1. EuSpRIG 2004Computational Models of Spreadsheet Development1/23 Computational Models of Spreadsheet Development Basis for Educational Approaches Karin Hodnigg, Markus Clermont, Roland T. Mittermeir University of Limerick, University of Klagenfurt

2. EuSpRIG 2004Computational Models of Spreadsheet Development2/23 Motivation To raise spreadsheet quality, spreadsheet education has to start from the right paradigms.

3. EuSpRIG 2004Computational Models of Spreadsheet Development3/23 Outline Conventional Programming Paradigm and Spreadsheet Programming Characteristics Implementing the Spreadsheet Paradigm Spreadsheet Semantics (Project-Screen-Paradigm)

4. EuSpRIG 2004Computational Models of Spreadsheet Development4/23 Conventional Programming Paradigm and Spreadsheet Programming Characteristics

5. EuSpRIG 2004Computational Models of Spreadsheet Development5/23 The Spreadsheet Paradigm Spreadsheets ≠ conventional program Data dependencies –Data Flow Program (?) –Graph Reduction Problem (?) A1 A2B2 B4 A3

6. EuSpRIG 2004Computational Models of Spreadsheet Development6/23 Data Flow Semantics A1 A2 B4 A3 Data Flow Semantics Spreadsheet Semantics 100 A2B2 B4 A3 100 1 1 10 B2 0.2 100 c A1 c c A2B2 B4 1 10 100

7. EuSpRIG 2004Computational Models of Spreadsheet Development7/23 Graph Reduction Semantics A1 A2B2 B4 A3 A2B2 B4 A3 A1 Graph Reduction Semantics Spreadsheet Semantics 1 10100 0.2 100 10 1 0.2

8. EuSpRIG 2004Computational Models of Spreadsheet Development8/23 The Whole Picture Local Evaluation (of a cell‘s value) –graph reduction (DDG), possibly lazy Global Evaluation (changes in a cell) –change tokens (reverse DDG), eager Scope –Conventional scope hierarchy local and global (classes/blocks) –Spreadsheet’s scope complete sheet portion of sheet (possibly) affected by change

9. EuSpRIG 2004Computational Models of Spreadsheet Development9/23 Implementing the Spreadsheet Paradigm

10. EuSpRIG 2004Computational Models of Spreadsheet Development10/23 Evaluation Strategies -Excel© value recalculation and change -Propagation (Formula Evaluator): list -Gnumeric –Distinct data structures for single dependencies (=A1) range dependencies (=SUM(A1:A5)) –Every (transitively) dependent cell in a list (“evaluation queue”) –Recursive traversing of the given expression tree

11. EuSpRIG 2004Computational Models of Spreadsheet Development11/23 Circular References No explicit control flow  no loop concept Microsoft Excel© –Ignorable warning, cell provides 0-value –“Iteration incorporation”: control panel cell re-evaluation after any change Gnumeric –Calculated without any warning –Re-evaluation on change in the transitive closure –Some functions support circular references

12. EuSpRIG 2004Computational Models of Spreadsheet Development12/23 Development Process Specialties Automated fill of cells (click and drag) –Automated generation of formulas / constants Drag-and-Drop Copy-and-Paste Adaptation of parameters –of formulas –of references

13. EuSpRIG 2004Computational Models of Spreadsheet Development13/23 Copy/Paste and movement heuristics Copy-and-Paste of cells ≠ Moving cells range references > distinct cell references > geometrical pattern

14. EuSpRIG 2004Computational Models of Spreadsheet Development14/23 Spreadsheet Semantics and the Projection-Screen-Model

15. EuSpRIG 2004Computational Models of Spreadsheet Development15/23 Relationships and Visibility Cell level  functional paradigm –Computations in cells are mainly covered. Global “picture”  ? –proposed model has to meet the basis. Visibility and Scope –(Transitively) dependant cells –Implicit visibility of cells Source cell does not “see” the target cell. Cell “sees” all referenced cells.

16. EuSpRIG 2004Computational Models of Spreadsheet Development16/23 The Projection Screen Model

17. EuSpRIG 2004Computational Models of Spreadsheet Development17/23 The Projection Screen Model with Aggregation

18. EuSpRIG 2004Computational Models of Spreadsheet Development18/23 Patterns resulting from development process Many-handed figures Queue on a Staircase Flying Carpets Recursive Images

19. EuSpRIG 2004Computational Models of Spreadsheet Development19/23 Patterns: Many-handed figures

20. EuSpRIG 2004Computational Models of Spreadsheet Development20/23 Patterns: Queue on a staircase

21. EuSpRIG 2004Computational Models of Spreadsheet Development21/23 Patterns: Flying Carpets

22. EuSpRIG 2004Computational Models of Spreadsheet Development22/23 Patterns: Recursive Images

23. EuSpRIG 2004Computational Models of Spreadsheet Development23/23 Summary Spreadsheet System Implementations are in conflict –with established paradigms –with each other No established conceptual model –global vs. local viewpoint Proper training to raise spreadsheet quality needed! - “Project-Screen”-Model

24. EuSpRIG 2004Computational Models of Spreadsheet Development24/23 “The limits of my language means the limits of my world.” – Ludwig Wittgenstein –

25. EuSpRIG 2004Computational Models of Spreadsheet Development25/23 Thank you