Lecture-10_Selection Python_24-25

COMP101 Introduction to Programming 2024-25

• Lecture-10: Selection in Python

Terminology

• Python selection syntax uses three commands:

  • if:

  • elif:

  • else:

• Important notes:

  • A colon (:) must follow if, elif, and else.

  • All code after the colon must be indented.

    • Default indentation is 4 spaces; changing this can create confusion.

  • Unlike other languages, Python does not use:

    • then

    • case or switch

    • { } delimiters

Selection Syntax

• Standard structure: if - elif - else

  • Usually contains one if at the start and one else at the end.

  • Multiple elif statements can be included as necessary.

• Tests return a boolean value (True or False).

• Supported operators in tests:

  • Comparison Operators: <, >=, <=, >, !=, ==

  • Boolean Operators: and, or, not

  • break and continue are used specifically for iteration.

If Statement Mechanics

• Example usage with dice roll:

  • Accept user input:

    number = int(input(\"Enter a throw of a die, 1 to 6: \\"))

  • The structure follows:

    • If the number is 6:

      if (number == 6):
          print(\"Throw again\")

    • Continuation:

      • Ends with a sequence statement (left-aligned):

        print(\"End of game\")

• Note: The if-statement must evaluate True or False; binary operators assist in this.

If Statement Examples

• If the condition is true, all indented lines execute:

  if (num > 0):
      print(\"Test is True. Number is positive\")

  • If false, all indented code is ignored.

if ... else Structure

• Align else with if, ensure the colon follows else:

  if (num > 0):
      print(\"Test is True. Number is positive\")
  else:
      print(\"Test is False. Number is negative\")

Handling Logical Errors

• For erroneous input (0):

  • If (num > 0) evaluates to False, else code executes.

  • Address any logical inaccuracies:

    • Modify the condition to num >= 0 to include zero as a valid input.

  • Two solutions discussed:

    • Modify the boolean test

    • Introduce elif to create stronger logic.

Utilizing elif

• elif statements enable additional checks between if and else, requiring a unique test condition.

• Proper structure:

  if (test):
      # block
  elif (another_test):
      # block
  else:
      # block

Structuring the Selection Statement

• Ensure to stick with the single if at the beginning and a single else at the end, with zero or more elif clauses in between.

• Good programming practices:

  • Effective use of if - elif - else enhances robustness by validating multiple conditions.

Summary Points

• Understand how each block executes based on the test value.

• Keep your input and conditions clear to avoid confusion.

• A comprehensive structure enhances logic and verification in programming.

Handling Errors and Debugging

• Maintain proper syntax and test conditions.

• Review and differentiate between expected output and logical accuracy.

• Good practices ensure smooth execution and fewer runtime errors in Python programs.