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Spreadsheet
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=== Cells === A '''"cell"''' can be thought of as a box for holding [[data]]. A single cell is usually referenced by its column and row (C2 would represent the cell containing the value 30 in the example table below). Usually rows, representing the [[dependent and independent variables|dependent variables]], are referenced in [[decimal notation]] starting from 1, while columns representing the [[dependent and independent variables|independent variables]] use 26-adic [[bijective numeration]] using the letters A-Z as numerals. Its physical size can usually be tailored to its content by dragging its height or width at box intersections (or for entire columns or rows by dragging the column- or row-headers). {| border="1" class="wikitable" |+ My Spreadsheet ! !! A !! B !! C !! D |- !01 | Sales || 100000 || 30000 || 70000 |- !02 | Purchases || 25490 || 30 || 200 |} An array of cells is called a ''sheet'' or ''worksheet''. It is analogous to an array of [[Variable (programming)|variables]] in a conventional [[computer program]] (although certain unchanging values, once entered, could be considered, by the same analogy, [[constant (computer science)|constants]]). In most implementations, many worksheets may be located within a single spreadsheet. A worksheet is simply a subset of the spreadsheet divided for the sake of clarity. Functionally, the spreadsheet operates as a whole and all cells operate as [[global variable]]s within the spreadsheet (each variable having 'read' access only except its containing cell). A cell may contain a [[Value (computer science)|value]] or a [[formula]], or it may simply be left empty. By convention, formulas usually begin with '''=''' sign. ==== Values ==== A value can be entered from the computer keyboard by directly typing into the cell itself. Alternatively, a value can be based on a formula (see below), which might perform a calculation, display the current date or time, or retrieve external data such as a stock quote or a database value. <blockquote>'''The Spreadsheet ''Value Rule''''' Computer scientist [[Alan Kay]] used the term ''value rule'' to summarize a spreadsheet's operation: a cell's value relies solely on the formula the user has typed into the cell.<ref>{{cite journal | last1=Kay | first1=Alan | author-link=Alan Kay | title=Computer Software | journal =Scientific American | volume =251 | issue =3 | pages =52–59 |date=September 1984 | doi=10.1038/scientificamerican0984-52 | pmid=6390676 | last2=Goldstein | first2=JL| bibcode=1984SciAm.251c..52K }} – Value Rule</ref> The formula may rely on the value of other cells, but those cells are likewise restricted to user-entered data or formulas. There are no 'side effects' to calculating a formula: the only output is to display the calculated result inside its occupying cell. There is no natural mechanism for permanently modifying the contents of a cell unless the user manually modifies the cell's contents. In the context of programming languages, this yields a limited form of first-order [[functional programming]].<ref>{{cite journal|last1=Burnett |first1=Margaret|author1-link=Margaret Burnett |last2=Atwood |first2=J. |last3=Walpole Djang |first3=R. |last4=Reichwein |first4=J. |last5=Gottfried |first5=H. |last6=Yang |first6=S. |title=Forms/3: A first-order visual language to explore the boundaries of the spreadsheet paradigm |journal=Journal of Functional Programming |volume=11 |issue=2 |pages=155–206 |date=March 2001 |doi=10.1017/S0956796800003828|s2cid=18730312}}</ref> </blockquote> ==== Automatic recalculation ==== A standard of spreadsheets since the 1980s, this optional feature eliminates the need to manually request the spreadsheet program to recalculate values (nowadays typically the default option unless specifically 'switched off' for large spreadsheets, usually to improve performance). Some earlier spreadsheets required a manual request to recalculate since the recalculation of large or complex spreadsheets often reduced data entry speed. Many modern spreadsheets still retain this option. Recalculation generally requires that there are no [[circular dependency|circular dependencies]] in a spreadsheet. A [[dependency graph]] is a graph that has a vertex for each object to be updated, and an edge connecting two objects whenever one of them needs to be updated earlier than the other. Dependency graphs without circular dependencies form [[directed acyclic graph]]s, representations of partial orderings (in this case, across a spreadsheet) that can be relied upon to give a definite result.<ref>{{citation |mode=cs1 | last1=Al-Mutawa | first1=H. A. | last2=Dietrich | first2=J. | last3=Marsland | first3=S. | last4=McCartin | first4=C. | contribution=On the shape of circular dependencies in Java programs | doi=10.1109/ASWEC.2014.15 | pages=48–57 | publisher=IEEE | title=23rd Australian Software Engineering Conference | year=2014| isbn=978-1-4799-3149-1 | s2cid=17570052 }}</ref> ==== Real-time update ==== This feature refers to updating a cell's contents periodically with a value from an external source—such as a cell in a "remote" spreadsheet. For shared, Web-based spreadsheets, it applies to "immediately" updating cells another user has updated. All dependent cells must be updated also. ==== Locked cell ==== Once entered, selected cells (or the entire spreadsheet) can optionally be "locked" to prevent accidental overwriting. Typically this would apply to cells containing formulas but might apply to cells containing "constants" such as a kilogram/pounds conversion factor (2.20462262 to eight decimal places). Even though individual cells are marked as locked, the spreadsheet data are not protected until the feature is activated in the file preferences. ==== Data format ==== A cell or range can optionally be defined to specify how the value is displayed. The default display format is usually set by its initial content if not specifically previously set, so that for example "31/12/2007" or "31 Dec 2007" would default to the cell format of ''date''. Similarly adding a % sign after a numeric value would tag the cell as a [[percentage]] cell format. The cell contents are not changed by this format, only the displayed value. Some cell formats such as "numeric" or "currency" can also specify the number of [[decimal place]]s. This can allow invalid operations (such as doing multiplication on a cell containing a date), resulting in illogical results without an appropriate warning. ==== Cell formatting ==== Depending on the capability of the spreadsheet application, each cell (like its counterpart the "style" in a [[word processor]]) can be separately formatted using the [[Attribute (computing)|attributes]] of either the content (point size, color, bold or italic) or the cell (border thickness, background shading, color). To aid the readability of a spreadsheet, cell formatting may be conditionally applied to data; for example, a negative number may be displayed in red. A cell's formatting does not typically affect its content and depending on how cells are referenced or copied to other worksheets or applications, the formatting may not be carried with the content. ==== Named cells ==== [[File:Named Variables in Excel.PNG|thumb|400px|Use of named column variables ''x'' & ''y'' in [[Microsoft Excel]]. Formula for y=x<sup>2</sup> resembles [[Fortran]], and ''Name Manager'' shows the definitions of ''x'' & ''y''.]] In most implementations, a cell, or group of cells in a column or row, can be "named" enabling the user to refer to those cells by a name rather than by a grid reference. Names must be unique within the spreadsheet, but when using multiple sheets in a spreadsheet file, an identically named cell range on each sheet can be used if it is distinguished by adding the sheet name. One reason for this usage is for creating or running macros that repeat a command across many sheets. Another reason is that formulas with named variables are readily checked against the algebra they are intended to implement (they resemble Fortran expressions). The use of named variables and named functions also makes the spreadsheet structure more transparent. ===== Cell reference ===== In place of a named cell, an alternative approach is to use a cell (or grid) reference. Most cell references indicate another cell in the same spreadsheet, but a cell reference can also refer to a cell in a different sheet within the same spreadsheet, or (depending on the implementation) to a cell in another spreadsheet entirely, or a value from a remote application. A typical '''cell reference''' in "A1" style consists of one or two case-insensitive letters to identify the column (if there are up to 256 columns: A–Z and AA–IV) followed by a row number (e.g., in the range 1–65536). Either part can be relative (it changes when the formula it is in is moved or copied), or absolute (indicated with $ in front of the part concerned of the cell reference). The alternative "R1C1" reference style consists of the letter R, the row number, the letter C, and the column number; relative row or column numbers are indicated by enclosing the number in square brackets. Most current spreadsheets use the A1 style, some providing the R1C1 style as a compatibility option. When the computer calculates a formula in one cell to update the displayed value of that cell, cell reference(s) in that cell, naming some other cell(s), causes the computer to fetch the value of the named cell(s). A cell on the same "sheet" is usually addressed as: <code> =A1 </code> A cell on a different sheet of the same spreadsheet is usually addressed as: =SHEET2!A1 (that is; the first cell in sheet 2 of the same spreadsheet). Some spreadsheet implementations in Excel allow cell references to another spreadsheet (not the currently open and active file) on the same computer or a local network. It may also refer to a cell in another open and active spreadsheet on the same computer or network that is defined as shareable. These references contain the complete filename, such as: ='C:\Documents and Settings\Username\My spreadsheets\[main sheet]Sheet1!A1 In a spreadsheet, references to cells automatically update when new rows or columns are inserted or deleted. Care must be taken, however, when adding a row immediately before a set of column totals to ensure that the totals reflect the values of the additional rows—which they often do not. A [[circular reference]] occurs when the formula in one cell refers—directly, or indirectly through a chain of cell references—to another cell that refers back to the first cell. Many common errors cause circular references. However, some valid techniques use circular references. These techniques, after many spreadsheet recalculations, (usually) converge on the correct values for those cells. ===== Cell ranges ===== Likewise, instead of using a named range of cells, a range reference can be used. Reference to a range of cells is typical of the form (A1:A6), which specifies all the cells in the range A1 through to A6. A formula such as "=SUM(A1:A6)" would add all the cells specified and put the result in the cell containing the formula itself.
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