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Lecture 21Lecture 21

Using & Managing Data:Using & Managing Data:SpreadsheetsSpreadsheets

(S&G, (S&G, §§§§11.111.1––11.2)11.2)

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Some Important Applications ofComputers

• Spreadsheets

• Databases

• Symbolic & numeric computation

• Networks

• Artificial Intelligence

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Relation to Algorithms

• Algorithmic problem solving is used inthese applications

• Algorithmic problem solving uses theseapplications

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Spreadsheet• Visual system for representing and

manipulating tabular data• Often used for:

– budgets & other financial data– grades

• Useful for keeping dependant dataconsistent

• Examples: Visicalc, Excel, Lotus 1-2-3

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Example Spreadsheet

RowCursor

Column Cells

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Cells• Each cell has a “name”

– e.g. “D2” is the cell in column D and row 2• Cells can contain character data

– e.g., names, column headings• Cells can contain numeric data in various

formats– e.g., integer, real, currency, date, time

• Cells can contain formulas– e.g., “D2*E2”

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Relative & Fixed Cell References• A name of the form “D2” is a relative

reference to the cell in column D and row 2• A name of the form “$D$2” is a fixed

reference to the cell in column D and row 2• When a formula is copied into another cell:

– the fixed references refer to the same cells asthey originally did

– the relative references refer to cells in the samerelative positions

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Example

• Suppose in G2 we have the formula:D2 * (1 + $C$8 / 100) * E2

• When we copy it to G3 it will be:D3 * (1 + $C$8 / 100) * E3

• When we copy it to G4 it will be:D4 * (1 + $C$8 / 100) * E4

• And so forth

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Run Excel Spreadsheet Example

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Beginning Spreadsheet

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Formula Entered in F2

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Result of Formula in F2

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Formula Copied into Rest ofColumn

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Result of Formulas in Column F

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Example Pie Chart of Column F

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Scatter Chart of Columns D vs. C

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Compute Sum of Column F

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Resulting Sum

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Formula for Projected Base PayIncrease

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Resulting New Pay

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Formula Relativized to Row 3

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Formula Relativized to Row 4

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Change Format of Column G

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Resulting Reformated Column G

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Insert Column for Merit Raises

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New Formula to Include MeritRaises

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Resulting Projected New Total

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Bar Chart of Old and New Pay

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Goal Seeking: Adjust Base Rateso Total = $12,000

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Calculated Base Rate to MeetGoal

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Computer Science Issues

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Update Strategies

• Reevaluate every cell• Reevaluate only those cells containing

formulas– works on small spreadsheets

• Reevaluate only those cells that depend onthe changed cell– problem: indirect dependencies (ripple effect)

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Indirect Dependency

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Result

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Problem of Indirect Dependencies• Suppose change D2

– Could look for all cells with a formula involving D2, andreevaluate those

– But suppose F9 depends on F2, and F2 depends on D2• Bad solution:

1. check all cells & update those directly dependant on D22. keep a list of them3. check all cells & update those directly dependent on those

in list4. keep a list of these, and return to (3)5. continue until there are no changes

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Better Solution• Keep with each cell a list of all other cells

directly dependant on it• Update this list whenever a formula is

changed• Whenever a cell’s value is changed:

– reevaluate all the cells that depend directly on it– reevaluate all the cells that depend directly on

those cells– and so forth

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Event-Driven Programming• Each cell contains either:

– data– or a formula, which is a little program– these formulas are interpreted directly (not compiled

into machine code)• Whenever a cell is modified:

– it creates an event– which may trigger other events– must end (no circular dependencies)

• There is no iteration except that provided throughthe built-in functions & tools