep understand of behavior of
  programs
• preparation for future
• an introduction to computer science
• CS is more than programming

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...a class for everyone

• Computers impact lives
• Essential tool in all science and engineering
  endeavors
• control expts, run simulations, process results,
  optimize designs
• We'll start from the beginning
• more fundamental/foundational than taught in high
  school

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The basic idea behind computing

• We can write a precise, mechanical specification
  of a process
• We can build machines to follow the specification
• We can reason rigorously about the behavior of
  these machines

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Model
big idea!

• A good conceptual model allows us to predict the
  effects of our actions. Without a good model we
  operate by rote, blindly, we do operations as we
  were told to do them we can't fully appreciate
  why, what effects to expect, or what to do if
  things go wrong. As long as things work
  properly, we can manage. When things go wrong,
  however, or when we come upon novel situations,
  then we need a deeper understanding, a good
  model.

Donald Norman
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Programming
Job 1

• Build a strong conceptual model of computation
• using the Scheme programming language
• Understand the meaning of programs
• Reason rigorously about their behavior
• Appreciate complexity
• and learn how to manage it
• emphasize managing complexity and understanding
  the model over micro-optimization
• human time is (usually) the limiting factor

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Poll

• Who has not programmed before?

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For those that have

• keep active mind on the course
• watch for differences
• unlearn what you have learned
• expose you to new ways of thinking
• valuable even if you never program in Scheme
  again
• paradigms, techniques, idioms carry over to
  (almost) any language

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Computer Language(s)

• Fundamental ideas behind all languages
• enable to pick up new ones easily
• Beyond syntax
• Use Scheme as programming language
• start functional
• extend from math functions
• introduce objects
• introduce assignment (imperative)

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Why not C/C/Java?

• Scheme is not a practical language
• but
• very simple, consistent syntax
• very powerful
• other languages keep reinventing Scheme ideas
• very consistent
• easy to precisely understand computational model
• can incorporate most programming paradigms
• great textbook!

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NOT!
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http//mitpress.mit.edu/sicp
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OR
(exact same book, different cover, cheaper!)
http//mitpress.mit.edu/sicp
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Another reason we use Scheme

• "Because it lets us do things we can't do in
  other languages."
• -- Paul Graham

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Two examples

• Example 1 sum all the numbers from 1 to n
• for some number n you specify
• easy to do in Scheme or (say) C
• or any other language

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Two examples

• Example 2
• Write a function that takes a number n and
  returns a function which adds n to its argument
• In Scheme
• (define (addn n) (lambda (x) ( x n)))
• you don't need to understand this yet
• In C
• "You just can't write it."

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Two examples

• "Programming languages teach you not to want what
  they don't provide."
• We want you to know what features programming
  languages can provide
• even if the language you're using doesn't
  provide them
• Want you to be educated consumers of languages
• or maybe even producers...

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Yet Another Reason

• Many new computer languages are very closely
  related to Scheme
• Erlang (massively concurrent programming)
• Ocaml (typed functional programming)
• Haskell (lazy functional programming)
• More conventional languages have borrowed
  features from Scheme
• Python, Ruby, C
• Therefore, Scheme is a good foundation for
  learning new languages

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Notations and Model

• (start into details)

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Infix notation

• infix operator between operands
• 3 4

operator
operands
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Infix notation

• Examples
• 3 4
• 3 4 5
• 3 4 5 22
• sometimes use parentheses to specify order of
  evaluation
• (3 4) 5
• (3 ((4 - 5) 2))2

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Assignment

• Assignment gives a symbol a particular value
• x 3

symbol
value
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Assignment

• Examples
• y 4
• z x y
• w z 2

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Evaluation

• x 3
• y 4
• z x y
• How to evaluate to get z?
• Substitute symbol's value for symbol in RHS
• ? z 3 4 7
• w z 2
• w 7 2 9

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Functions

• Function definition
• f(x) x2 - 2x 4
• f(x,y) 3x - 4y 2
• a(r) pi r2

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Function evaluation

• Substitute numbers for arguments, simplify
• f(x) x2 - 2x 4
• f(3) 32 - 23 4 9 - 6 4 7
• f(x,y) 3x - 4y 2
• f(3,2) 33 - 42 2 3
• a(r) pi r2
• a(1) pi 12 3.1415926.

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Prefix notation

• Prefix operator precedes the operands
• Familiar from function usage
• f(3, 2)

operands
operator
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Prefix notation

• Examples
• f(x) operator f operand x
• f(3) operator f operand 3
• f(x,y) operator f operands x, y
• f(3,2) etc.
• a(r)
• a(1)

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Alternative prefix notation

• (f 3 2)
• means the same thing
• parentheses on the outside
• separate with spaces, not commas
• space is significant!

operands
operator
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Scheme

• abandons infix only uses (2nd) prefix notation
• math Scheme
• 3 4 ( 3 4)
• 3 45 ( 3 ( 4 5))
• 3 4 - 522 ( 3 (- 4 ( 5 (expt 2 2))))
• f(3) (f 3)
• f(x,y) (f x y)
• f(3,2) (f 3 2)

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Yes, it's weird

• This notation takes getting used to
• Lots of parentheses
• DrScheme editor will help you with these
• but completely consistent
• no special rules to remember
• all operators are prefix
• no "operator precedence"
• Usually takes 2-3 weeks before comfortable

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Scheme combinations

• look like this
• (operator operand1 operand2 )
• Delimited by parentheses
• First element is the operator
• Rest are operands
• In general, arbitrary number of operands
• but will depend on operator

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Scheme combinations

• Examples
• ( 2 3)
• ( 2 3 4)
• (abs -7)
• May be nested
• (abs ( (- x1 x2) (- y1 y2)))

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Parentheses

• NOTE Parentheses are not optional
• Everything has parentheses
• They have a very specific meaning
• first item in parenthesized set is the operator.
• rest are operands
• Cant just wrap more parentheses around things
• why not?
• This is different from
• mathematical use
• use in most other programming languages

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Parentheses

• Scheme is a dialect of the programming language
  Lisp
• Lisp means either
• LISt Processing
• Lots of Irritating Stupid Parentheses
• depending on who you ask

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Assignment in Scheme

• Assignment is also a prefix operation
• math Scheme
• x 3 (define x 3)
• y 4 (define y 4)
• z x y (define z ( x y))
• w z 2 (define w ( z 2))

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Scheme function definition

• Translate a(r) pi r2
• (define a (lambda (r) ( pi (expt r 2))))

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lambda???

• (lambda (r) ( pi (expt r 2)))
• lambda means this combination represents a
  procedure (function)
• from the Greek letter lambda (l)
• we'll see why later
• Somewhat cumbersome well see a shortcut next
  lecture

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More function definitions

• f(x) x2 - 2x 4
• (define f (lambda (x)
• ( (expt x 2) ( -2 x) 4)))
• f(x,y) 3x - 4y 2
• (define f (lambda (x y)
• ( ( 3 x) ( -4 y) 2)))
• N.B. some operators (like ) can have a variable
  number of operands

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Comments

• Often useful to leave notes for
• your future self
• others who may read/modify your code
• Can do this by using comments
• words and symbols you tell the machine to ignore
• In Scheme
• comments start with a semicolon (usually two)
• and go to the end of the line
• this is a comment

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Scheme as a calculator

• Now know enough to use Scheme interpreter as a
  (programmable) calculator
• Assignment 1 will walk you through how to invoke
  the Scheme interpreter (DrScheme)
• More importantly
• Now you know (almost) all the syntax in Scheme!

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Big Ideas

• Automated processing of information
• Precise specification
• Computational Model
• Rigorous thinking about model
• Complexity and its management

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Finally...

• Some more words of motivation from an illustrious
  speaker

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Saturday Night Live (1993)
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Note

• Any resemblence between Matt Foley and the CS 1
  lecturers is purely accidental -)