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3.6.1 Index Constraints and Discrete Ranges

1
An index_constraint determines the range of possible values for every index of an array subtype, and thereby the corresponding array bounds. 

Syntax

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index_constraint ::=  (discrete_range {, discrete_range})
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discrete_range ::= discrete_subtype_indication | range

Name Resolution Rules

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{type of a discrete_range} The type of a discrete_range is the type of the subtype defined by the subtype_indication, or the type of the range. {expected type (index_constraint discrete_range) [partial]} For an index_constraint, each discrete_range shall resolve to be of the type of the corresponding index. 
4.a
Discussion: In Ada 95, index_constraints only appear in a subtype_indication; they no longer appear in constrained_array_definitions.

Legality Rules

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An index_constraint shall appear only in a subtype_indication whose subtype_mark denotes either an unconstrained array subtype, or an unconstrained access subtype whose designated subtype is an unconstrained array subtype; in either case, the index_constraint shall provide a discrete_range for each index of the array type. 

Static Semantics

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{bounds (of a discrete_range)} A discrete_range defines a range whose bounds are given by the range, or by the range of the subtype defined by the subtype_indication.

Dynamic Semantics

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{compatibility (index constraint with a subtype) [partial]} An index_constraint is compatible with an unconstrained array subtype if and only if the index range defined by each discrete_range is compatible (see 3.5) with the corresponding index subtype. {null array} If any of the discrete_ranges defines a null range, any array thus constrained is a null array, having no components. {satisfies (an index constraint) [partial]} An array value satisfies an index_constraint if at each index position the array value and the index_constraint have the same index bounds. 
7.a
Ramification: There is no need to define compatibility with a constrained array subtype, because one is not allowed to constrain it again.
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{elaboration (index_constraint) [partial]} The elaboration of an index_constraint consists of the evaluation of the discrete_range(s), in an arbitrary order. {evaluation (discrete_range) [partial]} The evaluation of a discrete_range consists of the elaboration of the subtype_indication or the evaluation of the range.
NOTES
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45  The elaboration of a subtype_indication consisting of a subtype_mark followed by an index_constraint checks the compatibility of the index_constraint with the subtype_mark (see 3.2.2).
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46  Even if an array value does not satisfy the index constraint of an array subtype, Constraint_Error is not raised on conversion to the array subtype, so long as the length of each dimension of the array value and the array subtype match. See 4.6.

Examples

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Examples of array declarations including an index constraint: 
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Board     : Matrix(1 .. 8,  1 .. 8);  --  see 3.6
Rectangle : Matrix(1 .. 20, 1 .. 30);
Inverse   : Matrix(1 .. N,  1 .. N);  --  N need not be static 
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Filter    : Bit_Vector(0 .. 31);
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Example of array declaration with a constrained array subtype: 
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My_Schedule : Schedule;  --  all arrays of type Schedule have the same bounds
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Example of record type with a component that is an array: 
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type Var_Line(Length : Natural) is
   record
      Image : String(1 .. Length);
   end record;
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Null_Line : Var_Line(0);  --  Null_Line.Image is a null array

Extensions to Ada 83

18.a
{extensions to Ada 83} We allow the declaration of a variable with a nominally unconstrained array subtype, so long as it has an initialization expression to determine its bounds. 

Wording Changes from Ada 83

18.b
We have moved the syntax for index_constraint and discrete_range here since they are no longer used in constrained_array_definitions. We therefore also no longer have to describe the (special) semantics of index_constraints and discrete_ranges that appear in constrained_array_definitions.
18.c
The rules given in RM83-3.6.1(5,7-10), which define the bounds of an array object, are redundant with rules given elsewhere, and so are not repeated here. RM83-3.6.1(6), which requires that the (nominal) subtype of an array variable be constrained, no longer applies, so long as the variable is explicitly initialized. 
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