How to Think LikeHow to Think Likea Computer Programmera Computer Programmer

Beginning PythonBeginning Python

OutlineOutline

•What Is Computer Science?What Is Computer Science?

•Computer ProgrammingComputer Programming

•Languages: Expressing Languages: Expressing things to the Computerthings to the Computer

•Fundamental ConceptsFundamental Concepts

•The Process of The Process of ProgrammingProgramming

•What Is Computer Science?What Is Computer Science?

•Computer ProgrammingComputer Programming

•Languages: Expressing Languages: Expressing things to the Computerthings to the Computer

•Fundamental ConceptsFundamental Concepts

•The Process of The Process of ProgrammingProgramming

What is Computer Science?What is Computer Science?

• Not using a Not using a computercomputer

• Not just Not just programmingprogramming

• A field at the A field at the intersection of intersection of Math, Engineering, Math, Engineering, etcetc

The AlgorithmThe Algorithm• Fundamental inventions in Fundamental inventions in

formal thinking:formal thinking:– Formal Logic (Athens, ~4 C. AD)Formal Logic (Athens, ~4 C. AD)– Mathematical Proof (Greece, Mathematical Proof (Greece,

~585 AD)~585 AD)– Secular Observation of NatureSecular Observation of Nature

(~600 AD)(~600 AD)– Arabic Numerals, including ZeroArabic Numerals, including Zero

(India, ~8 C. AD)(India, ~8 C. AD)– Algebra (al-Khwarizmi, ~810 AD)Algebra (al-Khwarizmi, ~810 AD)– Probability (late 1500's)Probability (late 1500's)– Scientific Method (early 1600s)Scientific Method (early 1600s)

• Now: Now: The AlgorithmThe Algorithm

Computer ScientistsComputer Scientists• Who are Computer Scientists?Who are Computer Scientists?

• Computer Scientists study Computer Scientists study lots of thingslots of things– e.g., "What are Algorithms e.g., "What are Algorithms

Theoretically Capable of?Theoretically Capable of?““– Can Machines Think?Can Machines Think?

• Today we're going to look at Today we're going to look at the applied version: the applied version: ProgrammingProgramming

Computer ProgrammersComputer Programmers

““Their rumpled clothes, their Their rumpled clothes, their unwashed and unshaven faces, and unwashed and unshaven faces, and their uncombed hair all testify that their uncombed hair all testify that they are oblivious to their bodies and they are oblivious to their bodies and to the world in which they move. to the world in which they move. These are computer bums, These are computer bums, compulsive programmers.compulsive programmers.””

Computer Science as the New Computer Science as the New LiteracyLiteracy• The modes of thought that come from CS The modes of thought that come from CS

are influencing a huge number of fieldsare influencing a huge number of fields

• Modeling & SimulationModeling & Simulation– We can create all sorts of worlds inside the We can create all sorts of worlds inside the

computer to work withcomputer to work with

What does a computer What does a computer do?do?• Computers internally just keep doing the Computers internally just keep doing the

same thing over and over:same thing over and over:– Get the next instructionGet the next instruction– Do whatever it saysDo whatever it says– Go back to step #1Go back to step #1

• But its internal instructions are in binary -- But its internal instructions are in binary -- too hard for people to understand easilytoo hard for people to understand easily

Simplifying how we instruct the Simplifying how we instruct the machinemachine

High-level language

Assembly Language

Machine Code

Compiler

Assembler

Matrix::Compute(double* values, int size) { for(int i=0; i<size; i++) { for(int j=0; j<size; j++) { if(i<j && values[i*size+j]<0.0) values[i*size+j] = values[j*size+i]; values[i*size+j] = 2*values[i*size+j]; }

pushl %ebpmovl %esp,%ebpmovb hi_temp,%aladdb lo_temp,%almovb $0,%ahadcb $0,%ahmovb $2,%blidivb %blmovb %al,av_templeaveret

100101010110101101010101001010101011110100001101010011101010111010110001101010010011010101010101010101101111010101010100111101010101010111010101010110111010101101101011010111010001010100001010101011000100001010101010111110101010101011111111

Natural LanguagesNatural Languages

• Computers donComputers don’’t understand Englisht understand English

• Need to deal withNeed to deal with– AmbiguityAmbiguity– RedundancyRedundancy– LiteralnessLiteralness

• So, we express what a computer should So, we express what a computer should do in a do in a formal languageformal language

Formal LanguagesFormal Languages• A program usually just a A program usually just a

document -- a written list of document -- a written list of instructionsinstructions

• You saw some of this with You saw some of this with HTML -- A markup language is HTML -- A markup language is sort-of a programming sort-of a programming languagelanguage

Poetry: Ambiguity and metaphor are important

Prose: Literalness and structure important

Program: Formal, precise structure

PB&J

Robot…

Programming LanguagesProgramming Languages

• Some simple ones are built Some simple ones are built in to programs you use in to programs you use every dayevery day– Excel is a programming Excel is a programming

language (and contains language (and contains another)another)

– Word Macro LanguageWord Macro Language

• There are also General There are also General Purpose Programming Purpose Programming LanguagesLanguages

PseudocodePseudocode

• There are lots of details to programming There are lots of details to programming languageslanguages

• WeWe’’ll also use something called ll also use something called ““pseudocodepseudocode””

• Most of the way to a programming languageMost of the way to a programming language– Excludes some detailsExcludes some details– With some plain English mixed inWith some plain English mixed in

Some Core ConceptsSome Core Concepts• (A quick taste of what you'll be (A quick taste of what you'll be

learning in Python)learning in Python)

• StateState

• ExpressionsExpressions

• VariablesVariables

• ControlControl

StateState

ExpressionExpressionss

> print > print ““HelloHello””

HelloHello

> print (5+3)> print (5+3)88

>print (5*5-2)>print (5*5-2)2323

VariablesVariables• Not exactly like AlgebraNot exactly like Algebra

• In Algebra x=5 (or 5=x) expresses a truthIn Algebra x=5 (or 5=x) expresses a truth

• In a program, x=5 updates something in the In a program, x=5 updates something in the computercomputer’’s memorys memory– (and 5=x wouldn(and 5=x wouldn’’t work: itt work: it’’s not a valid statement)s not a valid statement)

> x=0> print (x)0> x=5> print (x)5

Control StructuresControl Structures

• Lets you make decisionsLets you make decisions

• for, while, …for, while, …

if(x>5): print (“x is big”)

Else: print (“x is small”)

ProgramsPrograms

““The computer The computer programmer is a creator programmer is a creator of universes for which he of universes for which he alone is responsible. alone is responsible. Universes of virtually Universes of virtually unlimited complexity can unlimited complexity can be created in the form of be created in the form of computer programs.computer programs.””

ComplexityComplexity

Controlling complexity is the Controlling complexity is the essence of computer essence of computer programmingprogramming

-Brian Kernigan -Brian Kernigan

The ProcessThe Process

But what doBut what do you you DO?DO?

• DesignDesign– RequirementsRequirements

• Use casesUse cases

– PseudocodePseudocode– Formal CodeFormal Code– TestTest– DebugDebug

DesignDesign• Most programming is Most programming is

not done sitting in not done sitting in front of a computerfront of a computer

• You need to really You need to really understand what the understand what the program will doprogram will do

• How it interacts with How it interacts with the userthe user

• And how it is And how it is structured internallystructured internally

Mostly, when you see programmers, they Mostly, when you see programmers, they aren't doing anything. One of the attractive aren't doing anything. One of the attractive things about programmers is that you things about programmers is that you cannot tell whether or not they are working cannot tell whether or not they are working simply by looking at them. Very often simply by looking at them. Very often they're sitting there seemingly drinking they're sitting there seemingly drinking coffee and gossiping, or just staring into coffee and gossiping, or just staring into space. What the programmer is trying to space. What the programmer is trying to do is get a handle on all the individual and do is get a handle on all the individual and unrelated ideas that are scampering unrelated ideas that are scampering around in his or her head.around in his or her head.

Charles M Strauss Charles M Strauss

TestTest

• Testing a complicated program is difficultTesting a complicated program is difficult

• Some people write the tests before the Some people write the tests before the programsprograms

““Programming is like sex, one mistake and Programming is like sex, one mistake and you have to support it for the rest of your you have to support it for the rest of your lifelife..””

DebugDebug• At the beginning, much of At the beginning, much of

your timeyour time

• In many ways more In many ways more challenging than codingchallenging than coding

• When you have When you have eliminated the eliminated the impossible, whatever impossible, whatever remains, remains, however however improbableimprobable, must be , must be the truth.the truth. – Sherlock – Sherlock HolmesHolmes

•Millionaire!Millionaire!

Beginning PythonBeginning Python

What is Python?What is Python?

• A REAL Programming Language!A REAL Programming Language!• Developed in 1990 by Guido Van Rossum in Developed in 1990 by Guido Van Rossum in

the Netherlandsthe Netherlands• Named after Monte PythonNamed after Monte Python’’s Flying Circus!s Flying Circus!• Openly available at no charge!Openly available at no charge!• Freely distributed on the WebFreely distributed on the Web• LotLot’’s of Libraries of Python Routines availables of Libraries of Python Routines available• Increasingly popular for writing scripts for the Increasingly popular for writing scripts for the

WebWeb

How does Python work?How does Python work?

• It is an INTERPRETED Language, (Like HTML It is an INTERPRETED Language, (Like HTML and Java) so that it is easy to write and and Java) so that it is easy to write and execute programs in an interactive modeexecute programs in an interactive mode

• It has a very clear and easy SyntaxIt has a very clear and easy Syntax

• It is portable so it runs on many different It is portable so it runs on many different Operating Systems and Machines (Windows, Operating Systems and Machines (Windows, MacOS, Linux, Unix, etc)MacOS, Linux, Unix, etc)

What are we going to do What are we going to do with Python in CS2?with Python in CS2?• Learn some basic commands and Learn some basic commands and

capabilities of the languagecapabilities of the language– Printing data, evaluating expressions, Printing data, evaluating expressions,

inputting data, variables, loops, decisions, inputting data, variables, loops, decisions, functions, boolean expressionsfunctions, boolean expressions

• Learn how to use the programming Learn how to use the programming environmentenvironment

• Learn how to write simple but useful Learn how to write simple but useful programsprograms

What tools will you require?What tools will you require?

• You will need the Python Interpreter and You will need the Python Interpreter and programming environmentprogramming environment

– Already installed on the computers in the LABAlready installed on the computers in the LAB– Can be downloaded and installed on your Can be downloaded and installed on your

personal machine from:personal machine from:•www.python.org•Hint…to be safe install the 32 bit version Hint…to be safe install the 32 bit version

for your OSfor your OS

• You will need the Python TutorialYou will need the Python Tutorial

General ConventionsGeneral Conventions

• Python is Case Sensitive!…Be Careful!Python is Case Sensitive!…Be Careful!– Anything you name (variables, etc) must be Anything you name (variables, etc) must be

referred to exactly as initially typedreferred to exactly as initially typed

• Python Commands are always in lowercase!Python Commands are always in lowercase!

• Comments can be inserted anywhere by Comments can be inserted anywhere by using a using a ““## ”” before the comment text before the comment text

• Some commands require indentation to Some commands require indentation to work properlywork properly

VariablesVariables

•You can think of variables as labeled jars that store different types of data. While there are several kinds of variables, today we're only going to look at two:

String VariablesNumber Variables

String VariablesString Variables

• Strings are literal collections of text charactersStrings are literal collections of text characters

• Strings must be enclosed in either single or Strings must be enclosed in either single or double quotesdouble quotes

• Strings may be manipulated by a variety of Strings may be manipulated by a variety of operators (repetition, concatenation, etc)operators (repetition, concatenation, etc)

• Concatenation exampleConcatenation example– word = word = ““abcabc””– word = word + word = word + ““defdef””– print (word) print (word)

you will get: you will get: abcdefabcdef

More String OperationsMore String Operations

• String RepetitionString Repetition– word = word = ““Yo! Yo! ””– print (word* 5)print (word* 5)

•Results in Results in Yo!Yo!Yo!Yo!Yo!Yo!Yo!Yo!Yo!Yo!

String VariablesString Variables

• String - A string variable is a string of alphanumeric characters and allowed symbols that are contained within quotation marks. For example, "Hello world, I'm 102 years old today!" is an example of a string.

• Strings are basically used for storing text

• Example: greeting = “Good Morning Sunshine!”

Number VariablesNumber Variables

• Number - A number variable couldn't be more straightforward because all number variables store are numbers. You don't store them within quotes like strings. Instead, numbers can just be written as they are. If you want to store the number 9 in a variable, you just write 9

• Example: my_bank_balance = 9Example: my_bank_balance = 9

or…. pi=3.14or…. pi=3.14

Our First ProgramOur First Program

• A program that prints a greeting!A program that prints a greeting!

– print (print (““Hello, World!Hello, World!””))

• When this program runs, it outputs… When this program runs, it outputs… Hello, World!Hello, World!

• The text inside of the quotes in the program The text inside of the quotes in the program is referred to as a String, or String Variableis referred to as a String, or String Variable

Another way to achieve the sameAnother way to achieve the sameresultresult

– greeting = greeting = ““Hello, World!Hello, World!””– print (greeting)print (greeting)

• The output is The output is Hello, World!Hello, World!

• Note that we assigned the variable Note that we assigned the variable name name greetinggreeting with the value of the with the value of the literal string inside of the quotes.literal string inside of the quotes.

Expressions Expressions

• Another more complicated program:Another more complicated program:– print (print (““2 plus 2 is2 plus 2 is””, 2+2), 2+2)– print (print (““3 times 3 is3 times 3 is””, 3*3), 3*3)– print (print (““35 divided by 5 is35 divided by 5 is””, 35/5), 35/5)

• The output will be:The output will be:2 plus 2 is 42 plus 2 is 43 times 3 is 93 times 3 is 9

35 divided by 5 is 735 divided by 5 is 7

Operations on NumbersOperations on Numbers

• OperationOperation Symbol Symbol ExampleExample

• ExponentiationExponentiation **** 5 ** 2 == 255 ** 2 == 25

• MultiplicationMultiplication ** 2 * 3 == 62 * 3 == 6

• DivisionDivision // 14 / 3 == 4 14 / 3 == 4 14 / 3.0 == 4.6666 14 / 3.0 == 4.6666

• RemainderRemainder %% 14 % 3 == 2 14 % 3 == 2

• AdditionAddition + 1 + 2 == 3+ 1 + 2 == 3

• Subtraction Subtraction -- 4 - 3 == 14 - 3 == 1

More Variable Assignment ExamplesMore Variable Assignment Examples

• value = 4*8value = 4*8

• net_price =4.56net_price =4.56

• repeat_word = repeat_word = ““wordword”” * 5 * 5 • Variables are created and recognized dynamically, that is Variables are created and recognized dynamically, that is

when they appear in your program with a value assigned when they appear in your program with a value assigned to themto them

• Variables must be assigned before you refer to them Variables must be assigned before you refer to them otherwise an error will occurotherwise an error will occur

• The values that are assigned to variables in Python are The values that are assigned to variables in Python are called called Objects, each object having a reserved place in Objects, each object having a reserved place in memory with a pointer connecting it to the assigned memory with a pointer connecting it to the assigned variablevariable

Variable manipulationVariable manipulation

• Variables can be used in conjunction with Variables can be used in conjunction with themselves to affect certain operations;themselves to affect certain operations;– Example:Example:

•speed=3speed=3

•speed=speed+4speed=speed+4

•speed=speed*3speed=speed*3

•print (speed) print (speed) WHAT is the output of this program? WHAT is the output of this program?

Printing variablesPrinting variables

• Variables may be printed by themselves or in Variables may be printed by themselves or in successionsuccession

– ExampleExample

number = 9number = 9bignumber = 55bignumber = 55print (number)print (number)print (bignumber)print (bignumber)print (number , bignumber)print (number , bignumber)

(puts them on the same line)(puts them on the same line)

Inputting Data into your Inputting Data into your programprogram• We will focus on We will focus on user inputuser input which is the simplest which is the simplest

type and suitable for our purposestype and suitable for our purposes

• Input command syntax is:Input command syntax is:

speed = input (speed = input (““enter speed enter speed ““)) OrOr name = raw_input (name = raw_input (““enter your name: enter your name: ““))

WhatWhat’’s the obvious difference between these input s the obvious difference between these input commands?commands?

Ok, so letOk, so let’’s write a simple s write a simple program with the stuff weprogram with the stuff we’’ve ve covered so farcovered so far# A simple program to check our travel progress# A simple program to check our travel progress

name = raw_input ("enter your name: ") name = raw_input ("enter your name: ")

speed = input ("enter your average speed: ") speed = input ("enter your average speed: ")

time = input ("enter your exact travel time in hours: time = input ("enter your exact travel time in hours: ")")

print (name) + " your distance travelled is exactly print (name) + " your distance travelled is exactly " ,(speed*time), " miles”" ,(speed*time), " miles”

print "Travel safely!!! The End"print "Travel safely!!! The End"

•Python DemoPython Demo

““That's what's cool about That's what's cool about working with computers. working with computers. They don't argue, they They don't argue, they remember everything and remember everything and they don't drink all your they don't drink all your beer.beer.””