Problem Frames 7 - Model domains and real worlds

1. Problem Frames 7 - Model domains and real worlds

2. Adding a “model” domain • Technique for information display problems • A problem frame variant • A way of decomposing a problem into two subproblems • Makes the problem simpler • Describes the machine more accurately

3. Information Display:problem frame diagram Real world C3 RW!C1 C Information machine Display - Real world Display Y4 IM!E2 C

4. Information Display with model Real world C3 RW!C1 C Modeling machine Model - Real world Model Y6 MM!E5 X DM!E7 Model Y6 X Display - Model Display machine Display Y4 IM!E2 C

5. Decomposing • Invent the model • Split requirements into requirements for building the model, and requirements for displaying the results • Make two specifications

6. Composition • Two requirements should compose to form the original requirements • Except for extra model phenomena • Two specifications should compose to form the original specification • Compositions should be easy, because • One creates model, the other reads it

7. Why use a model? • Lets machine remember phenomena from the past • Model determines the questions that can be answered • Model indicates memory that is needed • Lets machine carry out some calculations incrementally

8. Why use a model? • Can model defined terms as if they correspond to separate phenomena • Can model processes of a conceptual domain as if they were physical entities • Can capture and embody inference rules • Can provide surrogates for private phenomena of the modelled domain

9. Model imperfections • Model makes assumptions • Continuously varying phenomena are modeled by discrete samples • Samples are gathered at different times • Time lag: sample is gathered after event happened

10. Model Imperfections • Incompleteness: missing samples or values • Value doesn’t exist • Value is unknown • Errors

11. Example: Experimental voltages • Measure voltages at 32 points in a circuit • Display voltages as columns side by side on the screen • Display average voltage over all the points

12. More experimental voltages • Display the average voltage of each point since the experiment began • Discrete • Finite number of samples • Time lag • Model: for each point, a count and a total

13. More experimental voltages • Average over last 5 minutes • Must keep set of samples for 5 minutes • Maximum voltage • Keep maximum for each point

14. Example: Payroll System • Inputs: Time cards, New employee form, benefit choice, raise, W4 form • Outputs: Pay checks, W1 form, W2 form, check and forms to insurance company

15. Payroll problem diagram fitted to information display frame Payroll forms c a C Requirements for payroll Payroll System Output d b C a: Pf! {time cards, new employee form, benefit choice, raise, W4} C1 b: PS! {paychecks, W1, W2, checks and forms to insurance} E2 c: Pf! {time cards, new employee form, benefit choice, raise, W4} C3 d: O! {paychecks, W1, W2, checks and forms to insurance} Y4

16. Real world C3 RW!C1 C Payroll input DB - Real world Payroll DB PI!E5 Y6 X Payroll DB Y6 MD!Y7 X Reports machine Reports -DB Reports RM!E2 Y4 C

17. Payroll DB Y6 PD!Y7 X Checkwriting machine Paychecks - DB Paycheck CM!E2 Y4 C Payroll DB Y6 PD!Y7 X W2 machine W2 - DB W2 WM!E2 Y4 C

18. Model Domains are common • Compilers (Transformation) • symbol table, abstract syntax tree • Robotics (controlled behavior) • map, where we are on the map • Commanded behavior, Workpieces • undo, selections

19. Conclusion • Models are useful for information display • Models are probably useful for other problem frames • Models are an example of a problem variant, and an example of decomposing problems and composing specifications