CS 105: COLLECTION TYPES Max Fowler (Computer Science) - PowerPoint PPT Presentation

CS 105: COLLECTION TYPES Max Fowler (Computer Science) https://pages.github-dev.cs.illinois.edu/cs-105/web/ June 21, 2020

Video Series Three Topics  Strings and Concatenation, String Formatting  Sequence Types (strings, lists, tuples), Immutable vs Mutable  Dictionaries

Strings (and Concatenation)

Remember Python's string representation  len() function let's us get a string's length  my_str = "CS 105"  my_str_len = len(my_str)  …What is the value in my_str_len?  Did you guess 6?

Reminder – string concatenation  If we have two inputs…  num1 = input() #Let's say 4  num2 = input() #Let's say 2  What is num1 + num2?  …42, of course!

Perils of string concatenation  We would like "New" + "York" to be "New York"  Instead, it is "NewYork"  Concatenation directly puts two sequences together – and there is no white space in "New" or "York"

strings are a Sequence Type  Sequences are ordered collections of things  Strings are specifically ordered collections of characters 'C' 'S' ' ' '1' '0' '5'  All Sequences  Can use len() operator  Can be concatenated with +  Can have multiple concatenation with *  "Max" * 5 = "MaxMaxMaxMaxMax"  Can be index with []  string variables are used to hold text information

Strings can be indexed  Indexing starts at 0  my_str = "CS 105"  print(my_str[3]) 0 1 2 3  It prints 1

Strings can also be formatted  Why format strings?  Image trying to make a multi-line message:  message = "Welcome " + first_name + " " + last_name  message += " to your first day of work at " + company + "."  Formatting gives us 'holes' we can fill in with data! (and reuse)  format_string = "Welcome {} {} to your first day of work at {}."  message = format_string.format(first_name, last_name, company)

Format Strings  a_string.format(parameters)  "We can {1} the {0}".format("order", "control")  "We can {0} info in our format. {0} as much as we want.".format("repeat")  "Format our money ${0:.2f}".format(25.223112)  More detail (and nicer) than book: https://pyformat.info/

Video Question – What use of format produces the following string using the variable price and num?  "You bought 3 cheesecakes for $25.25." price = 25.2432123 num = 3

Sequence Types in General And Immutable vs Mutable

Like strings, Lists are Sequence Types  Lists are different from strings in that they can contain ANY kind of objects as elements my_list = ["a string", 100, 3.1415]  Sequence operations still apply! len(my_list) my_list[2] my_list + [72, "Bob"] #Even concatenation!

Like strings, List indices start at 0 index 0 index 1 index 2 index 3 [10,20] “Strawberry” “Blueberry” 53

One of the more confusing differences  Strings are immutable: str1 str2 "hi" "hi" str1 = "hi" str2 = str1 "hi!" str1+="!" https://www.screengeek.net/2019/12/19/avengers-endgame-thanos- flaw/

Lists are mutable  list1 = ['a', 'b', 'c', 'd']  list2 = list1  list1.append('e')  What is the value of list2 after these lines?

Lists provide a number of modification functions  list1.append('f')  list1.insert(2, 'z')  list1.remove('c')  list1.pop(0)  list1.clear()  Change the value of existing elements  list1[1] = "an existing value changed"

What if you want an immutable list? 18  It's called a tuple tuple1 = ('a', 'b', 'c', 'd')

Video Question – How would you remove "Doughnut" from this list using pop? dessert_list = Cookie Cheesecake Doughnut Icecream Cake Cotton Candy

Dictionaries

What is a dictionary?  https://www.merriam-webster.com/words-at-play/an-explanation-of- why-we-sometimes-truncate-definitions

Lists vs Dictionaries A collection of values with their own labels and no A collection of real order! values in order

Dictionaries – key value pairs  sample_dict = {1: "Value one", "two": 2}  To access a value…  sample_dict[one]  To change a value  sample_dict["two"] = "New value"  To add a new key- value pair…  sample_dict[3] = "Like this!"

Dictionaries can be changed, but they don't use the same methods…  We don't append, we just use []  We don't pop or remove. In order to get rid of the key-value pair with the key "gold" below, what do we do?  sample_dict = {"gold": 25, "silver":35, "jade":10}  del sample_dict["gold"]

Video Question – Why might you use a dictionary?