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

===== Description =====

Evaluates an 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.

This functor is one of five in the [[calculate_functors|calculator family]]. Use it to compute a single parameter — such as a global threshold, a weighted sum of scalar inputs, or a value derived from a table — that can then be passed as input to other functors.

The **abbreviated syntax** (also called **shorthand**) for this functor is ''$''. See [[ego_script#calculator_functor_shorthand|Calculator functor shorthand]] in the EGO Script documentation for the full syntax of both the verbose and abbreviated forms, and [[calculate_functors|Calculate Functors — Complete Operator Documentation]] for the complete expression language reference.

===== Inputs =====

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

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

^ Name ^ Type ^ Default ^ Description ^
| Default Value | [[Real Value Type]] | None | Value returned if the expression produces an algebraic error. When provided, acts as a catch-all fallback equivalent to wrapping the entire expression in the ''?'' operator. |

===== Output =====

^ 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.

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