View source for Domain Expressions

''new to [[Analytica 4.3]]''

[[image:DomainExpressionPopup.png]]

The [[Domain Attribute|domain]] of a variable specifies the space of possible values for each atomic cell of the result.  Using the expression domain view (see above screenshot), arbitrary Analytica expressions can be employed to compute this space.  The computed domain may even vary along one or more indexes.

The computed domain makes use of special ''domain specification functions'' ([[Continuous]], [[Discrete]], [[Integer]], [[Boolean]] and [[Discrete]]).  In the simplest case, a domain expression contains a single call to one of these functions, e.g.:

 [[Continuous]](lb:-1, ub:1, nullOk:false )

More complex cases (such as in the above screenshot), arbitrary logic may be employed to compute the appropriate domain.  The return value(s) for such an expression is typically a call to one of the 5 domain specification functions.

In addition to the 5 domain specification functions, a domain expression may return [[Null]] (for ''Automatic'', meaning ''unspecified''), or an unindexed list for a set of explicit values.  In general, it is preferred to return explicit values using a call to [[Discrete]], listing the possible values as parameters, e.g.:

 [[Discrete]]('Democrat', 'Republican', 'Independent', 'Peace and Freedom', 'Green Party', 'Libertarian', 'Tea Party')

When the domain expression consists of a single identifier, this is taken to mean ''Copy from index'', which copies the possible values from the index value of the indicted variable.  This particular case is thus different from the behavior of the definition attribute that contains a single identifier, where the mid or prob value of the variable is returned.  You can compute the domain within the definition of another variable, but to use it you would need to write the domain expression as: <code>[[Mid]](theComputedDomain)</code>, where ''theComputedDomain'' is the variable containing the computed domain expression in its definition.

The domain expression itself is always evaluated in [[Evaluation Modes|mid mode]].

A computed domain is stored in a special system attribute called '''DomainValue'''.  It would be very rare for your own Analytica expressions to make use of this information, but certain classes of meta-inference algorithms might want to do so.  These algorithms can access this using <code>(DomainValue of «obj»)</code>.  The encoding makes use of data structures containing parsed calls to the domain specification functions.  The functions [[ParsedExprParameters]] and [[ParsedExprFunctions]] can be used.  If you attempt to evaluate a domain specification function directly (such as in a definition), you will also get a similar parsed structure back.

= Domain Specification Functions =

== Boolean() ==
:Parameters: '''Boolean(''nullOk'')'''

Specifies that the only values allowed are 0, 1, and [[Null]].  If «nullOk» is specified as false, then the only allowed values are 0 and 1.

<code>Boolean()</code> is equivalent to <code>[[Integer]](0,1)</code>, so it is predominantly a convenience function.  

Frontline solver engines, used by Analytica Optimizer, do distinguish between boolean and integer variables, so it is conceivable that the internal algorithms used to solve optimization problems may differ slightly when <code>[[Integer]](0,1)</code> is used rather than <code>Boolean()</code>.

== Continuous() ==
:Parameters: '''Continuous(''lb,ub,nullOk'')'''

Specifies a real-valued space of possible values.  Optional lower and upper bounds («lb» and «ub») are used to identify a closed interval of the real-number line.

[[Null]] values are considered acceptable by default unless the «nullOk» is specified as false.

'''Examples:'''

 Continuous()

 Continuous(lb:0)

 Continuous(0,1)

 Continuous(ub:100, nullOk:false)

== Discrete() ==
:Parameters: '''Discrete(''allowed..., type, nullOk'')'''

Specifies that the space of possible values is discrete (i.e., not continuous).  When the set of «allowed» values is unspecified, this is interpreted as meaning that the set of values is non-continuous, but the exact full set of possible values is unspecified (i.e., the ''Discrete'' option on the domain menu).  When one or more «allowed» values is specified, this is interpreted as meaning that the listed values are the only values considered to be acceptable; however, [[Null]] is also considered to be acceptable unless «nullOk» is specified explicitly.

The «type» parameter, which must be used in named-parameter syntax, may be used to specify the acceptable data types for the discrete values.  The parameter recognizes the following values: 'any', 'text', 'number', 'handle', and a subset of types can be specified by providing a list of these values to the parameter.  The list-entry GUI will only operate when the type is limited to 'all', 'text' or 'number', but bounds checking and other facilities can make use of the other types.

'''Examples:'''

 Discrete()

 Discrete('item 1','item 2','item 3','item 4')

 Discrete(type:'number')

 Discrete(type:['number','text'])

 Discrete(1,2,'n/a',nullOk:false)

== GroupedInteger() ==

Parameters: '''GroupedInteger(''groupName'')

This domain type is intended specifically for a particular class of optimization problems.  When a variable is of grouped-integer type, it means in any feasible solution, each scalar value belonging to the same «groupName» must have a value between ''1'' and ''N'', where ''N'' is the total number of items in the same group, and all these scalar values must be different.

In a non-optimized case (such as just for bounds checking), the domain type is treated simply as equivalnet to <code>Integer(lb:1)</code>.