Lecture 4

Python Basics I

Byeong-Hak Choe

bchoe@geneseo.edu

SUNY Geneseo

January 31, 2025

Python Basics

Python Basics

Variables Are Names, Not Places

• A value is datum (literal) such as a number or text.

• There are different types of values:

  • 352.3 is known as a float or double;
  • 22 is an integer;
  • “Hello World!” is a string.

Python Basics

Values, Variables, and Types

a = 10
print(a)

• A variable is a name that refers to a value.
  • We can think of a variable as a box that has a value, or multiple values, packed inside it.
• A variable is just a name!

Python Basics

Values, Variables, and Types

• Sometimes you will hear variables referred to as objects.

• Everything that is not a literal value, such as 10, is an object.

Python Basics

Variable in data.frame

• Definition: A data.frame is a table-like data structure used for storing data in a tabular format with rows and columns.
• Structure: Consists of:
  • Variables (Columns)
  • Observations (Rows)
  • Values (Cells): Individual data points within each cell of the data.frame.

Python Basics

Assignment ( = )

# Here we assign the integer value 5 to the variable x.
x = 5   

# Now we can use the variable x in the next line.
y = x + 12  
y

• In Python, we use = to assign a value to a variable.

• In math, = means equality of both sides.

• In programs, = means assignment: assign the value on the right side to the variable on the left side.

Python Basics

Code and comment style

• The two main principles for coding and managing data are:
  • Make things easier for your future self.
  • Don’t trust your future self.
• The # mark is Google Colab’s comment character.
  • The # character has many names: hash, sharp, pound, or octothorpe.
  • # indicates that the rest of the line is to be ignored.
  • Write comments before the line that you want the comment to apply to.
• Consider adding more comments on code cells and their results using text cells.

Python Basics

Assignment

• In programming code, everything on the right side needs to have a value.
  • The right side can be a literal value, or a variable that has already been assigned a value, or a combination.
• When Python reads y = x + 12, it does the following:
  1. Sees the = in the middle.
  2. Knows that this is an assignment.
  3. Calculates the right side (gets the value of the object referred to by x and adds it to 12).
  4. Assigns the result to the left-side variable, y.

Python Basics

Variables Are Names, Not Places

list_example = [10, 1.23, "like this", True, None]
print(list_example)
type(list_example)

• The most basic built-in data types that we’ll need to know about are:
  • integers 10
  • floats 1.23
  • strings "like this"
  • booleans True
  • nothing None
• Python also has a built-in type of data container called a list (e.g., [10, 15, 20]) that can contain anything, even different types

Python Basics

Types

• The second column (Type) contains the Python name of that type.

• The third column (Mutable?) indicates whether the value can be changed after creation.

Python Basics

Brackets

• There are several kinds of brackets in Python, including [], {}, and ().

• []
• {}
• ()

vector = ['a', 'b']
vector[0]

• [] is used to denote a list or to signify accessing a position using an index.

{'a', 'b'}  # set
{'first_letter': 'a', 'second_letter': 'b'}  # dictionary

• {} is used to denote a set or a dictionary (with key-value pairs).

num_tup = (1, 2, 3)
sum(num_tup)

• () is used to denote
  • a tuple, or
  • the arguments to a function, e.g., function(x) where x is the input passed to the function.

Python Basics

Operators

string_one = "This is an example "
string_two = "of string concatenation"
string_full = string_one + string_two
print(string_full)

• All of the basic operators we see in mathematics are available to use:

• + for addition
• - for subtraction

• * for multiplication
• ** for powers

• / for division
• // for integer division

• These work as you’d expect on numbers.
• These operators are sometimes defined for other built-in data types too.
  • We can ‘sum’ strings (which really concatenates them).

Python Basics

Operators

list_one = ["apples", "oranges"]
list_two = ["pears", "satsumas"]
list_full = list_one + list_two
print(list_full)

• It works for lists too:

string = "apples, "
print(string * 3)

• We can multiply strings!

Python Basics

Operators

Q. Classwork 4.1

Using Python operations only, calculate below: \[\frac{2^5}{7 \cdot (4 - 2^3)}\]

Python Basics

Casting Variables

orig_number = 4.39898498
type(orig_number)

mod_number = int(orig_number)
mod_number
type(mod_number)

• Sometimes we need to explicitly cast a value from one type to another.

  • We can do this using built-in functions like str(), int(), and float().
  • If we try these, Python will do its best to interpret the input and convert it to the output type we’d like and, if they can’t, the code will throw a great big error.

Python Basics

Tuples and (im)mutability

• A tuple is an object that is defined by parentheses and entries that are separated by commas, for example (15, 20, 32). (They are of type tuple.)

• Tuples are immutable, while lists are mutable.

• Immutable objects, such as tuples and strings, can’t have their elements changed, appended, extended, or removed.

  • Mutable objects, such as lists, can do all of these things.

• In everyday programming, we use lists and dictionaries more than tuples.

Python Basics

Dictionaries

cities_to_temps = {"Paris": 28, "London": 22, "New York": 36, "Seoul": 29}

cities_to_temps.keys()
cities_to_temps.values()
cities_to_temps.items()

• Another built-in Python type that is enormously useful is the dictionary.
  • This provides a mapping one set of variables to another (either one-to-one or many-to-one).
  • If you need to create associations between objects, use a dictionary.
• We can obtain keys, values, or key-value paris from dictionaries.

Python Basics

Running on Empty

• Being able to create empty containers is sometimes useful, especially when using loops.

• The commands to create empty lists, tuples, dictionaries, and sets are lst = [], tup=(), dic={}, and st = set() respectively.

• Q. What is the type of an empty list?

Google Colab Settings

Turn off AI Assistance

• On Google Colab
  1. From the top-right corner, click ⚙️
  2. Click “AI Assistance” from the side menu.
  3. Disable all options.