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## Spreadsheet
A One of the first commercial uses of computers was in processing payroll and other financial records, so the programs (and, indeed, the programming languages themselves) were designed to generate reports in the standard "spreadsheet" format bookkeepers and accountants used. The more available and affordable computers themselves became in the last quarter of the 20th century, the more software became available for them, and programs to keep financial records and generate spreadsheet reports were always in demand. Those spreadsheet programs can be used to tabulate many kinds of information, not just financial records, so the term "spreadsheet" has developed a more general meaning as information (= data = facts) presented in a rectangular table, usually generated by a computer. ## Spreadsheet programmingThe cells are functionally equivalent to variables in a sequential programming model. Cells often have a formula, a set of instructions which can be used to compute the value of a cell. Formulas can use the contents of other cells or external variables such as the current date and time. It is often convenient to think of a spreadsheet as a mathematical graph, where the nodes are spreadsheet cells, and the edges are references to other cells specified in formulas. This is often called the dependency graph of the spreadsheet. References between cells can take advantage of spatial concepts such as relative position and absolute position, as well as named locations, to make the spreadsheet formulas easier to understand and manage. Spreadsheets usually attempt to automatically update cells when the cells on which they depend have been changed. The earliest spreadsheets used simple tactics like evaluating cells in a particular order, but modern spreadsheets compute a minimal recomputation order from the dependency graph. Later spreadsheets also include a limited ability to propagate values in reverse, altering source values so that a particular answer is reached in a certain cell. Since spreadsheet cells formulas are not generally invertable, though, this technique is of somewhat limited value. Many of the concepts common to sequential programming models have analogues in the spreadsheet world. For example, the sequential model of the indexed loop is usually represented as a table of cells, with similar formulas. Cyclic dependency graphs produce the traditional construct known as the infinite loop. Most spreadsheets allow iterative recalculation in the presence of these cyclic dependencies, whch can be either directly controlled by a user or which stop when threshold conditions are reached. The power of spreadsheets derives largely from the fact that human beings have a well developed intuition about spaces, and a well developed notion of dependency between items. Thus, many people find it easier to perform complex calculations in a spreadsheet than writing the equivalent sequential program. ## Origins of the SpreadsheetThe computerized spreadsheet was invented by Pardo and Landau, who applied for a patent on it in 1970. The spreadsheet concept was reinvented by Dan Bricklin. Legend has it that Bricklin was watching his university professor create a table of calculation results on a blackboard. When the professor found an error, he had to tediously erase and rewrite a number of sequential entries in the table, triggering Bricklin to think that he could replicate the process on a computer, using a blackboard/spreadsheet paradigm to view results of underlying formulas. His idea became VisiCalc, the first spreadsheet, and the "killer application" that turned the personal computer from a hobby for computer enthusiasts into a business tool.
- VisiCalc (still available - http://www.bricklin.com/history/vcexecutable.htm)
- MultiPlan
- Lotus Improv
Programs still in production
- Microsoft Excel
- Quattro Pro
- Lotus 1-2-3
- Gnumeric
- KSpread
- OpenOffice.org Calc
- Framework
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