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.