02 Python and Data

Python #

Recap #

  • Interactive Mode
  • Scripts - files with .py extension
  • Syntax and structure
    • Syntax: rules, how code must be written.
    • Semantics: Meaning of a statement or instruction.
  • print()
  • input()

Variables #

A variable is a name that stores a value, acting as a reference to that value in memory. It allows you to reuse and manipulate data in your programs. Example:

x = 10       # Integer
y = "Hello"  # String
z = 3.14     # Float

x = "Now I'm a string!"     # Reassignment

Rules for Variable Names:

  • Must start with a letter or an underscore (_).
  • Cannot start with a number.
  • Can contain letters, numbers, and underscores.
  • Cannot use Python keywords (e.g., if, else, class).
  • For now, stick only to lowercase characters (i.e. result and not Result).

Snake Case #

Snake case is a naming convention where words are written in lowercase and separated by underscores (_). It is commonly used in Python for variable and function names to improve readability. E.g. valid_name.

Comments #

# this is the first comment
spam = 1  # and this is the second comment
          # ... and now a third!
text = "# This is not a comment because it's inside quotes."

Statements and Expressions #

  • Statement is a command. It does something, but not necessarily return a value. E.g. print
  • Expression is a combination of values, variables, operators, function calls that evaluates to a value. E.g. 5, 2+2, pow(3, 2) + (3/4)

You can execute several statements (expressions) in one line. Don’t do that! Stick to one idea/task/expression per line. That way it will be easier to understand.

Data #

Data refers to information or values that a computer program processes, stores, or manipulates. It can be as simple as a single number or as complex as a collection of multimedia files. In programming, data is categorized into data types, which define the kind of value and what operations can be performed on it.

Kinds of data types:

  • Primitive: Basic, indivisible data types like integers, floats, and booleans.
  • Structured: Organized collections of data, like arrays, lists, and objects.

Hand in hand with data types go operators. An operator is a symbol or keyword in programming that performs an action or operation on data (called operands). For example, + is an operator that adds two numbers.

Numeric #

  • Integer: int
  • Float: float
  • Complex: complex
>>> x = 42 
>>> y = 3.14

>>> # Check types
>>> type(x)
<class 'int'>
>>> type(y)
<class 'float'>

>>> # Perform basic arithmetic operations
>>> x + y 
45.14
>>> x - y
38.86
>>> x * y 
131.88
>>> x / y  # Division (always returns float)
13.375796178343949

>>> # Floor division and modulus (go hand in hand)
>>> x // 5  # Integer division
8
>>> x % 5  # Remainder
2

>>> # Exponentiation
>>> 2 ** 3  # 2 raised to the power of 3
8

>>> # Working with floats
>>> z = 7.5 / 2 
>>> z
3.75

>>> # Rounding floats
>>> round(z, 1)  # Round to 1 decimal place
3.8

>>> # Type conversions (casting)
>>> int(z)  # Convert float to integer (truncates)
3
>>> float(x)  # Convert integer to float
42.0

>>> # Handling very large numbers
>>> big_number = 12345678901234567890
>>> big_number * 2
24691357802469135780

>>> # Scientific notation for small numbers
>>> small_number = 1e-10  # Equivalent to 0.0000000001
>>> small_number
1e-10

>>> # Some built-in functions
>>> abs(-42)  # Absolute value
42
>>> pow(2, 3)  # 2 raised to the power of 3 (same as 2 ** 3)
8

For some float numbers computers cannot represent them exactly. Therefore, never compare floats (but use greater-than or lesser-than):

a = 0.1 + 0.2
b = 0.3

# Direct comparison
if a == b:
    print("a and b are equal")
else:
    print("a and b are NOT equal")

print(f"a: {a}, b: {b}")

Text #

  • String: str
  • Existing strings cannot be changed. You can only use them to create new strings out of the existing ones.
>>> # Let's start with basic strings
>>> name = "Alice"
>>> greeting = "Hello"

>>> # Check the type of a string
>>> type(name)
<class 'str'>

>>> # Concatenate strings
>>> full_greeting = greeting + ", " + name + "!"
>>> full_greeting
'Hello, Alice!'

>>> # Repeating strings
>>> excited_greeting = full_greeting * 2
>>> excited_greeting
'Hello, Alice!Hello, Alice!'

>>> # Accessing characters by index
>>> full_greeting[0]  # First character
'H'
>>> full_greeting[-1]  # Last character
'!'

>>> # Slicing strings
>>> full_greeting[0:5]  # First 5 characters
'Hello'
>>> full_greeting[7:]  # Everything from index 7 onwards
'Alice!'
>>> full_greeting[:5]  # Everything up to index 5 (exclusive)
'Hello'

>>> # String length
>>> len(full_greeting)
13

>>> name.startswith("Al")
True
>>> name.endswith("e")
True

>>> # Converting case
>>> full_greeting.upper()  # Uppercase
'HELLO, ALICE!'
>>> full_greeting.lower()  # Lowercase
'hello, alice!'
>>> # The function as `object.function` is called a `method`. More on methods in the OOP class.

>>> # Stripping whitespace
>>> padded = "   hello, world!   "
>>> padded.strip()
'hello, world!'
>>> padded.lstrip()
'hello, world!   '
>>> padded.rstrip()
'   hello, world!'

>>> # Splitting strings
>>> sentence = "Python is awesome"
>>> words = sentence.split()  # Split into a list of words (we'll deal with lists later)
>>> words
['Python', 'is', 'awesome']

>>> # Joining strings
>>> "-".join(words)  # Combine words with a hyphen
'Python-is-awesome'

>>> # Checking substrings
>>> "awesome" in sentence  # Is "awesome" part of the sentence?
True
>>> "boring" in sentence
False

>>> # Replacing parts of a string 
>>> # does not change existing string but creates a new string!
>>> sentence.replace("awesome", "fantastic")
'Python is fantastic'

>>> # Formatting strings - string interpolatoin
>>> age = 25
>>> formatted = f"My name is {name} and I am {age} years old."
>>> formatted
'My name is Alice and I am 25 years old.'

>>> # Advanced formatting (see: https://pyformat.info/)
>>> pi = 3.14159
>>> f"The value of pi is approximately {pi:.2f}"  # Format to 2 decimal places
'The value of pi is approximately 3.14'
  • Strings are texts in quotes (") or apostrophes (') on one line.
  • Multiline strings begin and end with triple quotes or apostrophes. E.g.:
>>> long_text = """Line 1
... Line 2
... ...
... and here is the END"""
>>> long_text
'Line 1\nLine 2\n...\nand here is the END'
>>>
  • \n is new line character.
  • All characters starting with \ are called escape sequences and contain special characters.

input() always returns a string, even if the user enter digits. It’s your job to convert it to number. Example:

>>> number = input("Please enter a number: ")
Please enter a number: 42
>>> number
'42'
>>> type(number)
<class 'str'>
>>> number = int(number)
>>> type(number)
<class 'int'>
>>> number
42

Homework (graded) #

Calculator #

Create a program that will read three numbers, store them in three distinct variables and then create the sum of them. This will be stored in another variable. Then it will print the variable result saying: “result is N” where N is the sum.

Meter #

Create a program that will let the user input his name. The program will then say how many characters are there in his name.

Extra points: if spaces are inserted, substract their count from the result, so that “Peter Pan” will return 8 and not 9.

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