Programming By Strengthening Of Invariants

Bart Massey 2013-11-07

Overview

• Misc notes
• Quiz
• Programming by strengthening of invariants
• In-class programming demo

Misc Notes

• Daily WTF CSOD is http://thedailywtf.com/Articles/Interesting-Year-Length-Logic.aspx

Quiz

• In-class quiz

Programming By Strengthening Of Invariants

• Upon input, your routine may have complicated state

  • Messy parameters

  • Unknown global and module state

• Idea: Program by successively handling messy side cases until you get to the main case

  • With each handled case, return from the routine

  • Within a case, use break and continue inside loops to get to return points

• End result: Code that reads cleanly top-to-bottom, with each section performing a clear and useful function

Establishing Invariants

• At the start of each handled case, indicate with a comment what is being handled and why

• Implicitly (else explicitly) be able to give an account of invariants (things that must be true) after every statement

• Invariants should get stronger and stronger, making the code easier and easier

Demo

• In-class demo of programming by strengthening invariants