Lecture IV - Handling Data in more than one Dimension

Programming with Python

Author
Affiliation

Dr. Tobias Vlćek

Kühne Logistics University Hamburg - Fall 2024

Quick Recap of the last Lecture

Functions

  • Functions are reusable blocks of code that perform specific tasks
  • They can accept inputs (parameters) and return outputs
  • def followed by the function name, parameters and a colon
  • Help in organizing code and reducing repetition

. . .

def greet(name):
    return f"Welcome to this lecture, {name}!"

print(greet("Students"))
Welcome to this lecture, Students!

Scope

  • Scope determines the visibility and lifetime of variables
  • Variables defined inside a function are local to that function
  • Variables defined outside of all functions are global
  • They can be accessed from anywhere in the program

. . .

def greet(name):
    greeting = f"Welcome to this lecture, {name}!"
    return greeting

print(greeting) # This will cause an error

>Question: Why does this cause an error?

Classes

  • Classes are blueprints for creating objects
  • They encapsulate data (attributes) and behavior (methods)
  • Help in organizing code and creating objects with similar structures

. . .

class Lectures:  
    def __init__(self, name, length_minutes):
        self.name = name
        self.length = length_minutes

    def duration(self):
        return f"Lecture '{self.name}' is {self.length} minutes long!"

lecture_4 = Lectures("4. Data in more than one dimension", 90)
print(lecture_4.duration())
Lecture '4. Data in more than one dimension' is 90 minutes long!

Tuples

What are Tuples?

  • Tuples are ordered collections of items
  • They are immutable (cannot be changed after creation)
  • Help in storing multiple items in a single variable
  • Created using the tuple() function or the () syntax

. . .

my_tuple = (1, 2, 3, 4, 5)
print(my_tuple)
(1, 2, 3, 4, 5)

Tuple Operations

  • Tuples support the same operations as strings
  • We can use indexing and slicing to access elements
  • We can use the + operator to concatenate tuples
  • We can use the * operator to repeat a tuple

. . .

>Question: What will the following code print?

my_tuple = (1, 2, 3)    
print(my_tuple[1:3])
print(my_tuple + (4, 5, 6))
print(my_tuple * 2)
(2, 3)
(1, 2, 3, 4, 5, 6)
(1, 2, 3, 1, 2, 3)

Tuple Methods

  • Tuples support the following methods:
    • count(x): Returns the number of times x appears in the tuple
    • index(x): Returns the index of the first occurrence of x

. . .

>Question: What will this code print?

my_tuple = (1, 2, 3, 2, 4, 2)
print(my_tuple.count(2))
print(my_tuple.index(3))
3
2

Tuple Data Types

  • Tuples can contain elements of different data types
my_tuple = ("Peter", 25, "Hamburg")
print(my_tuple)
('Peter', 25, 'Hamburg')

. . .

# This works as well
my_tuple = "Peter", 25, "Hamburg"
print(my_tuple)
('Peter', 25, 'Hamburg')

. . .

Note

We can also create tuples by listing the elements separated by commas.

Tuples from Functions

  • Functions can return tuples
  • This is useful if we want to return multiple values from a function

. . .

def get_student_info(name, age, city):
    return name, age, city

student_info = get_student_info("Peter", 25, "Hamburg")
print(student_info)
('Peter', 25, 'Hamburg')

. . .

>Question: How would you access the age from the tuple?

Tuple Unpacking

  • Allows us to assign the elements of a tuple to variables
  • The number of variables must match the number of elements
  • Use the * operator to assign the remaining elements to a variable

. . .

def get_student_info(name, age, city):
    return name, age, city
name, *rest = get_student_info("Peter", 25, "Hamburg")
print(f"Name: {name}")
print(f"Other info: {rest}")
Name: Peter
Other info: [25, 'Hamburg']

. . .

Warning

The output is positional, so we have to be careful with the order of the variables.

Lists

What are Lists?

  • Lists are ordered collections of items
  • They are mutable (can be changed after creation)
  • Created using the list() function or the [] syntax
  • They support the same operations as strings and tuples
  • Have much more methods and are more versatile than tuples

. . .

my_list = [1, 2, 3, 4, 5]
print(my_list)
[1, 2, 3, 4, 5]

. . .

>Question: Any idea why lists support more methods?

List Methods

  • Common methods for lists:
    • count(x): Returns the number of times x appears in the list
    • append(x): Adds an element x to the end of the list
    • insert(i, x): Inserts an element x at index i
    • remove(x): Removes the first occurrence of element x
    • index(x): Returns the index of the first occurrence of x
    • pop([i]): Removes the element at index i and returns it
    • sort(): Sorts the list in ascending order
    • reverse(): Reverses the list

Lists in Action

>Task: Solve the following problem using lists:

# Imagine the following shoppping list for this weekend
shopping_list = ["cider", "beer", "bread", "frozen_pizza"]

. . .

  • First, add some apples to the list for a healthy option
  • Next, remove the cider as you already have some at home
  • Sort all items in the list alphabetically
  • Print each item of the list on a new line

. . .

Tip

You can use the methods and loops we learned so far to solve the problem.

Sets

What are Sets?

  • Sets are unordered collections of unique elements
  • They are mutable (can be changed after creation)
  • Created using the set() function or the {} syntax
  • Supports + and * operations like lists and tuples
  • Unlike lists and tuples, sets do not support indexing

. . .

my_set = {1, 2, 2, 5, 5}
print(my_set)
{1, 2, 5}

Set Methods

  • Common methods for sets:
    • add(x): Adds an element x to the set
    • remove(x): Removes an element x from the set
    • discard(x): Removes an element x from the set if it is present
    • pop(): Removes and returns an arbitrary element from the set
    • update(other): Adds all elements from other to the set

Set Theory

  • Additional methods are derived from set theory
    • union(other): New set with elements from both sets
    • intersection(other): New set with common elements
    • isdisjoint(other): True if no elements in common
    • issubset(other): True if subset of other

. . .

Tip

There are more methods for sets! If you are working intensively with sets, keep that in mind.

Sets in Action

>Task: Solve the following problem using sets:

# You have a list of friends from two different groups
friends_group_1 = ["Neo", "Morpheus", "Trinity", "Cypher"]
friends_group_2 = [ "Smith", "Apoc", "Cypher", "Morpheus"]

. . .

  • First, find the mutual friends in both groups
  • Then create a new set of all friends from both groups
  • Count the number of friends in total
  • Print each item of the set on a new line

. . .

Tip

Notice, there is a small error in the given code that you have to fix.

Dictionaries

What are Dictionaries?

  • Dictionaries are unordered collections of key-value pairs
  • They are mutable (can be changed after creation)
  • Keys must be unique and immutable
  • Values can be of any type
  • Created using the dict() function or the {} syntax
  • As sets we cannot access them by index

. . .

who_am_i = {"name": "Tobias", "age": 30, "city": "Hamburg"}
print(who_am_i)
{'name': 'Tobias', 'age': 30, 'city': 'Hamburg'}

Key-Value Pairs

  • We can access them by their keys, though!
  • You can think of them as a set of key-value pairs

. . .

who_am_i = {"name": "Tobias", "age": 30, "city": "Hamburg"}
print(who_am_i["name"])
Tobias

. . .

Note

Note, how we can use the [] operator to access the value of a key?

Dictionary Operations

  • Common operations and methods:

  • in operation to check if a key is in the dictionary

  • for loop to iterate over the dictionary

  • keys() method to return a view of the dictionary’s keys

  • values() method to return a view of the dictionary’s values

  • pop(key[, default]) to remove a key and return its value

Dictionaries in Action

>Task: Solve the following problem using dictionaries:

# Create a dictionary with the following information about yourself: name, age, city
i_am = {}

. . .

  • Add your favorite color and food to the dictionary
  • Remove the city from the dictionary
  • Print your name and age in a formatted sentence

Overview of new Data Types

Comparison between Data Types

  • Tuple: Immutable, ordered, duplicates allowed
  • List: Mutable, ordered, duplicates allowed
  • Set: Mutable, unordered, no duplicates
  • Dictionary: Mutable, unordered, no duplicates, key-value pairs

. . .

Tip

This impacts your code, the operations you can perform and the speed of your program. Thus, it makes sense to understand the differences and choose the right data type for the task.

When to use which?

  • Tuples: store a collection of items that should not be changed
  • Lists: store a collection of items that should be changed
  • Sets: store a collection of items that should not be changed and duplicates are not allowed
  • Dictionaries: store a collection of items that should be changed, duplicates are not allowed and require key-value pairs

. . .

Tip

You can convert between the data types using tuple(), list(), set() and dict(). Note, that this is not always possible, e.g. you cannot convert a list to a dictionary without specifying a key.

Speed Differences

  • Lists are the most versatile, but slowest
  • Tuples are generally faster than lists
  • Sets are generally faster than lists and tuples
  • Dictionaries depend, but are generally faster than lists and tuples
Code 
import timeit

# Number of elements in each data structure
n = 10000000

# Setup for each data structure, including the test function
setup_template = """
def test_membership(data_structure, element):
    return element in data_structure
data = {data_structure}
"""

setups = {
    'Tuple': setup_template.format(data_structure=f"tuple(range({n}))"),
    'List': setup_template.format(data_structure=f"list(range({n}))"),
    'Set': setup_template.format(data_structure=f"set(range({n}))"),
    'Dictionary': setup_template.format(data_structure=f"{{i: i for i in range({n})}}")
}

# Measure time for each data structure
print(f"Time taken for a single membership test in {n} elements (in seconds):")
print("-" * 75)
for name, setup in setups.items():
    stmt = f"test_membership(data, {n-1})"  # Test membership of the last element
    time_taken = timeit.timeit(stmt, setup=setup, number=1)
    print(f"{name:<10}: {time_taken:.8f}")
print("-" * 75)
print("Note, that theses values are machine dependent and just for illustration!")
Time taken for a single membership test in 10000000 elements (in seconds):
---------------------------------------------------------------------------
Tuple     : 0.05766171
List      : 0.05379279
Set       : 0.00000304
Dictionary: 0.00000408
---------------------------------------------------------------------------
Note, that theses values are machine dependent and just for illustration!

Comprehensions

  • Comprehensions provide a concise way to create data structures
    • Tuple comprehensions: (x for x in iterable)
    • List comprehensions: [x for x in iterable]
    • Set comprehensions: {x for x in iterable}
    • Dictionary comprehensions: {x: y for x, y in iterable}

. . .

Tip

The iterable can be any object that can be iterated over, e.g. a list, tuple, set, dictionary, etc.

Iterables

  • We have already introduced those!
  • We can use the for loop to iterate over an iterable

. . .

shopping_list = ["cider", "beer", "bread", "frozen_pizza"]
for item in shopping_list:
    print(item)
cider
beer
bread
frozen_pizza

. . .

who_am_i = {"name": "Tobias", "age": 30, "city": "Hamburg"}
for key, value in who_am_i.items():
    print(f"{key}: {value}")
name: Tobias
age: 30
city: Hamburg

Nesting

  • We can nest data structures within each other
  • This is useful if we want to store more complex data

. . .

normal_list = [1, 2, 3, 4, 5]
nested_list = ["Hello, World!", normal_list, (1,2)]

print(nested_list)
print(nested_list[2])
['Hello, World!', [1, 2, 3, 4, 5], (1, 2)]
(1, 2)

. . .

Tip

You can also nest lists within lists within lists, etc.

I/O

Input/Output

  • A common task in programming is to interact with users
  • Remember the input() function from the first lecture?
  • It is a classical example of user input
  • An example of output is the print() function

. . .

name = input("Please enter your name: ")    
print(f"Hello, {name}!")

. . .

Note

Thus, we have already worked with I/O in Python!

Reading and Writing Files

  • We also need to interact with data
  • File handling in Python is quite simple:
    • Use open(file_name, mode) to open a file
    • Modes: "r" (read), "w" (write), "a" (append)
  • Basic operations:
    • Read: file.read()
    • Write: file.write(content)
    • Close: file.close()

File Handling in Action

file = open("hi.txt", "w") # This creates a file called "hi.txt"
file.write("Hello, World!") # This writes "Hello, World!" to the file
file.close() # This closes the file
print("File successfully written")
File successfully written

. . .

>Question: Any ideas how to read the file?

. . .

file = open("hi.txt", "r") # This opens the file "hi.txt"
content = file.read() # This reads the content of the file
file.close() # This closes the file
print(content) # This prints the content of the file
Hello, World!

. . .

Tip

Close files with file.close() to free up system resources and ensure data is properly saved.

Easier File Handling with with

  • We can also use the with statement to open a file
  • This ensures the file is properly closed after its handling finishes
  • It’s a good practice to use it when working with files
with open("hi_again.txt", "w") as file:
    file.write("Hello again, World!")

print("File successfully written")
File successfully written

. . .

>Task: Open the file hi_again.txt and print its content using with

Working with other file types

  • Naturally, we also want to work with other file types!
  • Reading and writing CSV files is a common tasks in data analysis
  • Excel files are used in many applications and companies
  • We will see how to do this later in the course

. . .

Note

And that’s it for todays lecture!
We now have covered the basics of tuples, sets, lists and dictionaries as well as some basic file handling. For now, just remember that advanced reading and writing is possible and that there are libraries that help with this.

Literature {.title}

Interesting Books

  • Downey, A. B. (2024). Think Python: How to think like a computer scientist (Third edition). O’Reilly. Link to free online version
  • Elter, S. (2021). Schrödinger programmiert Python: Das etwas andere Fachbuch (1. Auflage). Rheinwerk Verlag.

. . .

Tip

Nothing new here, but these are still great books!

. . .

For more interesting literature to learn more about Python, take a look at the literature list of this course.