Control table: Difference between revisions

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[[File:Control table.png|thumb|220px|A simple control table that directs program flow according to the value of a single input value. Each entry holds a possible input value and an associated subroutine to invoke.]]

A '''control table''' is a table [[data structure]] (i.e. [[Array (data structure)|array]] of [[Record (computer science)|record]]s) used to direct the [[control flow]] of a [[computer program]]. [[Software]] that uses a control table is said to be ''table-driven''.<ref>''Programs from decision tables'', Humby, E., 2007,Macdonald, 1973 ... Biggerstaff, Ted J. Englewood Cliffs, NJ : Prentice-Hall {{ISBN|0-444-19569-6}}</ref><ref>{{Cite web |url=http://www.dkl.com/wp-content/uploads/2016/05/DataKinetics-Table-Driven-Design.pdf |title=Archived copy |access-date=17 May 2016 |archive-date=10 June 2016 |archive-url=https://web.archive.org/web/20160610160908/http://www.dkl.com/wp-content/uploads/2016/05/DataKinetics-Table-Driven-Design.pdf |url-status=dead }}</ref> A control table encodes both the [[Parameter (computer programming)|parameters]] to a [[conditional (programming)|conditional expression]] and a [[Function (computer programming)|function]] [[reference (computer science)|reference]]. An [[interpreter (computing)|interpreter]] processes a table by evaluating the conditional expression for input data and invoking the selected function. Using a control table can reduce the need for repetitive code that implements the same logic. The two-dimensional nature of most tables makes them easier to view and update than the one-dimensional nature of program code.

In general, the mapping of input parameters can be via any data structure. A common data structure is the [[lookup table |lookup]] which provides relatively high performance but at a relatively high memory footprint. An [[associative array]] can minimize memory use at the cost of more lookup time.

Typical uses for a control table include:

* Transform an input value to an [[associative array |index]], for branching or [[pointer (computer programming)|pointer]] [[lookup table |lookup]]

* Transform an input value to a program name, relative [[subroutine]] number, [[label (programming language)|program label]] or program [[offset (computer science)|offset]], to alter [[control flow]]

* Control a [[main loop]] in [[event-driven programming]] using a [[control variable (programming)|control variable]] for [[state transition]]s

* Control the program cycle for [[online transaction processing]] applications

How the associated behavior is referenced varies. Some languages provide a direct function reference (i.e. [[pointer (computer programming)|pointer]]) that can be used to invoke a function directly, but some languages do not. Some languages provide for [[goto |jumping]] to a ___location (i.e.[[label (programming language)|label]]). As a fallback, any language allows for mapping input to an index that can then be used to branch to a particular part of the code.

A relatively advanced use as instructions for a [[virtual machine]] processed by an interpreter {{endash}} similar to [[bytecode]] but usually with operations implied by the table structure itself.

A control table is often used as part of a higher-level algorithm. It can control the [[main loop]] of an [[event-driven programming |event-driven program]]. A relatively advanced use asis instructions for a [[virtual machine]] processed by an interpreter {{endash}} similar to [[bytecode]] but usually with operations implied by the table structure itself insteaad of encoded in the table data.

==Data structure==

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* {{Annotated link |Keyword-driven testing}}

* {{Annotated link |Threaded code}}

* {{Annotated link |Truth table}}

==Notes==