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.
Historically, the word spreadsheet is derived from an accounting term. Spreadsheets in accounting are large sheets of paper with a printed grid, used to organize figures for review by managers and supervisors. The first spreadsheet programs attempted to provide an electronic equivalent. They appear to have been very successful. The spreadsheet has become a central component in the office applications suite.
Spreadsheet programs
Programs of historical interest
Programs still in production
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