1. Lecture #3 – Agenda Cell phones off & name signs out –I’ll judge signs on Wednesday next week Quick review & Questions Activity Problem solving

2. Quick review A bit can have one of two values (0 or 1) A sequence of bits is called a bit string Internal representations are in terms of bit strings Bit strings have no intrinsic meaning Bit strings are interpreted according to some representation scheme

3. Compilation “10110110” could represent a number, or a character, part of a digital photo, part of a song, or even a machine-language instruction. Programs are written in “high-level” programming languages, which must be translated into machine-language equivalent. Translation is done by a “compiler”. A compiler is a computer program (!)

4. Questions?

5. Moving on… We will return to low-level issues later in the semester, and also in later courses. This brief low-level discussion gives context for upcoming topics. Now we turn to some higher-level issues.

6. I have a question for you! What did you have for breakfast today?

7. Activity The goal of this short activity is to demonstrate two things: 1.objects have behaviors 2.sending message to objects can trigger those behaviors

8. Activity details 3 volunteers, a.k.a. objects When I tell you to…you should… STARTbegin doing jumping jacks STOPstop jumping, and stand normally anything elsedo nothing When I tell you to…you should… STARTstart counting slowly, 0…1…2… etc, but always starting at zero STOPstop counting, but remember where you left off CONTINUEkeep counting up from where you left off anything elsedo nothing When I tell you to…you should… anything at alldo nothing

9. Activity Review Why did we do this activity? What is an object?

10. Problem solving How do we go about solving a problem? Simplification through abstraction is key. –A process of iterative refinement –Start with little detail in solution –Refine solution, adding more detail

11. Example Describe the game of soccer. http://en.wikipedia.org/wiki/Soccer

12. Executable Model Conceptual Model Apply same idea to developing software solutions:

13. The conceptual model A model of the problem domain Problem domain consists of: –objects properties capabilities –relationships between objects

14. PROGRAM CODE Executable Model MACHINE EXECUTABLE Initial Conceptual Model Problem Domain Iterative refinement of model Compilation A more refined picture (iterative refinement in action!)