Attrib of Obj


Attrib Of Obj

Returns the value of attribute «Attrib» of object «Obj». «Attrib» and «Obj» may each be an identifier, a text value containing identifiers, or a handle to the attribute or object. For example:

Units OF Time → 'Years'
'Units' OF 'Time' → 'Years'
'Units' OF HandleFromIdentifier('Time') → 'Years'

If the requested attribute is not set for that object, it returns Null. This works if «Attrib» or «Obj» are arrays of attribute or object names or handles.

Computing «obj»

Sometimes you may want to use an expression for «obj» rather than just a literal identifier , especially when doing Meta-Inference. For example:

Decision A := 'Yes'
Variable B := Handle(A)
Class OF B → Variable

You might have expected that the second expression would return Decision, the class of variable 'A' whose handle is the value of B. But the parser naturally assumes you want the attribute of the object B, not its value. If you want to get the value of the variable or expression in «Obj», you need use an expression that is not just the name of a variable. You can simply put parentheses around the identifier:

Class OF (B) → Decision

This can be particularly useful if you want to get an attribute of a list of objects:

INDEX Vars := ListOfHandles(X, Y, Z)
Class OF (Vars) → Array(Vars, [Decision, Variable, Index])

It's fine to use the name of a variable (or more complex expression) for «attrib». If «attrib» is not simply the identifier of an attribute, it evaluates the expression. It expects to find a handle to an attribute, or a text value with the identifier of an attribute, or an array of such handles or text values, for example:

Variable Attribs := ['Class', 'Identifier', 'Value']
Attribs OF A→ Array(Attribs, ['Decision', 'A', 'Yes'])

You can use OF to get attributes of a function:

Description OF Evaluate

returns the description of the evaluate function, while

Description OF Evaluate('Va1')

returns the description of Va1.

The OF operator parses in a right-associative fashion, so that: Class OF Value OF B→ Decision parses as:

Class OF (Value OF B)→ Decision

OF binds more tightly than arithmetic operators, but less tightly than the Subscript/Slice Operator. So, for example, to access an attribute of a local index of an object, parens are not necessary:

Index I := 1..2;
Description OF I := "A simple index";
Var A := I^2;
Description OF A.I & "=" & A

The last line parses as

((Description OF (A.I)) & "=") & A

Assigning Values to Attributes

You can set the value of attributes using the syntax:

attrib OF obj := expr

As with the standard Assignment Operator :=, you can use this only in the OnClick attribute of a Button, OnChange of a Variable, Script attribute (obsolete), or a Function called from one of these. You may also assign to an attribute of a Local variable within a Definition.

You may not assign to read-only attributes, like Inputs, and Outputs. You can assign a handle to a module IsIn, which has the effect of moving the object into that module. You can assign to the Value of a variable, but that is dangerous because it may be inconsistent with its Definition.

As with the simple construct, «attrib» OF «obj» described above, «attrib» and «obj» may each be a simple identifier, a text value containing an identifier, or a handle to the attribute or object. And if «obj» is a variable X containing a handle or text value referring to the object of interest, you must put parentheses around it Title OF (X) if you want the Title of what X refers to instead of X itself.

However, for attribute assignment, «attrib» and «obj» must be atoms, not arrays. It does NOT array abstract.

For most attributes, «expr» should to evaluate to text, even in this case where it appears the attribute contains numbers:

NodeColor OF Va1 := '16000,8000,65535'

You may only assign a text value to the definition attribute, e.g.:

Definition OF Va1 := "A + B"

When you assign directly to a variable:

Va1 := "A + B"

it sets the definition to the text value 'A+B' (with quotes), rather than the expression A+B (without quotes) that depends on the values of A and B.

You may assign to attributes of a local index(declared using Index..Do).

To remove an attribute value, assign Null to it:

Units of Va1 := Null

Note: Internally, Analytica distinguishes between an attribute set to the special value Null, and an attribute that has no value. However, it is not possible to set an attribute value to Null from an Analytica expression (although you can set value to lists or arrays containing Null elements).

No auto propagation of changes to attributes other than Definition and Identifier

Analytica maintains dependencies between variables specified in their Definitions, as reflected in the influence arrows. So, if you change the definition of variable, A, it invalidates the value of any variable B that depends on A, and recomputes B when needed. It also automatically propagates any change to an identifier to update any definitions that use that identifier. This works for CheckAttribute, OnClick, and OnChange attributes. But not other attributes! So if you (or some Analytica code) change, say, the Units of A, it will not invalidate or recompute B if it depends on the Units of A:

A := 20
Units OF A := 'KW'
B := IF Units OF A = 'KW' THEN 1000*A ELSE IF Units of A = 'MW' THEN 1M*A ELSE A
B → 20000 { Because Units of A are KW }
Units OF A := 'MW'
B → 20000 { Changing Units of A did not cause B to be recomputed }

See Also

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