Decision table - AbsoluteAstronomy.com

Decision tables are a precise yet compact way to model complicated logic.

Decision tables, like flowchart

Flowchart

A flowchart is a type of diagram that represents an algorithm or process, showing the steps as boxes of various kinds, and their order by connecting these with arrows. This diagrammatic representation can give a step-by-step solution to a given problem. Process operations are represented in these...

s and if-then-else and switch-case

Switch statement

In computer programming, a switch, case, select or inspect statement is a type of selection control mechanism that exists in most imperative programming languages such as Pascal, Ada, C/C++, C#, Java, and so on. It is also included in several other types of languages...

statements, associate conditions with actions to perform, but in many cases do so in a more elegant way.

In the 1960s and 1970s a range of "decision table based" languages such as Filetab

Filetab

Filetab is a decision table-based computer programming language.Filetab has a long history, originally designed in the late 1960s and descended from the DETAB programming...

were popular for business programming.

Structure

**The four quadrants**
Conditions	Condition alternatives
Actions	Action entries

Each decision corresponds to a variable, relation or predicate whose possible values are listed among the condition alternatives.
Each action is a procedure or operation to perform, and the entries specify whether (or in what order) the action is to be performed for the set of condition alternatives the entry corresponds to.
Many decision tables include in their condition alternatives the don't care symbol, a hyphen. Using don't cares can simplify decision tables, especially when a given condition has little influence on the actions to be performed.
In some cases, entire conditions thought to be important initially are found to be irrelevant when none of the conditions influence which actions are performed.

Aside from the basic four quadrant structure, decision tables vary widely in the way the condition alternatives and action entries are represented. Some decision tables use simple true/false values to represent the alternatives to a condition (akin to if-then-else), other tables may use numbered alternatives (akin to switch-case), and some tables even use fuzzy logic or probabilistic representations for condition alternatives. In a similar way, action entries can simply represent whether an action is to be performed (check the actions to perform), or in more advanced decision tables, the sequencing of actions to perform (number the actions to perform).

Example

The limited-entry decision table is the simplest to describe. The condition alternatives are simple Boolean values, and the action entries are check-marks, representing which of the actions in a given column are to be performed.

A technical support company writes a decision table to diagnose printer problems based upon symptoms described to them over the phone from their clients.

The following is a balanced decision table.

**Printer troubleshooter**
		Rules
Conditions	Printer does not print	Y	Y	Y	Y	N	N	N	N
	A red light is flashing	Y	Y	N	N	Y	Y	N	N
	Printer is unrecognised	Y	N	Y	N	Y	N	Y	N
Actions	Check the power cable			X
	Check the printer-computer cable	X		X
	Ensure printer software is installed	X		X		X		X
	Check/replace ink	X	X			X	X
	Check for paper jam		X		X

Of course, this is just a simple example (and it does not necessarily correspond to the reality of printer troubleshooting), but even so, it demonstrates how decision tables can scale to several conditions with many possibilities.

Software engineering benefits

Decision tables, especially when coupled with the use of a domain-specific language, allow developers and policy experts to work from the same information, the decision tables themselves.

Tools to render nested if statements from traditional programming languages into decision tables can also be used as a debugging tool

Decision tables have proven to be easier to understand and review than code, and have been used extensively and successfully to produce specifications for complex systems.

Program embedded decision tables

Decision tables can be, and often are, embedded within computer programs and used to 'drive' the logic of the program. A simple example might be a lookup table

Lookup table

In computer science, a lookup table is a data structure, usually an array or associative array, often used to replace a runtime computation with a simpler array indexing operation. The savings in terms of processing time can be significant, since retrieving a value from memory is often faster than...

containing a range of possible input values and a function pointer

Function pointer

A function pointer is a type of pointer in C, C++, D, and other C-like programming languages, and Fortran 2003. When dereferenced, a function pointer can be used to invoke a function and pass it arguments just like a normal function...

to the section of code to process that input.

**Static decision table**
Input	Function Pointer
'1'	Function 1 (initialize)
'2'	Function 2 (process 2)
'9'	Function 9 (terminate)

Multiple conditions can be coded for in similar manner to encapsulate the entire program logic in the form of an 'executable' decision table or control table

Control table

Control tables are tables that control the program flow or play a major part in program control. There are no rigid rules concerning the structure or content of a control table - its only qualifying attribute is its ability to direct program flow in some way through its 'execution' by an associated...

Structure

Example

Software engineering benefits

Program embedded decision tables

See also

Further reading