CS 105 Lecture 4: Functions and Conditionals Craig Zilles (Computer Science) https://go.illinois.edu/cs105fa19 September 20, 2019
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 2. Functions • Indentation / Code Blocks • Parameters and Arguments • Return Values 3. Boolean Expressions • Relational operators: ==, !=, <, >, <=, >= • Boolean operators: and, or, not 4. Conditionals • if, elif, else • Nesting 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
Us User-de define ned d Func unctions ns using recipes in recipes 6
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() 7
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. 8
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) 9
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() 10
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 11
Paramet eters, Arguments, Ret eturn Values def welcome_message(first, last): message = "Welcome " + first + " " + last message += " to CS 105!" return message welcome_message("Harry", "Potter") 12
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 13
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!") 14
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) 15
An Annou ounce cements ts • Quiz 1 next week • Taken at home on PrairieLearn • To be done Alone! 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 + Exam • Which of these Boolean 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
Ho How was Ex Exam m 0 for you? u? • A) Great, no problems at all! • B) Fine • C) Whatever • D) Not a fan • E) Terrible, I hated it! 18
Ex Exam m 0 • Seemed to go pretty smoothly 19
Bool Booleans • I don't understand what Booleans are and how to use them, why they're used, etc. • There are two Boolean values: True , False • Used for making decisions: • Do something or don't do something 20
Bool Boolean Expression ons • Expressions that evaluate to True or False (1 + 6) < (2 + 5) • A) True • B) False • C) Raises an error 21
Re Relational and membership operators • When to use = and when use == ? 22
Suppose young is a Boolean variable why does if young == true: not work when if young does? 23
Re Relational Ops on non-numbe numbers • Why lower case letters are greater than upper case letters? • People often normalize case before comparisons thing1.lower() < thing2.lower() 24
Tr Truthy and Fa Falsy • Python will convert non-Boolean types to Booleans if "hello": • You can force conversion using bool() function. • All values are truthy (convert to True ) except: Falsy values 25
Bool Boolean op operator ors • Why is x==3 or 4 always True? I am still confused with this concept. • "If you've finished your homework and done your chores then you can go out." • Binary operators: and or • Unary operator: not • Operate on Booleans (or coerce to Booleans) 26
Pr Precedence • I'm still confused about the sequence of the operations when it has both Boolean operators and other kinds of operators. • Order of evaluation is confusing. When I was comparing 'and' or 'or' to a symbol, it is hard to tell which one i should evaluate first. • Relational operators evaluate before Boolean ops. and evaluates before or • x == 7 and y == 3 or x > 12 and y < -12 • Avoid relying on operator precedence; use parentheses 27
x==3 or 4 28
Sh Shor ort t Ci Circu cuiti ting • i am confused about the concept of short circuit • Python is lazy (which is a good thing if you understand it) • It won't evaluate Boolean expressions it doesn't need to is True True or anything() is False False and anything() • Python won't evaluate the anything() part • Can use this to avoid running code that would get errors (len(my_str) > 10) and (my_str[10] == 'a') 29
Wh What does this program output? print('hello') and print('there') • A) it raises an error • B) hello • C) there • D) hello there • E) it prints nothing, but raises no errors 30
Con Conditi tion onals • if, elif, else • I'm confused about when to use multiple distinct if statements or if-elif-else. • I'm not too sure about the concept of "elif" • I don't understand how each of the components of the chapter can be used in real world cases. A thing I like to do to help me better grasp the concepts, is imagine them happening in this world. So giving me more mundane scenarios of where we would be using these things would help a lot. 31
Sh Shape of of "d "deci cision on tr tree": ": if w/o o else Asked my TA to send email to all students in the class that didn't take Exam 0. • Step 1: make a set of all students that took Exam 0 • Step 2: check each student in class if in the set if student in exam0_takers: send_email(student) 32
Sh Shape of of "d "deci cision on tr tree": ": if w/else Company sends recruiting invitations to students with Python in their resume, sends 'nack' email to others if 'python' in resume.lower(): send_invitation(student) else: send_polite_decline(student) 33
Ch Choos oosing betw tween many alternati tives Final exam location based on first letter of netid: [a-j] Loomis 100 [k-o] DCL 1320 [p-z] English 214 first_char = netid.lower()[0] if first_char <= 'j': location = 'Loomis 100' elif first_char <= o: location = 'DCL 1320' else: location = 'English 214' 34
Mul Multi-wa way branches in general? If you were choosing between 6 possibilities, how many elif statements would you have: A) 1 B) 2 C) 3 D) 4 E) 5 35
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