Variables

Introduction to Variables in Python

• Python supports various value types, including integers (ints), floating-point numbers (floats), and strings (str).

• Variables are used to store and recall calculated values for future use.

Example: Calculating Costs for Guacamole

• Scenario: Preparing for a potluck and calculating expenses for ingredients.

• Items and Prices:

  • Avocados: 2

  • Price per avocado: $1.78

  • Lemon: $0.77

  • Onion: $1.29

  • Ground oregano: $5.99

• Subtotal Calculation:

  • Initial subtotal: $11.61 (calculated from added prices)

  • Sales tax in Alberta: 5%

  • Total calculation requires recalling previous values.

Understanding Variables

• Definition: Variables allow naming a stored value for easy recall.

• Assignment Statement: The equal sign (=) is called an assignment statement, where the right side is computed and stored as the left side's identifier.

• Identifiers: Referred to as names in this context.

  • Example assignments:

    • avocado = 1.78

    • lemon = 0.77

    • onion = 1.29

    • oregano = 5.99

Calculating Subtotal Using Variables

• Subtotal Initialization:

  • Start subtotal at 0: subtotal = 0

• Updating Subtotal: Each item can be added sequentially:

  • subtotal = 2 * avocado

  • subtotal = subtotal + lemon

  • subtotal = subtotal + onion

  • subtotal = subtotal + oregano

• The updated subtotal can be printed at any stage.

Final Calculations

• Tax Calculation:

  • tax = 0.05 * subtotal

• Total Calculation:

  • total = subtotal + tax

• Formatting Output:

  • To format total to two decimal places: print("%.2f" % total)

• String formatting will be elaborated in future videos.

Common Mistakes with Variables

• Case Sensitivity: Variable names are case-sensitive. Example: Using total and Tax leads to a NameError (incorrect capitalization).

• Spelling Errors: Misspelling an identifier results in a NameError. Example: subtotal vs. subtotl.

Valid Identifier Rules

• Starting Characters: Identifiers cannot start with a digit (e.g., 2avocado is invalid).

• Spaces: Cannot contain spaces (e.g., sub total is invalid).

• Special Characters: Hyphens are invalid (e.g., sub-total); underscores can be used instead (e.g., sub_total).

• Camel Case: Use camel case for multi-word identifiers (e.g., subTotal).

Reserved Keywords

• Identifiers cannot be keywords in Python (e.g., None is invalid as an identifier).

• To check for keywords use: help('keywords').

Conclusion

• This video covered the use of variables for computation storage, reassignment of values, and common pitfalls.

• Understanding these concepts is crucial for effective programming in Python.