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<H1> 3.6.1 Index Constraints and Discrete Ranges</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> An <FONT FACE="Arial, Helvetica">index_constraint</FONT>
determines the range of possible values for every index of an array subtype,
and thereby the corresponding array bounds. </DIV>
<H4 ALIGN=CENTER>Syntax</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">index_constraint<A NAME="I1874"></A>
::= </FONT> (<A NAME="I1875"></A><FONT FACE="Arial, Helvetica">discrete_range</FONT> {, <A NAME="I1876"></A><FONT FACE="Arial, Helvetica">discrete_range</FONT>})</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">discrete_range<A NAME="I1877"></A>
::= </FONT><I>discrete_</I><A NAME="I1878"></A><FONT FACE="Arial, Helvetica">subtype_indication</FONT> | <A NAME="I1879"></A><FONT FACE="Arial, Helvetica">range</FONT></DIV>
<H4 ALIGN=CENTER>Name Resolution Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Normal"> <A NAME="I1880"></A>The type of a <FONT FACE="Arial, Helvetica">discrete_range</FONT>
is the type of the subtype defined by the <FONT FACE="Arial, Helvetica">subtype_indication</FONT>,
or the type of the <FONT FACE="Arial, Helvetica">range</FONT>. <A NAME="I1881"></A>For
an <FONT FACE="Arial, Helvetica">index_constraint</FONT>, each <FONT FACE="Arial, Helvetica">discrete_range</FONT>
shall resolve to be of the type of the corresponding index. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B>In Ada 95,
<FONT FACE="Arial, Helvetica">index_constraint</FONT>s only appear in
a <FONT FACE="Arial, Helvetica">subtype_indication</FONT>; they no longer
appear in <FONT FACE="Arial, Helvetica">constrained_array_definition</FONT>s.
</FONT></DIV>
<H4 ALIGN=CENTER>Legality Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Normal"> An <FONT FACE="Arial, Helvetica">index_constraint</FONT>
shall appear only in a <FONT FACE="Arial, Helvetica">subtype_indication</FONT>
whose <FONT FACE="Arial, Helvetica">subtype_mark</FONT> denotes either
an unconstrained array subtype, or an unconstrained access subtype whose
designated subtype is an unconstrained array subtype; in either case,
the <FONT FACE="Arial, Helvetica">index_constraint</FONT> shall provide
a <FONT FACE="Arial, Helvetica">discrete_range</FONT> for each index
of the array type. </DIV>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Normal"> <A NAME="I1882"></A>A <FONT FACE="Arial, Helvetica">discrete_range</FONT>
defines a range whose bounds are given by the <FONT FACE="Arial, Helvetica">range</FONT>,
or by the range of the subtype defined by the <FONT FACE="Arial, Helvetica">subtype_indication</FONT>.
</DIV>
<H4 ALIGN=CENTER>Dynamic Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Normal"> <A NAME="I1883"></A>An <FONT FACE="Arial, Helvetica">index_constraint</FONT>
is <I>compatible</I> with an unconstrained array subtype if and only
if the index range defined by each <FONT FACE="Arial, Helvetica">discrete_range</FONT>
is compatible (see <A HREF="AA-3-5.html">3.5</A>) with the corresponding
index subtype. <A NAME="I1884"></A>If any of the <FONT FACE="Arial, Helvetica">discrete_range</FONT>s
defines a null range, any array thus constrained is a <I>null array</I>,
having no components. <A NAME="I1885"></A>An array value <I>satisfies</I>
an <FONT FACE="Arial, Helvetica">index_constraint</FONT> if at each index
position the array value and the <FONT FACE="Arial, Helvetica">index_constraint</FONT>
have the same index bounds. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>There is
no need to define compatibility with a constrained array subtype, because
one is not allowed to constrain it again.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Normal"> <A NAME="I1886"></A>The elaboration of an <FONT FACE="Arial, Helvetica">index_constraint</FONT>
consists of the evaluation of the <FONT FACE="Arial, Helvetica">discrete_range</FONT>(s),
in an arbitrary order. <A NAME="I1887"></A>The evaluation of a <FONT FACE="Arial, Helvetica">discrete_range</FONT>
consists of the elaboration of the <FONT FACE="Arial, Helvetica">subtype_indication</FONT>
or the evaluation of the <FONT FACE="Arial, Helvetica">range</FONT>.
</DIV>
<DIV Class="NotesHeader"><FONT SIZE=-1>NOTES</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>43 The elaboration of a <FONT FACE="Arial, Helvetica">subtype_indication</FONT>
consisting of a <FONT FACE="Arial, Helvetica">subtype_mark</FONT> followed
by an <FONT FACE="Arial, Helvetica">index_constraint</FONT> checks the
compatibility of the <FONT FACE="Arial, Helvetica">index_constraint</FONT>
with the <FONT FACE="Arial, Helvetica">subtype_mark</FONT> (see <A HREF="AA-3-2-2.html">3.2.2</A>).</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>44 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
<A HREF="AA-4-6.html">4.6</A>. </FONT></DIV>
<H4 ALIGN=CENTER>Examples</H4>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <I>Examples of
array declarations including an index constraint: </I></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<DIV Class="Examples"><TT>Board : Matrix(1 .. 8, 1 .. 8); <I>-- see <A HREF="AA-3-6.html">3.6</A></I><BR>
Rectangle : Matrix(1 .. 20, 1 .. 30);<BR>
Inverse : Matrix(1 .. N, 1 .. N); <I>-- N need not be static </I></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<DIV Class="Examples"><TT>Filter : Bit_Vector(0 .. 31);</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<DIV Class="Wide" Style="margin-bottom: 0.4em"> <I>Example of array
declaration with a constrained array subtype: </I></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15</FONT></DIV>
<DIV Class="Examples"><TT>My_Schedule : Schedule; <I>-- all arrays of type Schedule have the same bounds</I></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>16</FONT></DIV>
<DIV Class="Wide" Style="margin-bottom: 0.4em"> <I>Example of record
type with a component that is an array: </I></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>17</FONT></DIV>
<DIV Class="Examples"><TT><B>type</B> Var_Line(Length : Natural) <B>is</B><BR>
<B>record</B><BR>
Image : String(1 .. Length);<BR>
<B>end</B> <B>record</B>;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>18</FONT></DIV>
<DIV Class="Examples"><TT>Null_Line : Var_Line(0); <I>-- Null_Line.Image is a null array</I></TT></DIV>
<H4 ALIGN=CENTER>Extensions to Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>18.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><A NAME="I1888"></A>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.
</FONT></DIV>
<H4 ALIGN=CENTER>Wording Changes from Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>18.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>We have moved the syntax for <FONT FACE="Arial, Helvetica">index_constraint</FONT>
and <FONT FACE="Arial, Helvetica">discrete_range</FONT> here since they
are no longer used in <FONT FACE="Arial, Helvetica">constrained_array_definition</FONT>s.
We therefore also no longer have to describe the (special) semantics
of <FONT FACE="Arial, Helvetica">index_constraint</FONT>s and <FONT FACE="Arial, Helvetica">discrete_range</FONT>s
that appear in <FONT FACE="Arial, Helvetica">constrained_array_definition</FONT>s.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>18.c</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>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. </FONT></DIV>
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