Lecture 4: Functions and Conditionals

Craig Zilles (Computer Science)

September 20, 2019

https://go.illinois.edu/cs105fa19

CS 105

Are you sitting next to someone to talk to for the clicker

questions?

Today I'm using: pythontutor.com

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Big Picture (Muddiest Points)

• 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.

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Today1. Warmup2. 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

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Negative Indexing

What is the value of the above expression?

A) 'a'B) 'b'C) 'c'D) 'd'E) 'e'

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"abcde"[-2]

User-defined Functionsusing recipes in recipes

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User-defined Functions• 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()

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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.

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Code Blocks• Need a way to tell Python "this statements are related"• Python uses indentation

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Code Block A

Code Block B(Execution determined by control flow construct)

Control flow construct:

Code Block C(Same indentation as A)

Indentation• 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

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def test():print('first')print('second')

test()

def test():print('first')

print('second')

test()

def test():print('first')print('second')

test()

What does this program output?

• A) it raises an error

• B)

• C)

• D)

• E)

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first

firstsecond

secondfirst

firstsecond

def test():print('first')

print('second')

test()

Parameters, Arguments, Return Values

def welcome_message(first, last):

message = "Welcome " + first + " " + last

message += " to CS 105!"

return message

welcome_message("Harry", "Potter")

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Function Compositiondef f(x):

return 3 * x

What value is returned by f(f(2))?A) 3B) 6C) 9D) 12E) 18

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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!")

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Functions vs. 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)

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Announcements• Quiz 1 next week

• Taken at home on PrairieLearn• To be done Alone!

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How much total time did you spend in the past week on CS 105?• Lecture + Lab = 3 hours• Readings + Preflights + HW + Office hours + Exam

• Which of these Boolean expressions are True for youA) hours_spent < 6 hoursB) 6 hours <= hours_spent < 9 hoursC) 9 hours <= hours_spent < 11 hoursD) 11 hours <= hours_spent < 13 hoursE) hours_spent >= 13 hours

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How was Exam 0 for you?• A) Great, no problems at all!• B) Fine• C) Whatever• D) Not a fan• E) Terrible, I hated it!

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Exam 0• Seemed to go pretty smoothly

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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

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Boolean Expressions• Expressions that evaluate to True or False

(1 + 6) < (2 + 5)

• A) True• B) False• C) Raises an error

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Relational and membership operators

• When to use = and when use == ?

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Suppose young is a Boolean variable

why doesif young == true:not work whenif youngdoes?

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Relational Ops on non-numbers• Why lower case letters are greater than upper case

letters?

• People often normalize case before comparisons

thing1.lower() < thing2.lower()

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Truthy and 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:

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Falsy

valu

es

Boolean operators• 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)

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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 ishould evaluate first.

• Relational operators evaluate before Boolean ops.• and evaluates before orx == 7 and y == 3 or x > 12 and y < -12• Avoid relying on operator precedence; use parentheses

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x==3 or 4

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Short Circuiting• 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

True or anything() is TrueFalse and anything() is False

• 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')

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What does this program output?

• A) it raises an error

• B)

• C)

• D)

• E) it prints nothing, but raises no errors

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hello

there

hellothere

print('hello') and print('there')

Conditionals• 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.

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Shape of "decision tree": if w/o elseAsked 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)

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Shape of "decision tree": if w/elseCompany 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)

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Choosing between many alternativesFinal 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'

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Multi-way branches in general?If you were choosing between 6 possibilities, how many elif statements would you have:

A) 1B) 2C) 3D) 4E) 5

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