CS 105 Lecture 4: Functions Craig Zilles (Computer Science) https://go.illinois.edu/cs105fa19 February 16, 2020
Are you sitti ting next t to someone to talk to for th the clicker questi tions? To Today I'm using: pythontutor.com 2
Bi Big Pictu cture (Muddiest t Poi oints ts) • if the challenge questions are too hard can we leave some of it? • In general, I'd say that there is no single concept that confuses me most as of right now. I kind of understand why I need to do what I need to do in coding, but I am still foggy as to where each and every little character goes, and what makes something a syntax error and what doesn't. 3
To Today 1. Warmup • Negative indices 2. Functions • Indentation / Code Blocks • Parameters and Arguments • Return Values & None 3. Functions in Excel 4. Next week's reading: Booleans & Conditionals 4
Neg Negative e Indexi xing "abcde"[-2] What is the value of the above expression? A) 'a' B) 'b' C) 'c' D) 'd' E) 'e' 5
Se Sets ts • When to use: • You have a collection of similar things • You want to know if something is in the set or not • You want to do set operations (e.g., intersection) • Finding birthdays shared by two or more people in the class 6
Di Dictionaries es • A mapping type : given a piece of info, find related info • Key-Value pairs • Keys must be unique , values can be repeated • What is it used for? • UIN -> student record • Cell phone number -> cell tower (service provider) 7
Wh What type of data collection is this? {"Craig", "Anant", "Sofia", "Chinny"} A) Dictionary B) List C) Set D) String E) Tuple 8
Us User-de define ned d Func unctions ns using recipes in recipes 9
Us User-de define ned d Func unctions ns • Sequences of operations for use elsewhere in program • Function definition says what to do def <name> (): <body> • Function calls/invocations actually run the function <name>() 10
Us User-de define ned d Func unctions ns • Sequences of operations for use elsewhere in program • Function definition says what to do def get_input_and_print(): name = input("Your name?\n") print("Hello " + name + "!") • Function calls/invocations actually run the function get_input_and_print() 11
Re Representative Muddiest Points • The sections on parameters and returns made no sense to me. I don't understand when to indent something, or what to return and when. • I feel like I do not understand what code block and indentations stand for in Python. 12
Cod Code Bl Block ocks • Need a way to tell Python "this statements are related" • Python uses indentation Code Block A Control flow construct: Code Block B (Execution determined by control flow construct) Code Block C (Same indentation as A) 13
Indentation In tion • In other prog. languages, indentation is just good style • In Python, it is syntactic and semantic • These three programs are all different • Text is same, white space and behavior is different def test(): def test(): def test(): print('first') print('first') print('first') print('second') print('second') print('second') test() test() test() 14
Wh What does this program output? def test(): print('first') print('second') • A) it raises an error test() • B) first • C) first second • D) first second • E) second first 15
An Annou ounce cements ts • We have reading assignments due every Saturday • Please don't make this a source of drama • Muddiest Points should be about most recent reading • Tutoring available! cs105-tutor@illinois.edu • Lab this week: Debugging! (and intro. to Colab) • Quiz 1 this week (Thursday - Saturday ) • Taken at home on PrairieLearn • To be done Alone! • Practice exam up soon (but do HW#4 first anyway) 16
Ho How much to total time di did y d you spe u spend i nd in n th the past t week on CS 105? • Lecture + Lab = 3 hours • Readings + Preflights + HW + Office hours + Exam0 • Which of these expressions are True for you A) hours_spent < 6 hours B) 6 hours <= hours_spent < 9 hours C) 9 hours <= hours_spent < 11 hours D) 11 hours <= hours_spent < 13 hours E) hours_spent >= 13 hours 17
Pa Parameters def welcome_message(first, last): # function body here … welcome_message("Harry", "Potter") • Passing parameters is just like an assignment statement • Binds new name to an existing value within the function • Code inside the function can access value using name • Name disappears when function is over 18
Wh What value is printed? def do_thing(v1, v2, v3): a = v2 b = v1 + 1 print(a * b) do_thing(3, 2, 0) A) 0 B) 6 C) 8 D) 9 E) any other number 19
Wh What value is printed? def do_thing(var1): var1 = 2 var1 = 1 do_thing(var1) print(var1) A) 0 B) 1 C) 2 D) any other number E) Error occurs 20
Wh What value is printed? def do_thing(var1): var1.append(4) var1 = [1, 2, 3] do_thing(var1) print(len(var1)) A) 0 B) 3 C) 4 D) any other number E) Error occurs 21
Re Return Values • If we need a function to produce a value, return will exit the function, and replace the function call with the returned value • Function calls can be part of arbitrary expressions • If there is no return, the function terminates when it reaches the bottom (and returns None) 22
Wh Which assigns x to 5? A) x = f1() def f1(): return 5 B) x = f2() def f2(): C) x = f3() print(5) D) All of the above def f3(): return print(5) E) None of the above 23
Paramet eters, Arguments, Ret eturn Values def welcome_message(first, last): message = "Welcome " + first + " " + last message += " to CS 105!" return message msg = welcome_message("Harry", "Potter") 24
Fu Function Composi sition def f(x): return 3 * x What value is returned by f(f(2)) ? A) 3 B) 6 C) 9 D) 12 E) 18 25
No None • I don't understand the value of None, and what it means when you don't have the return statement. • Would there ever be a time that we would need a function to return the value of "none"? • Mostly this is important to know because you might do something like this by accident: x = print("hi there!") 26
Fu Functions s vs. s. Methods • Methods are functions that are part of an object/type • They use dot notation • For example: my_list = [0, 1, 2, 3] my_list.append(22) • Functions, in contrast: len(my_list) 27
Fu Functions s in Excel el • Excel provides many useful "built-in" functions: • E.g., SUM(), AVERAGE(), MIN() • Take arguments: cells, cell ranges • Produce return values • Can be part of expressions & assignments 28
Wh What bugs are in the following code? def add_one(x): return x + 1 x = 2 x = x + add_one(x) A) No bugs. The code is fine. B) The function body is not indented. C) We use x as both a parameter and a variable, but we are not allowed to do that D) B and C 29
Ne Next week eek's s rea eading • Sometimes we want to "conditionally" execute code • Send email to a student only if they missed Exam 0 • Booleans: a variable type that is either True or False • Relational statements: Boolean from comparing 2 things • x < 7 • Boolean operators: using multiple relational statements • x < 7 and z > 2 • Conditional blocks: execute code only if condition is true if x < 7: print('hi') 30
Recommend
More recommend
