CS 111: Program Design I Lecture # 7: First Loop, Web Crawler, Functions

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1 CS 111: Program Desig I Lecture # 7: First Loop, Web Crawler, Fuctios Robert H. Sloa & Richard Warer Uiversity of Illiois at Chicago September 18, 2018

2 What will this prit? x = 5 if x == 3: prit("hi!") prit("bye!") A. Nothig B. "Hi!" C. "Bye!" D. "Hi!" ad "Bye!"

3 What will this prit? x = 5 if x == 5: prit("hi!") prit("bye!") A. Nothig B. "Hi!" C. "Bye!" D. "Hi!" ad "Bye!"

4 What will the value of z be after this code rus? def foo(x): if x!= 3: retur 1 retur 2 A. 1 B. 2 C. This will cause a error z = foo(-1)

5 What will the value of z be after this code rus? def foo(x): if x!= 3: prit(1) retur 2 A. 1 B. 2 C. This will cause a error or odd uexpected result z = foo(-1)

6 shiftig by k for very short strigs def shift(s, k): if le(s) == 1: retur rotate(s, k) if le(s) == 2: as = (rotate(s[0], k) + rotate(s[1], k)) retur as prit("sorry, ca't help you")

7 From legth 2 to 3 def shift(s, k): if le(s) == 1: retur rotate(s, k) if le(s) == 2: as = (rotate(s[0], k) + rotate(s[1], k)) retur as if le(s) == 3: retur (rotate(s[0], k) + rotate(s[1], k) + rotate(s[2], k)) prit("sorry, ca't help you")

8 Or eve 4 def shift(s, k): if le(s) == 1: retur rotate(s, k) if le(s) == 2: retur rotate(s[0], k) + rotate(s[1], k) if le(s) == 3: retur (rotate(s[0], k) + rotate(s[1], k) + rotate(s[2], k)) if le(s) == 4: retur (rotate(s[0], k) + rotate(s[1], k) + rotate(s[2], k) + rotate(s[3], k)) prit("sorry, ca't help you")

9 But This will be a real drag eve for 140 character tweets Imagie the 5-page report with 15,000 characters.... We eed for loops q for loops allow us to do same thig for every item i a sequece!

10 FIRST LOOK AT FOR LOOPS

11 for loop basic idea for c i st: <body that refers to c> execute body le(st) times, oce each with c beig each character of strig st i order (I geeral, st could be ay sequece, e.g., a list ad c becomes each item of st oce)

12 Example: d detector I [1]: d('i would like dodecarchy i the US') Out[1]: 3 I [2]: d('i am agaist the letter after c') Out[2]: 0 def d(iput): couter = 0 for symbol i iput:

13 Example: d detector I [1]: d('i would like dodecarchy i the US') Out[1]: 3 I [2]: d('i am agaist the letter after c') Out[2]: 0 def d(iput): couter = 0 for symbol i iput: if symbol == 'd': retur couter We choose the ame of a variable couter = couter + 1 ad we provide a sequece

14 This will prit? for x i "0123": prit(x) A. 0 D. 0 1 B C. 0 1 E. This will 2 ru 3 forever

15 Lab Hit 1: Build up aswer i for loop Very ofte build up aswer to retur iside for loop ad retur it outside loop, after its ed: def d(iput): couter = 0 for symbol i iput: if symbol == 'd': couter = couter + 1 retur couter

16 Example Retur strig with lower-case characters replaced by X, all other left uchaged def x_it(iput): """Example for lecture slides""" aswer = '' #Empty strig for c i iput: ext = c if ext.islower(): ext = 'X' aswer = aswer + ext retur aswer

17 Usig else istead def x_else_it(iput): """Example with if-else istead of if""" aswer = '' #Empty strig for c i iput: if c.islower(): aswer = aswer + 'X' else: aswer = aswer + c retur aswer

18 Lab Hits 2/Remider word = 'hi' word.upper() à "HI" "h".isalpha() à True 'h'.islower() à True 'h'.isupper() à False

19 Towards Crawlig the Web (MORE ABOUT) FUNCTIONS

20 Web crawler Oe log-term goal of course: build ad uderstad web crawler, program that will visit every page reachable from give start web page q Key compoet of, e.g., search egie May pieces, somewhat complicated q q Need a orgaizig priciple: fuctios! Also eed to do thigs over ad over: iteratio Will retur to crawler from time to time

21 Fuctios: defiitio & use Ca do 2 thigs with fuctio: Defie it; Call it Defiitio: def f_ame(parameters): q E.g., def strig_multiply(my_strig, um): Call (use) f_ame(parameters) q E.g, strig_multiply("hi", 3) q Rus fuctio f_ame o parameters

22 Note: Defiitio must have some ideted code after the def f_ame(): lie This is ot a legal fuctio defiitio: def othigess(): But def othigess(): retur q is legal (though useless) fuctio defiitio

23 Iput parameters (1) Most fuctios have 1 iput parameters (though legal & sometimes appropriate to create fuctio with o iput parameters). Example of built-i fuctio (techically specifically a method fuctio) with zero iputs: Strig method upper(): word = "hi" word.upper() à "HI"

24 Retur values fuctio may or may ot retur a value If eed termiology, call fuctio that returs value fruitful fuctio; fuctio that does't o-fruitful fuctio If (ad oly if) fuctio returs value, legal to assig ame to (retur value of) fuctio call: x = fruitful_f_ame(iputs)

25 Example Say we wat to fid absolute value of a umber (say -3) There is built-i Pytho fuctio called abs that fids absolute value I [1]: x = abs(-3) I [2]: x Out[2]: 3

26 Most famous built-i o-fruitful fuctio prit() prit does't retur ay value. q We do't use prit (or ay o-fruitful fuctio) for its retur value but for some other reaso

27 fuctios you write that retur somethig Must iclude a lie that begis retur

28 fuctio flow A fuctio's executio eds either whe 1. A retur statemet is executed, or 2. Last lie of code is executed q whichever comes first

29 What is wrog with this fuctio? def triple(x): retur 3 * x prit("triple the iput is", 3*x) A. You should ever use a prit statemet i a fuctio B. You must calculate the value of 3*x before you retur it C. You should ot have statemets after the retur D. A fuctio ca retur strig types but ot a umber E. Nothig

30 Fuctio as iput-output box z abs z x triple 3 * x

31 Parameters (1) (actual) parameter >>> y = abs(-3) >>> y 3

32 Parameters (2) Parameters i the ()s i def statemet called formal parameters Formal parameters are (like) variables : they're the thig that chages iside the fuctio Fuctios have 0 or more parameters Value(s) i fuctio call: actual parameter(s) q q Same umber as formal parameters Could be variables ad/or literal values

33 Formal vs. actual parameters def triple(x): retur 3 * x formal parameter I [1]: = 17 I [2]: triple() Out[2]: 51 I [3]: 4 + triple(20) Out[3]: 64

34 Formal vs. actual parameters def triple(x): retur 3 * x formal parameter (which happes to be x i this example) I [1]: = 17 I [2]: triple() Out[2]: 51 I [3]: 4 + triple(20) Out[3]: 64

35 Formal vs. actual parameters def triple(x): retur 3 * x I [1]: = 17 I [2]: triple() Out[2]: 51 I [3]: 4 + triple(20) Out[3]: 64 formal parameter (which happes to be x i this example) actual parameter

36 Parameters ad fuctio executio Like fuctios i high-school Algebra 2: q At time fuctio called, formal parameter takes o value of actual parameter Algebra 2: q if f(x) = 3x, the 4 + f(20) = 64 ad implicitly at least, the formal parameter x took o the value 20. Pytho: 4 + triple(x) q formal x boud to 3 for legth of ru of triple()

37 I eve more detail def triple(x): retur 3 * x formal parameter >>> 4 + triple(20)

38 I eve more detail def triple(x): retur 3 * x formal parameter x boud to 20 >>> 4 + triple(20) At poit where triple(20) is called, value 20 is assiged to triple's iteral x parameter, multiplicatio is doe gettig value 60, umber 60 is retured (ad triple is doe), ad iterpreter (commad lie) adds 4 ad 60

39 A Plea Please start projects due Suday ight way early q q We would love to help you Thursday afteroo We will try to check Piazza regularly from 6:30 pm Friday util 11 pm Suday, but wo't be early as reliable as Thursday or Friday at 2:15 pm

CS 111: Program Design I Lecture # 7: Web Crawler, Functions; Open Access

CS 111: Program Design I Lecture # 7: Web Crawler, Functions; Open Access CS 111: Program Desig I Lecture # 7: Web Crawler, Fuctios; Ope Access Robert H. Sloa & Richard Warer Uiversity of Illiois at Chicago September 13, 2016 Lab Hit/Remider word = "hi" word.upper() à "HI" Questio