Lecture 3: Collection Types and Functions

Craig Zilles (Computer Science)Al Harris (Computer Science)

September 13, 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)• Do we have to memorize all the functions of the lists,

dictionaries, removing an element, adding an element, etc. for exams and such?

• Is there a tip or way to remember what to use at specific times?

• I do not understand what is happening in each line of code and why, it is so confusing!

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Collections• I mainly had trouble differentiating between the

sequence types and what each one does.

• The most confusing part of the material for me was keeping straight the differences between lists, sets, and dictionaries. It would be helpful to have a more concise review of the different properties and applications of each one.

• What is the purpose of a dictionary? Is it permanently stored until instructed otherwise?

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Today1. Strings

• Concatenation2. Sequence Types (strings, lists, tuples)

• Indexing and Functions3. Immutable vs. Mutable4. Sets and Dictionaries5. Functions

• Parameters• Return Values

6. String Formating

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Strings• Last time, we showed Python's string representation:

• The len() function is just reading the string's lengthmy_str = "CS 105"my_str_length = len(my_str)

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String 6 001000011 001010011

TypeNumber ofcharacters

Characters(Stored using Unicode encoding)

000100000 000110001 000110000 000110101‘CS 105’

C S 1 0 5

What is string concatenation and how is that different than string literals?

What is the value of the above expression?

A) "Champaign-Urbana"B) "Champaign""Urbana"C) Champaign UrbanaD) "ChampaignUrbana"E) "Champaign Urbana"

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"Champaign" + "Urbana"

String Concatenation• string concatenation, why like "New" + "York" results "NewYork", I

thought it should be "New York"

• What's going on: "New" + "York"

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strings are a kind of Sequence Type

• Sequences are ordered collections • Strings are ordered collections of characters

• Sequences Types:• Have len() operator• Can be concatenated with +• Multiple concatenation with *

• "Craig" * 3 is "CraigCraigCraig"• Can be indexed with []

• Why used? To hold text information.10

Indexing• Sequences are 0 indexed:

my_str = "CS 105"my_char = my_str[1]

What is the value of my_char?a) 'C' b) 'S' c) ' ' d) '1'

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String 6 001000011 001010011

TypeNumber ofcharacters

Characters(Stored using Unicode encoding)

000100000 000110001 000110000 000110101‘CS 105’

C S 1 0 5

Lists are also Sequence Types

• Lists are sequences of any kind of objects ("elements")my_list = ["a string", 100, 3.14159]

len(my_list)my_list[2]my_list + [72, "Fred"]

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Sequence Type

string list

Why are commas allowed in lists/adding to a list but not in regular statements or in

int/floats?

What are lists used for?

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One big difference: strings vs. lists• Strings are immutable

str1 = "hi"str2 = str1str1 += "!"

What is the value of str2 after the above code runs?

A) "hi" B) "hi!" C) "!" D) an error occured

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A thing that i felt was most confusing in this current week's reading assignment was what variables you can change and which you can't. and whether it is a lot of things to change or just one single character.

Lists are mutablelist1 = ['a', 'b', 'c', 'd']list2 = list1list1.append('e')

What is the value of len(list2) after the above code runs?

A) 1 B) 3 C) 4 D) 5 E) an error occurred

Why are lists mutable?

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Lists provide 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"

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Announcements• Exam 0 ongoing

• Take the practice exam!

• Piazza etiquette• Use office hours if you are really confused

• Keep track of how much time you spend on CS 105 in the next week, please!

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Compositionlist1 = [1, 2]list1.append([3, 4])

What does list1 contain after the code above executes?A) [1, 2]B) [1, 2, 3]C) [1, 2, 3, 4]D) [1, 2, [3, 4]]E) [1, 2, [3, [4]]]

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Changing strings

str1 = "urbana"str1[5] = "e"

• What is the value of str1 after the code executes?A) "urbaea"B) "urbane"C) "urbanae"D) "ueeeee"E) an error occurred

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What if you want an immutable list?• It's called a tuple

tuple1 = ('a', 'b', 'c', 'd')

• Also tuples just seem like lists with extra restrictions.

• I am confused of what a tuple is and what the purpose of using it is.

• I was very confused with namedtuple(). I did not understand where to use it, how to use it, or why it is even used.

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Sets• How does a set differ from a list?• Set basics. "a set is an unordered collection of unique

elements", this is a definition from zyBooks- what does it mean by elements? Are those like variables? Values? I'm having a really hard time keeping all of the definitions for the terms straight.

• 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)

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Dictionaries• 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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What type of data collection is this?

{"Craig", "Anant", "Sofia", "Chinny"}

A) DictionaryB) ListC) SetD) StringE) Tuple

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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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String Formatting• I don't fully understand how to use string formatting and

why it is used?• The uses of conversion specifiers. Why and when do we

need to use those specifiers? The formatting is also confusing.

• Let's first tackle the why of string formatting.

• Next week's lab is about MadLibs.

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• Things like this are ugly and tedious:message = "Welcome " + first + " " + last

message += " to CS 105!"

• We'd rather make string with holes, and then say how to fill in the holes

base = "Welcome {} {} to CS 105!"

message = base.format(first, last)

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Python Format Strings• a_string.format(parameters)

• "a {} way".format("better")

• "can {1} the {0}".format("order", "control")

• "precision {0:.2f}".format(math.pi)

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