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Computational Models of Spreadsheet Development

Basis for Educational Approaches

Karin Hodnigg, Markus Clermont, Roland T. MittermeirUniversity of Limerick, University of Klagenfurt

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Motivation

To raise spreadsheet quality, spreadsheet education

has to start from the right paradigms.

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Outline

Conventional Programming Paradigm and Spreadsheet Programming Characteristics

Implementing the Spreadsheet Paradigm

Spreadsheet Semantics(Project-Screen-Paradigm)

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Conventional Programming Paradigm and Spreadsheet Programming Characteristics

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The Spreadsheet Paradigm

• Spreadsheets ≠ conventional program

• Data dependencies – Data Flow Program (?)

– Graph Reduction Problem (?)

A1

A2 B2

B4

A3

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Data Flow Semantics

A1

A2

B4

A3

Data Flow Semantics Spreadsheet Semantics

100

100A2 B2

B4

A3100

1

1

10

10

B2

0.2

100 100

c

A1A1

c

cA2 B2

B4

1

10 100

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Graph Reduction Semantics

A1

A2 B2

B4

A3A2 B2

B4

A3

A1

Graph Reduction Semantics Spreadsheet Semantics

1

10 100 100

0.2

10010010

1

0.2

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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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Implementing the Spreadsheet Paradigm

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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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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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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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Copy/Paste and movement heuristics

• Copy-and-Paste of cells ≠ Moving cells

range references > distinct cell references > geometrical pattern

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Spreadsheet Semantics and the Projection-Screen-Model

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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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The Projection Screen Model

value

valuevalue value

value

literalliteral

formula

formula

formula

literal

value

value

value

value value

value

literalliteral

formula

literal

formula

formula

value

valueaggregation

formula

aggregation range

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The Projection Screen Model with Aggregation

value

valuevalue value

value

literalliteral

formula

formula

formula

literal

value

value

value

value value

value

literalliteral

formula

literal

formula

formula

value

valueaggregation

formula

aggregation range

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Patterns resulting from development process

• Many-handed figures• Queue on a Staircase• Flying Carpets• Recursive Images

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Patterns: Many-handed figures

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Patterns: Queue on a staircase

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Patterns: Flying Carpets

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Patterns: Recursive Images

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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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“The limits of my languagemeans the limits of my world.”

– Ludwig Wittgenstein –

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Thank you