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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

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

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

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

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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

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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

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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

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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

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

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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

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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

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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

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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

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

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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.

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

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

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

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

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

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

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

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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

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

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EuSpRIG 2004Computational Models of Spreadsheet Development25/23 Thank you

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