Variables, Data Types, and Operators

# CHAPTER 4

Variables, Data Types, and Operators #

1. Chapter Introduction #

2. Core Data Types #

Python has four primary primitive data types. You can always check a variable's type using the built-in type() function.

# 1. Integer (Whole numbers, positive or negative)
quantity = 50
print(type(quantity))  # Output: <class 'int'>

# 2. Float (Decimal numbers)
price = 19.99
print(type(price))     # Output: <class 'float'>

# 3. String (Text, wrapped in single or double quotes)
product_id = "ITEM-4421"
print(type(product_id)) # Output: <class 'str'>

# 4. Boolean (True or False - MUST be capitalized)
is_active = True
print(type(is_active))  # Output: <class 'bool'>

3. Type Conversion (Casting) #

Often, data comes from a CSV file in the wrong format (e.g., a number is read as a string). You must convert (cast) it before doing math.

# A number stored as a string
string_num = "100"

# This will cause an error: string_num + 50 

# Convert string to integer
real_num = int(string_num)
print(real_num + 50)  # Output: 150

# Convert integer to string
new_string = str(real_num) + " dollars"
print(new_string)     # Output: 100 dollars

# Convert integer to float
decimal_num = float(real_num)
print(decimal_num)    # Output: 100.0

4. Arithmetic Operators #

Python acts as a powerful calculator using standard arithmetic operators.

x = 10
y = 3

print(x + y)  # Addition (13)
print(x - y)  # Subtraction (7)
print(x * y)  # Multiplication (30)
print(x / y)  # Division (3.333) -> ALWAYS returns a float
print(x // y) # Floor Division (3) -> Chops off the decimal, returns integer
print(x ** y) # Exponentiation (10 to the power of 3 = 1000)
print(x % y)  # Modulus (1) -> Returns the remainder of division

*Note on Modulus (%):* This is heavily used in data science to find even/odd numbers. If x % 2 == 0, the number is even.

5. Comparison Operators #

Comparison operators compare two values and ALWAYS return a Boolean (True or False). These are critical for filtering data.

a = 10
b = 20

print(a == b)  # Equal to (False) - Notice it is TWO equals signs!
print(a != b)  # Not equal to (True)
print(a > b)   # Greater than (False)
print(a < b)   # Less than (True)
print(a >= 10) # Greater than or equal to (True)

6. Logical Operators #

Logical operators are used to combine multiple comparison statements.

• and: Returns True if BOTH statements are true.

• or: Returns True if ONE of the statements is true.

• not: Reverses the result (True becomes False).

age = 25
income = 60000

# Using 'and'
is_target_demographic = (age > 18) and (income > 50000)
print(is_target_demographic) # True

# Using 'or'
is_student_or_rich = (age < 22) or (income > 100000)
print(is_student_or_rich) # False

7. Mini Project: Interactive Profit Calculator #

Let's use input() (from Ch 3) and type casting to build a calculator.

# 1. Get input (Remember, input() returns a STRING)
revenue_str = input("Enter total revenue: ")
expenses_str = input("Enter total expenses: ")

# 2. Cast strings to floats for math
revenue = float(revenue_str)
expenses = float(expenses_str)

# 3. Calculate
profit = revenue - expenses
profit_margin = (profit / revenue) * 100

# 4. Output
print(f"Total Profit: ${profit}")
print(f"Profit Margin: {profit_margin}%")

8. Common Mistakes #

• Confusing = with ==: A single equals sign = *assigns* a value (age = 20). Double equals == *compares* values (age == 20). Using = when you mean == is a very common beginner error.

• Forgetting to cast input: Doing math = input("Enter number: ") + 10 will crash because Python tries to add text and a number. You must do int(input()).

9. MCQs #

10. Interview Questions #

• Q: You import a CSV containing temperatures, and they all read as strings like "98.6". Write the code to convert the variable temp_str into a format you can use for math.

• Q: Explain how you would use the modulus operator (%) to determine if a customer ID number is even or odd.

11. Summary #