Lecture 7 Variables Lecturer M icha el Ba ll while statement - PDF document

10/21/19 Computational Structures in Data Computational Concepts Toolbox Science • Data type: values, literals, • Iteration: operations, – data-driven (list – e.g., int, float, string comprehension) • Expressions, Call – control-driven (for expression statement) UC Berkeley EECS Lecture 7 • Variables Lecturer M icha el Ba ll – while statement • Assignment Statement • Higher Order Functions Abstract Data Types • Sequences: tuple, list – Functions as Values – indexing – Functions with functions as argument • Data structures – Assignment of function • Tuple assignment values • Call Expressions • Recursion • Function Definition • Lambda - function valued Statement expressions • Conditional Statement October 21, 2019 http:cs88.org 10/21/19 UCB CS88 Fa19 L7 2 1 2 Announcements Today’s Lecture • Abstract Data Types – More use of functions! • Midterm Tonight! – Value in documentation and clarity • Monday 10/21 7-9pm, 155 Dwinelle • New Python Data Types • 1 page, double-sided handwritten cheat sheet – Dictionaries, a really useful too! 3 10/21/19 UCB CS88 Fa19 L7 4 3 4 Why ADTs? Abstract Data Type • “Self-Documenting” – contact_name(contact) Operations Object » Vs contact[0] – “0” may seem clear now, but what about in a week? 3 months? A new Data Constructors Type • Change your implementation – Maybe today it’s just a Python List Selectors Internal Representation – Tomorrow: It could be a file on your computer; a database in web Operations Implementation on that Internal representation External Representation Interface Abstraction Barrier! 5 10/21/19 UCB CS88 Fa19 L7 6 5 6 1

10/21/19 Creating Abstractions Reminder: Lists • Lists • Compound values combine other values – Constructors : together » list( … ) – date: a year, a month, and a day » [ <exps>,… ] – geographic position: latitude and longitude » [<exp> for <var> in <list> [ if <exp> ] ] – Selectors : <list> [ <index or slice> ] – Operations : in, not in, +, *, len, min, max • • Data abstraction lets us manipulate compound » Mutable ones too (but not yet) values as units • Isolate two parts of any program that uses data: – How data are represented (as parts) – How data are manipulated (as units) • Data abstraction: A methodology by which functions enforce an abstraction barrier between representation and use 7 10/21/19 UCB CS88 Fa19 L7 8 7 8 A Small ADT Creating an Abtract Data Type • Constructors & Selectors def point(x, y): # constructor return [x, y] • Operations – Express the behavior of objects, invariants, etc x = lambda point: point[0] # selector – Implemented (abstractly) in terms of Constructors and Selectors for the object y = lambda point: point[1] • Representation def distance(p1, p2): # Operator – Implement the structure of the object return ((x(p2) - x(p1)**2 + (y(p2) - y(p1))**2) ** 0.5 • An abstraction barrier violation occurs when a part of the program that can use the higher level functions uses lower level ones instead origin = point(0, 0) my_house = point(5, 5) – At either layer of abstraction campus = point(25, 25) • Abstraction barriers make programs easier to get distance_to_campus = distance(my_house, campus) right, maintain, and modify – Few changes when representation changes 9 10/21/19 UCB CS88 Fa19 L7 10 9 10 Clicker ?: Changing Representations? An Abstract Data Type: Key-Value Pair Assuming we update our selectors, what are valid • Collection of key-Value bindings representations for our point(x, y) ADT? – Key : Value • Many real-world examples Currently point(1, 2) is represented as [1, 2] – Dictionary, Directory, Phone book, Course Schedule, Facebook Friends, Movie listings, … Given some Key, What is the value associated with it? • A) [y, x] # [2, 1] • B) “X: ” + str(x) + “ Y: ” + str(y) # “X: 1 Y: 2” • C) str(x) + “ ” + str(y) # “1 2” • D) All of the above • E) None of the above 11 10/21/19 UCB CS88 Fa19 L7 12 11 12 2

10/21/19 Key-Value ADT A little application • Constructors – kv_empty: create an empty KV phone_book_data = [ – kv_add: add a key:value binding to a KV ("Christine Strauch", "510-842-9235"), ("Frances Catal Buloan", "932-567-3241"), – kv_create: create a KV from a list of key,value tuples ("Jack Chow", "617-547-0923"), • Selectors ("Joy De Rosario", "310-912-6483"), ("Casey Casem", "415-432-9292"), – kv_items: list of (key,value) tuple in KV ("Lydia Lu", "707-341-1254") – kv_keys: list of keys in KV ] – kv_values: list of values in KV • Operations phone_book = pb_create(phone_book_data) – kv_len: number of bindings – kv_in: presence of a binding with a key print("Jack Chows's Number: ", pb_get(phone_book, "Jack Chow")) – kv_display: external representation of KV print("Area codes") area_codes = list(map(lambda x:x[0:3], pb_numbers(phone_book))) print(area_codes) 10/21/19 UCB CS88 Fa19 L7 13 10/21/19 UCB CS88 Fa19 14 13 14 A Layered Design Process Example 1 • Build the application based entirely on the ADT • KV represented as list of (key, value) pairs interface – Operations, Constructors and Selectors • Build the operations in ADT on Constructors and Selectors – Not the implementation representation • Build the constructors and selectors on some concrete representation 10/21/19 UCB CS88 Fa19 L7 15 10/21/19 UCB CS88 Fa19 L7 16 15 16 Dictionaries Dictionary Example • Lists, Tuples, Strings, Range • Dictionaries – Constructors : » dict( <list of 2-tuples> ) » dict( <key>=<val>, ...) # like kwargs » { <key exp>:<val exp>, … } » { <key>:<val> for <iteration expression> } – Selectors : <dict> [ <key> ] » <dict>.keys(), .items(), .values() » <dict>.get(key [, default] ) – Operations : » Key in, not in, len, min, max » <dict>[ <key> ] = <val> 10/21/19 UCB CS88 Fa19 L7 17 10/21/19 UCB CS88 Fa19 L7 18 17 18 3

10/21/19 Clicker ?: Dictionaries Limitations • What is the result of the final expression? • Dictionaries are unordered collections of key- value pairs • Dictionary keys have two restrictions: my_dict = { ‘course’: ’CS 88’, semester = ‘Fall’ } – A key of a dictionary cannot be a list or a dictionary (or any my_dict[‘semester’] = ’Spring’ mutable type ) – Two keys cannot be equal; There can be at most one value for a given key my_dict[‘semester’] This first restriction is tied to Python's underlying implementation of dictionaries A) ‘Fall’ The second restriction is part of the dictionary B) ‘Spring’ abstraction C) Error If you want to associate multiple values with a key, store them all in a sequence value 19 20 19 20 Beware Example 3 • KV represented as dict • Built-in data type dict relies on mutation – Clobbers the object, rather than “functional” – creating a new one • Throws an errors of key is not present • We will learn about mutation shortly 10/21/19 UCB CS88 Fa19 L7 21 10/21/19 UCB CS88 Fa19 L7 22 21 22 C.O.R.E concepts Building Apps over KV ADT friend_data = [ Perform useful computations treating objects abstractly as ("Christine Strauch", "Jack Chow"), C ompute whole values and operating on ("Christine Strauch", "Lydia Lu"), them. ("Jack Chow", "Christine Strauch"), Abstract Data Type ("Casey Casem", "Christine Strauch"), Provide operations on the O perations ("Casey Casem", "Jack Chow"), abstract components that allow ("Casey Casem", "Frances Catal Buloan"), ease of use – independent of ("Casey Casem", "Joy De Rosario"), concrete representation. ("Casey Casem", "Casey Casem"), ("Frances Catal Buloan", "Jack Chow"), R epresentation Constructors and selectors that ("Jack Chow", "Frances Catal Buloan"), provide an abstract interface to ("Joy De Rosario", "Lydia Lu"), a concrete representation ("Joy De Lydia", "Jack Chow") Execution on a computing ] E valuation machine • Construct a table of the friend list for each person Abstraction Barrier 10/21/19 UCB CS88 Fa19 L7 23 10/21/19 UCB CS88 Fa19 L7 24 23 24 4