Introduction to CS111 Part 2: Big Ideas

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What is Cmputer Science? Intrductin t CS111 Part 2: Big Ideas CS111 Cmputer Prgramming Department f Cmputer Science Wellesley Cllege It s nt really abut cmputers. It s nt really a science.

Hal Abelsn A cmputer language is a nvel frmal medium fr expressing ideas abut methdlgy, nt just a way t get a cmputer t perfrm peratins.

1 What is Cmputer Science? Intrductin t CS111 Part 2: Big Ideas CS111 Cmputer Prgramming Department f Cmputer Science Wellesley Cllege It s nt really abut cmputers. It s nt really a science. It s abut imperative ( hw t ) knwledge as ppsed t declarative ( what is ) knwledge. Imperative knwledge is expressed via algrithms = cmputatinal recipes. Hal Abelsn A cmputer language is a nvel frmal medium fr expressing ideas abut methdlgy, nt just a way t get a cmputer t perfrm peratins. Prgrams are written fr peple t read, and nly incidentally fr machines t execute. -- Harld Abelsn and Gerald J. Sussman 1-2 Fur big ideas Big idea #1: Abstractin Fur imprtant cncepts are at the cre f this curse: 1. Abstractin; 2. Mdularity; 3. Divide, Cnquer and Glue; 4. Mdels These interrelated ideas are imprtant in almst every discipline, but they re at the cre f CS. We will illustrate these ideas in several ways. Our gal is t help yu think abut prblem slving in new ways. 1-3 User / Client Cntract / API Implementer / Designer 1-4

2 Big #2: Mdularity Large systems are built frm cmpnents called mdules. The interfaces between mdules are designed s they can be put tgether in a mixand-match way. Big #3: Prblem Slving Strategies Example: Divide-Cnquer-Glue Divide prblem P int subprblems. Cnquer each f the subprblems. Glue (cmbine) the slutins t the subprblems int a slutin S fr P. In cmputer prgramming, the gal is t design packages fr maximum reusability. P P1 P2 P3 P4 S1 S2 S3 S4 S 1-5 Big 1-6 What will yu build with these ideas? # 4: Mdels We ll start with numerical calculatins and graphics Need simple mdels t understand cmplex artifacts and behavirs. We ll draw lts f diagrams t predict what prgrams will d

3 Define functins t capture cmmn patterns Apply prblem slving strategies like Divide/ Cnquer/Glue t draw quilts Write text-based games with cnditinals Use the pwer f iteratin t generate graphics and write games Tic-tac-te Rck, paper, scissrs

4 Learn Data Structures: Lists and Dictinaries Perfrm Data Analysis and Visualizatin Prblem slving with Recursin Object-Oriented Prgramming

Pythn prgrams in Lab / Psets Jupyter ntebks fr handsn activities in Lecture argument bject

Pythn Intr Overview [Cntinues n Friday] The First Mdel: Variable as a Bx Values: 10

1415 (decimal number r flat), 'wellesley' (text r string) Types: numbers and text: int,

a value as a result) len('abc') * 'abc' + 'def' Built-in functins: max, min, len, int,

are names we make up (but, there are rules fr creating these names) A variable name shuld

5 On t Pythn! Unlearn what yu have learned class type variable functin flat Canpy t edit and execute Pythn prgrams in Lab / Psets Jupyter ntebks fr handsn activities in Lecture argument bject string instance parameter I dn t think that wrd means what yu think it means Pythn Intr Overview [Cntinues n Friday] The First Mdel: Variable as a Bx Values: 10 (integer), (decimal number r flat), 'wellesley' (text r string) Types: numbers and text: int, flat, str type(10) type('wellesley') Operatrs: + - * / % = Expressins: (they always prduce a value as a result) len('abc') * 'abc' + 'def' Built-in functins: max, min, len, int, flat, str, rund, print, raw_input Knwing the type f a value allws us t chse the right peratr when creating expressins fav = 17 # assign fav + 3 fav 17 # lkup # lkup and reassign fav = fav + 3 Variables are names we make up (but, there are rules fr creating these names) A variable name shuld appear fr the first time in an assignment statement. A value is stred in a bx. The variable labels the bx. When a variable is used in expressins, we lkup fr the bx with that name and read its value. We can reassign a (new) value t a bx. If we use a name in an expressin withut using it in an assignment first, we get a NameErrr. 1-20

6 Example with cs1graphics lecture01-graphics.ipynb Versin anntated t display crdinates fr reference pints. cs1graphics A library fr graphics frm cs1graphics imprt * paper = Canvas(400, 550, 'yellw') head = Circle(50, Pint(200,150)) paper.add(head) # The trs trs = Rectangle(100, 200, Pint(200, 300)) trs.setfillclr('gray') paper.add(trs) Defines names used in this prgram: Canvas, Rectangle, Text, etc. Assignment statement that: 1. Creates bject, assigns it t a variable 2. Displays an empty canvas Assignment statement (value and name) Statement: methd call t display circle bject n canvas windw Statement: methd call t change bject s internal state, als visible n the canvas. # SOME CODE OMMITTED 1-21 message = Text('If I nly had a heart!', 20) paper.add(message) Statement: methd call t change reference message.setfntclr('red') pint and display bject in new psitin. message.mvet(200, 25) 1-22 Expressins vs. Statements They always prduce a value: * /25 max(10, 20) int("100") fav fav + 3 "pie" + " in the sky Expressins are cmpsed f values, peratrs, variables, functins, and any cmbinatin f them. They perfrm an actin (that can be visible, invisible, r bth): print(10) age = 19 paper = Canvas(400, 550, 'yellw') paper.add(head) Statements may cntain expressins, which are evaluated befre the actin is perfrmed. print('she is ' + str(age) + ' years ld.') 1-23

Graphics Transformations and Layers

Graphics Transformations and Layers Graphics Transfrmatins and Layers CS111 Cmputer Prgramming Department f Cmputer Science Wellesley Cllege Review cs1graphics Things t knw: All ur graphics prgrams shuld start with: frm cs1graphics imprt