====== Calculate Value ======

===== Description =====

Evaluates an algebraic or logical expression once and returns a single scalar value. Unlike the map and lookup table calculators, there is no current cell, row, or spatial grid: the expression is evaluated in a purely scalar context, drawing only on tables, lookup tables, and scalar values connected through hooks.

This is one of Dinamica EGO's five calculator functors. Its abbreviated (shorthand) syntax is ''$''. See [[ego_script#calculator_functor_shorthand|Calculator functor shorthand]] in the EGO Script documentation for the verbose and abbreviated forms, and [[Calculate Functors|Calculate Functors — Complete Operator Documentation]] for the full expression language reference.

===== Inputs =====

^ Name  ^ Type  ^ Description  ^
| Expression  | [[Image Expression Type]]  | Algebraic or logical expression that evaluates to a single numeric result. Must be enclosed in square brackets ''[ ]'' in EGO Script.  |

===== Optional Inputs =====

^ Name  ^ Type  ^ Description  ^ Default Value  ^
| Default Value  | [[Real Value Type]]  | Value returned if evaluating the expression produces an algebraic error. When not provided, evaluation errors are propagated. This is an advanced port.  | .none  |

===== Outputs =====

^ Name  ^ Type  ^ Description  ^
| Result  | [[Real Value Type]]  | The computed scalar value.  |

===== Group =====

[[Functor List#Table | Table]]

===== Notes =====

==== Connecting data inputs ====

Tables, lookup tables, and scalar values referenced in the expression must be connected to the functor's ports before writing the expression, using hook functors:

  * Tables and lookup tables → [[Number Table]] → referenced in the expression as t1, t2, …, t100
  * Scalar values → [[Number Value]] → referenced as v1, v2, …, v100

Maps cannot be connected — there is no cell context in this functor. In the abbreviated syntax, tables are referenced as ''%variableName'' and values as ''$variableName''.

==== Available expression features ====

Image operators (''iX''), the line and column spatial keywords, neighbourhood functions, neighbourhood indexing, and convolution are not available — there is no cell or row context. Available features are arithmetic and logical operators and table and lookup table queries (''tX[...]'') in all single-key and multi-column forms. See [[Calculate Functors#The Expression Language|The Expression Language]] for the complete reference, or [[Image Expression Type]] for the navigable operator index.

All operations are performed using the precision defined by ImageExpression::Value; the final result is then converted to a double-precision real. See [[Calculate Value 2]] for a variant that instead performs all operations using double precision throughout.

==== Null value handling ====

Null propagation rules and defensive patterns are covered in [[Calculate Functors#Null Value Handling|Null Value Handling]] in the Calculate Functors documentation.

===== Internal Name =====

CalculateValue

===== Usage examples =====

  * [[Calculate Functors#CalculateValue practical examples|CalculateValue practical examples]] in the Calculate Functors documentation
  * [[lesson_8|Lesson 8: Inserting and getting a specific value into a table]]