Lecture 8: Optional 1 on 1 with a staff member to help just you - PDF document

No-laptop front Announcements http://www.cs.cornell.edu/courses/cs1110/2020sp zone on your left ok Lecture 8: • Optional 1 ‐ on ‐ 1 with a staff member to help just you Conditionals & Control Flow with course material. Sign up for a slot on CMS under (Sections 5.1 ‐ 5.7) “SPECIAL: one ‐ on ‐ ones“. CS 1110 • A1 first submission due Feb 19 Wedn at 11:59pm Introduction to Computing Using Python [E. Andersen, A. Bracy, D. Fan, D. Gries, L. Lee, 2 S. Marschner, C. Van Loan, W. White] Swap (Question) What is in p.x at the end of this code? import shapes p = shapes.Point3(1,2,3) A: 1 q = shapes.Point3(3,4,5) B: 2 C: 3 def swap_x(p, q): D: I don’t know 1 t = p.x Review: Objects are referenced Heap ap Space ace 2 p.x = q.x 3 q.x = t id1 id2 ‐ Must call constructor function to create object swap_x(p, q) Point3 Point3 ‐ Object variable stores ID of object Global al Space ace x 1 x 3 ‐ Multiple variables can reference same object p id1 y 2 y 4 z 3 q id2 z 5 4 Global p (Question) Methods: Functions Tied to Classes import shapes • Method : function tied to object What is in global p after calling swap? p = shapes.Point3(1,2,3) u id3  Method call looks like a function q = shapes.Point3(3,4,5) A: id1 call preceded by a variable name: B: id2 def swap(p, q): ⟨ variable ⟩ . ⟨ method ⟩ ( ⟨ arguments ⟩ ) C: I don’t know id3 1 t = p Heap ap Space ace 2 p = q Example : Point3 4 x 3 q = t import shapes id1 id2 2 y swap(p, q) u = shapes.Point3(4,2,3) Point3 Point3 u.greet() Global al Space ace 3 z x 1 x 3 “Hi! I am a 3-dimensional point p id1 y 2 located at (4,2,3)” y 4 z 3 q id2 z 5 Where else have you seen this?? 7 10 1

Example: String Methods Built-in Types vs. Classes • s 1 .upper() • s 1 .index(s 2 ) Built-in types Classes  Returns returns an upper case  Returns position of the first version of s 1 instance of s 2 in s 1 • Built-into Python • Provided by modules  error if s 2 is not in s 1 • s.strip() • Refer to instances as values • Refer to instances as objects • s 1 .count(s 2 )  Returns a copy of s with • Instantiate with literals • Instantiate w/ constructors white-space removed at ends  Returns number of times s 2 • Can ignore the folders • Must represent with folders appears inside of s 1 So far only about understanding objects ; later will create your own classes 11 12 Big Picture Big Picture Conditionals: Co nditionals: I If-Statements -Statements Format For Example Exampl Statements either affect data or control • DATA: change the value of a variable, create a # is there a new high score? if < boolean-expression >: variable, etc. < statement > if curr_score > high_score: if … Examples: high_score = curr_score < statement > print(“New high score!”) x = x + 1 name = “Alex” • CONTROL: tell python what line to execute next Execution : Examples: if ⟨ boolean-expression ⟩ is true, then execute all of the statements greet(name) indented directly underneath (until first non-indented statement) if name == “Alex”:  today’s Lecture 13 14 What are Bool What are Boolean expre expressions ons? What g What gets pr ts printed inted, Ro Round 1 und 1 Expressions that evaluate to a Boolean value. a = 0 a = 0 a = 0 a = 0 a = 0 print(a) a = a + 1 if a == 0: if a == 1: if a == 0: Boolean olean oper operati ations ns: is_student = True print(a) a = a + 1 a = a + 1 a = a + 1 is_senior = False if is_s is_student nt and and is is_sen _senior ior: print(a) print(a) a = a + 1 print(“Hi senior student!”) num_credits = 25 print(a) Boo Boolean var variables: Comp mparison arison o operations: erations: if is_st is_studen dent: if num_c num_credits dits > 24 24: print(“Hi student!”) print(“Are you serious?”) 15 16 2

What g What gets pr ts printed inted? ( (Question) uestion) Conditionals: If-Else-Statements Format Example a = 0 if a == 0: # new record? if < boolean-expression >: A: 0 a = a + 1 < statement > if curr_score > high_score: if B: 1 … if a == 0: print(“New record!”) C: 2 else : a = a + 2 else: el D: 3 < statement > print(“Try again next time”) a = a + 1 E: I do not know … Execution : print(a) if ⟨ boolean-expression ⟩ is true, then execute statements indented under if ; otherwise execute the statements indented under else 18 20 Conditionals: Co nditionals: “Co “Control F trol Flow” S ” Statements ements What g What gets pr ts printed inted, Ro Round 2 und 2 Branch Point: b a = 0 a = 0 a = 0 a = 0 if b : True Evaluate & Choose if a == 0: if a == 1: if a == 1: if a == 1: Statements: False s1 # statement s1 Execute a = a + 1 a = a + 1 a = a + 1 a = a + 1 s3 # statement else: else: else: else: s3 a = a + 2 a = a + 2 a = a + 2 a = a + 1 if b : b a = a + 1 a = a + 1 Flow False s1 Program only print(a) print(a) print(a) a = a + 1 True takes one path else : s1 s2 print(a) during an execution s2 (something will s3 s3 not be executed!) 21 22 Pr Progr ogram F m Flow ( ow (car lo ar locked cked, 0) , 0) Pr Progr ogram F m Flow ( ow (car no ar not lo t locked cked, 0) , 0) if determines which statement is executed next if determines which statement is executed next if if Global al Space ace Global al Space ace def get_in_car(car_locked): def get_in_car(car_locked): 1 if car_locked: 1 if car_locked: 2 print(“Unlock car!”) 2 print(“Unlock car!”) print(“Open the door.”) print(“Open the door.”) 3 3 car_locked = True car_locked = False get_in_car(car_locked) get_in_car(car_locked) 24 30 3

What does What does the the call frame call frame look look like next? (Q) like next? (Q) Progr Pr ogram F m Flow and Var ow and Variable ables Variables created inside if continue to exist past if : def max(x,y): if x > y: 1 a = 0 2 return x if a == 0: b = a + 1 3 return y print(b) max(0,3) Current call frame: max 1 …but are only created if the program actually executes that line of code x 0 y 3 35 41 Co Control F ntrol Flow a ow and V Variables ables ( (Q1) 1) Co Control F ntrol Flow a ow and V Variables ables ( (Q2) 2) def max(x,y): Value of maximum? def max(x,y): Value of maximum? """Returns: max of x, y""" """Returns: max of x, y""" A: 3 A: 3 # note: code has a bug! # note: code has a bug! B: 0 B: 0 # check if x is larger # check if x is larger C: Error! C: Error! if x > y: if x > y: D: I do not know D: I do not know bigger = x bigger = x return bigger return bigger maximum = max(3,0) maximum = max(0,3) 44 46 Progr Pr ogram F m Flow and Var ow and Variable ables Conditionals: If-Elif-Else-Statements def zero_or_one(a): Format Example if a == 1: if < Boolean expression >: # Find the winner make sure that ALL b = 1 < statement > if score1 > score2: if … if branches create if else: winner = “Player 1” elif < Boolean expression >: the variable elif score2 > score1: el < statement > b = 0 … winner = “Player 2” print(b) … el else: else : winner = “Players 1 and 2" < statement > … 48 49 4

If-E -Elif-El lif-Else ( e (Ques uestion) ion) Conditionals: If-Elif-Else-Statements Format Notes on Use a = 2 What gets printed? • No limit on number of elif if < Boolean expression >: if a == 2: A: 2 < statement >  Must be between if , else a = 3 B: 3 … • else is optional elif a == 3: C: 4 elif < Boolean expression >:  if - elif by itself is fine a = 4 D: I do not know < statement > • Booleans checked in order … print(a)  Once Python finds a true … < Boolean-expression >, skips else : over all the others < statement >  else means all < Boolean- … expression > are false 50 51 What g What gets pr ts printed inted, Ro Round 3 und 3 Ne Nested Co ed Conditionals nditionals a = 2 a = 2 def what_to_wear(raining, freezing): if raining: if if a == 2: if a == 2: if if freezing: a = 3 a = 3 print(”Wear a waterproof coat.”) elif a == 3: el if if a == 3: else: a = 4 a = 4 print(”Bring an umbrella.") pr print(a) a) pr print(a) a) else: if freezing: print(”Wear a warm coat!") else: print(”A sweater will suffice.") 53 55 Pr Progr ogram F m Flow and Testing ow and Testing Can use print statements # Put max of x, y in z to examine program flow print('before if’) if if x > y: 'before if’ print(‘inside if x>y’) “traces” or z = x ‘inside if x>y‘ “breadcrumbs” 'after if' el else: x must have been greater print(‘inside else (x<=y)’) than y z = y print('after if’) 57 5