# MetaIndex..Do

## MetaIndex I := expr Do body

This declaration creates a local index object that does not evaluate its index elements when performing associative lookup, such as with Subscript or @[I=x].

A MetaIndex..Do declaration is identical to a Index..Do declaration except that the local index object created as the MetaOnly attribute set to 1.

Local indexes declared using MetaIndex are used in Meta-Inference, and are a topic for advanced Analytica expert modelers. For most modeling tasks, use Index..Do.

## When to use

MetaIndex is most useful when performing meta-inference -- i.e. reasoning about variables. For example, if you want to work on a list of objects. If you used a simple Index, referring to that index will cause each variable mentioned to be evaluated. You usually don't need or want them to be evaluated when looking at the model structure, so a MetaIndex would be useful. See meta-inference to learn more about this advanced topic.

## Controlling the Index Name

It is possible to control the index name, which may be different from the local variable name referenced in the body. The index name can even be computed dynamically. The syntax for doing this is:

MetaIndex J / nameExpr := seqExpr do bodyExpr

Here «nameExpr» is evaluated and must result in a text string that contains a valid identifier. The «nameExpr» is then used in the A.I notation. The local name, «J» here, can be referenced within «bodyExpr».

One instance where this control over the name is required is when you write a user-defined function that creates an N-dimensional array, where the dimensionality, N, is not known in advance. Here you must loop over 1..N, creating a new local index during each iteration, but with a separate name each time.

## Examples

### Sorting Objects

The following creates a list of objects in module Report_A, sorted by identifier:

MetaIndex objs := contains of Report_A;
var v := objs;
Index Items := ['Identifier', 'Title', 'Description', 'Definition'];
var a := Items;
var info := a of v;
MetaIndex sortedObjects := sortIndex( info[Items = 'Identifier']);
info[ objs = sortedObjects]

The first line is the salient one in this example. Had this used the Index..Do declaration, rather than the MetaIndex..Do declaration, the variables in Report_A would have been evaluated when info[objs = sortedObject] was encountered (they would be evaluated only the first time an associative lookup on that index occurs). Here the evaluation is avoided.

### Avoiding An Ambiguity

Associative lookup using an Index can be ambiguous if it contains a variable whose value is an atom. MetaIndex avoids that ambiguity. For example:

Variable A := 5
Variable B := 2
Variable C := Handle(A)
Index I := [C, A, B, 2]
MetaIndex J := [A, B, 2, C]
@[I = 2] → 3 { Ambiguous, could be 3 or 4, first is returned }
@[J = 2] → 4 { not ambiguous }
@[I = Handle(A)] → 1 { Ambiguous, could be 1 or 2 }
@[J = Handle(A)] → 2 { not ambiguous }