2.1 Expressions Part 1

Chapter 2: Variables, Expressions, and Statements

Page 1: Introduction

  • Focus on expressions, variables, and statements in Python programming.

  • Material sourced from "Python for Everybody" course.

Page 2: Constants

  • Definition: Fixed values that do not change.

    • Numeric constants: typical numbers (e.g., integers, floating-point numbers).

    • String constants: text values denoted by single (') or double (") quotes.

  • Examples:

    • print(123) outputs 123.

    • print(98.6) outputs 98.6.

    • print('Hello world') outputs Hello world.

Page 3: Reserved Words

  • Definition: Keywords that have specific meanings in Python; cannot be used as variable names.

  • Examples of Reserved Words:

    • False, None, True, and, or, if, for, while, try, def, class, import

    • Reserved words help to maintain the syntax and structure of the programming language.

Page 4: Variables

  • Definition: A variable is a named storage location in memory for data that can be modified.

  • Variable Naming: Programmers choose variable names which are retrievable.

  • Example Usage:

    • Assigning values: x = 12.2, y = 14, later x = 100 updates x.

Page 5: Python Variable Name Rules

  • Naming Conventions:

    • Must start with a letter or underscore (_).

    • Allowed characters: letters, numbers, and underscores.

    • Case Sensitive (e.g., spam, Spam, SPAM are distinct).

  • Valid Examples: spam, eggs, spam23, _speed

  • Invalid Examples: 23spam, #sign, var.12

Page 6: Statements and Expressions

  • Assignment Statement: Stores a value in a variable, e.g., x = 2.

    • Assignment with Expression: x = x + 2, adds 2 to the current value of x.

  • Print Function: Displays information to output, e.g., print(x).

  • Components:

    • Variable: (e.g., x)

    • Operator: (e.g., =, +)

    • Constant: (e.g., 2)

    • Function: (e.g., print())

Page 7: Mnemonic Variable Names

  • Concept: Naming variables in a way that reminds programmers of their purpose (memory aids).

  • Comment: Careful naming can confused beginners as well-named variables may resemble reserved keywords.

Page 8: Analyzing Code Examples

  • Code Overview:

    • Assigning and calculating values using variables.

      • x1q3z9ocd = 35.0, x1q3z9afd = 12.50

      • x1q3p9afd = x1q3z9ocd * x1q3z9afd

      • Print output: print(x1q3p9afd).

    • Reformulated variables:

      • a = 35.0, b = 12.50, c = a * b, print(c).

    • More descriptive example:

      • hours = 35.0, rate = 12.50, pay = hours * rate, print(pay).

Page 9: Assignment Statements

  • Definition: Assigns a value to a variable using syntax variable = expression.

  • Example: x = 3.9 * x * (1 - x) evaluates and assigns the result to x.

Page 10: Variable Evaluation

  • Memory Location Concept: A variable is a location that stores values, e.g., if x = 0.6, evaluates the expression on the right and assigns results.

    • Evaluated from the right-hand side and stored on the left-hand variable.

Page 11: Updating Variables

  • A variable's value can be updated when a new calculation is made.

  • Example of updating: replacing old 0.6 value with a new value, 0.936.

  • Demonstrates that values in a variable are not fixed and can change with new evaluations.

Page 12: Conclusion

  • Summary of vital concepts of expressions, variables, and assignment in Python programming.