Variables and Operators

Prof Kathryn Kasmarik

Learning Objectives

  • After today’s lecture, you will be able to:

    • Define named variables and constants to hold data in a computer program.

    • Know some different data types.

    • Assign data into a variable.

    • Use augmented assignment.

    • Pass data from one part of a program to another.

    • Pass data from a human to a program and vice versa.

    • Differentiate between local variables and instance variables.

    • Understand the role of garbage collection.

Revision: UML Class Diagrams

  • Recall:

    • UML class diagrams illustrate the data and behaviors a program requires to successfully complete its tasks.

    • Example of a UML diagram structure:

    • Simple Pendulum

      • Variables:

      • _length (an instance variable)

      • _GRAVITY (a constant)

      • Methods:

      • calculate_period() -> returns a float

      • calculatelength(desiredperiod) -> returns a float

Program Data

Instance Variables

  • Definition:

    • Named pieces of data expected to be accessed by the entire class.

    • Defined within the ‘init’ method.

    • Naming Convention:

    • Utilizes the prefix _ to indicate protected access.

    • Example:

    • self._length

Constants

  • Definition:

    • Similar to variables, but their values are not intended to change.

  • Examples:

    • self._GRAVITY

    • math.pi

Naming Conventions

  • Python programmers utilize the convention of _names_like_this to signal that the data in the variable is private.

  • Important Considerations:

    • Should not be accessed from outside the class it’s declared in.

    • Use descriptive names for variables:

    • Helps in remembering the stored data.

    • Assists others in understanding your code.

  • Reserved Words:

    • Certain words cannot be used as variable names, such as:

    • if, for, which are part of the Python language syntax.

Data Types

  • Definition:

    • Data types are implied by the kind of data stored in a variable.

    • Notably, the data type of a variable can change dynamically.

    • Example data types include:

    • Integer (int)

    • Floating-point number (float)

    • String (string)

Assignment

  • The assignment operator (=) assigns a piece of data to a variable.

  • Methods of data assignment:

    • Hard coded values.

    • Transferred from another variable.

Assignment from a Calculation

  • Basic math operators available in Python include:

    • Addition: +

    • Subtraction: -

    • Division: /

    • Multiplication: *

    • Exponentiation: ** (power of)

  • Other available operators:

    • Built-in functions, e.g., abs(-5).

    • Library function usage, e.g., math.sqrt(.).

  • Order of Operations:

    • Follows conventional mathematical rules.

Augmented Assignment

  • Explanation of Long Form vs. Augmented Assignment:

    • Long form examples:

    • x = x + 1 vs. x += 1

    • y = y - 5 vs. y -= 5

    • z = z**2 vs. z **= 2

    • p = p / 2 vs. p /= 2

    • q = q*5 vs. q *= 5

Input from and Output to a Human

  • Example usage of input and output in Python:

if __name__ == "__main__":
    # Input example: asking for pendulum length
    length = float(input('How long should the pendulum be in metres?'))
    pendulum = SimplePendulum(length)  # Create a new pendulum object of given length
    current_period = pendulum.calculate_period()
    print(f'The current period of the pendulum is {current_period:.2f} seconds.')
    desired_period = float(input('How long should the period be in seconds? '))  # usually 1.0s on a clock!
    recommended_length = pendulum.calculate_length(desired_period)
    print(f'The recommended length for a period of {desired_period:.2f} seconds is {recommended_length:.2f} m')

Local and Instance Variables

Local Variables

  • Definition:

    • Named pieces of data used briefly within a function.

    • Cleaned up by the garbage collector post-usage.

    • Often used as input and output values in functions.

Instance Variables

  • Definitions and Examples:

    • Named pieces of data used persistently within an object.

Garbage Collection

  • Definition:

    • A process that unassigns or ‘cleans up’ data that is no longer in use.

    • Operates as a background process returning unused memory to the available memory pool.

Summary of Learning Outcomes

  • After today’s lecture, you can:

    • Define named variables and constants to hold data in a computer program.

    • Know some different data types.

    • Assign data into a variable.

    • Use augmented assignment.

    • Differentiate between local variables and instance variables.

    • Understand the role of garbage collection.

Textbook References

  • Chapter 2 includes:

    • Variables

    • Data types: (int, float, string)

    • Assignment syntax =

    • Formatted strings

    • Mathematical operators: *, **, /

    • Built-in methods: math.sqrt, print