Spreadsheet

A spreadsheet is a rectangular table (or grid) of information, often financial information. (It is, therefore, a kind of matrix.) The word came from "spread" in its sense of a newspaper or magazine item (text and/or graphics) that covers two facing pages, extending across the center fold and treating the two pages as one large one. The compound word "spread-sheet" came to mean the format used to present bookkeeping ledgers -- with columns for categories of expenditures across the top, invoices listed down the left margin, and the amount of each payment in the cell where its row and column intersect, for example -- which were traditionally a "spread" across facing pages of a bound ledger (= book for keeping accounting records) or on oversized sheets of paper ruled into rows and columns in that format and approximately twice as wide as ordinary paper.

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 programming

Just as the early programming languages were designed to generate spreadsheet printouts, programming techniques themselves have evolved to process tables (= spreadsheets = matrices) of data more efficiently in the computer itself. A spreadsheet program is designed to perform general computation tasks using spatial relationships rather than time as the primary organizing principle. Many programs designed to perform general computation use timing, the ordering of computational steps, as their primary way to organize a program. A well defined entry point is used to determine the first instructions, and all other instructions must be reachable from that point.

In a spreadsheet, however, a set of cells is defined, with a spatial relation to one another. In the earliest spreadsheets, this arrangments were a simple two-dimensional grid. Over time, the model has been expanded to include a third dimension, and in some cases a series of named grids. The most advanced examples allow inversion and rotation operations which can slice and project the data set in various ways.

The 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 Spreadsheet

The 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.

Spreadsheet programs

Programs of historical interest

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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