CS 105 Lecture 4: Functions Craig Zilles (Computer Science) - - PowerPoint PPT Presentation

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CS 105 Lecture 4: Functions Craig Zilles (Computer Science) - - PowerPoint PPT Presentation

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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

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Lecture 4: Functions

Craig Zilles (Computer Science) February 16, 2020 https://go.illinois.edu/cs105fa19

CS 105

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Are you sitti ting next t to someone to talk to for th the clicker questi tions? To Today I'm using: pythontutor.com

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Bi Big Pictu cture (Muddiest t Poi

  • ints

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.

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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

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Neg Negative e Indexi xing

What is the value of the above expression? A) 'a' B) 'b' C) 'c' D) 'd' E) 'e'

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"abcde"[-2]

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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

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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)

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Wh What type of data collection is this?

{"Craig", "Anant", "Sofia", "Chinny"} A) Dictionary B) List C) Set D) String E) Tuple

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Us User-de define ned d Func unctions ns

using recipes in recipes

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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>()

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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()

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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.

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Cod Code Bl Block

  • cks
  • 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)

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In Indentation 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

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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()

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Wh What does this program output?

  • A) it raises an error
  • B)
  • C)
  • D)
  • E)

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first first second second first first second

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

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An Annou

  • unce

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)

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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

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Pa Parameters

  • 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

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def welcome_message(first, last): # function body here … welcome_message("Harry", "Potter")

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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

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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

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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

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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)

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Wh Which assigns x to 5?

def f1(): return 5 def f2(): print(5) def f3(): return print(5)

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A) x = f1() B) x = f2() C) x = f3() D) All of the above E) None of the above

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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")

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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

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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!")

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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)

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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

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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

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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')

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