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<H1> 5.5 Loop Statements</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> [A <FONT FACE="Arial, Helvetica">loop_statement</FONT>
includes a <FONT FACE="Arial, Helvetica">sequence_of_statements</FONT>
that is to be executed repeatedly, zero or more times.] </DIV>
<H4 ALIGN=CENTER>Syntax</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">loop_statement<A NAME="I2965"></A>
::= </FONT><BR>
[<I>loop_</I><A NAME="I2966"></A><FONT FACE="Arial, Helvetica">statement_identifier</FONT>:]<BR>
[<A NAME="I2967"></A><FONT FACE="Arial, Helvetica">iteration_scheme</FONT>] <B>loop</B><BR>
<A NAME="I2968"></A><FONT FACE="Arial, Helvetica">sequence_of_statements</FONT><BR>
<B>end</B> <B>loop</B> [<I>loop_</I><A NAME="I2969"></A><FONT FACE="Arial, Helvetica">identifier</FONT>];</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">iteration_scheme<A NAME="I2970"></A>
::= </FONT><B>while</B> <A NAME="I2971"></A><FONT FACE="Arial, Helvetica">condition</FONT><BR>
| <B>for</B> <A NAME="I2972"></A><FONT FACE="Arial, Helvetica">loop_parameter_specification</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">loop_parameter_specification<A NAME="I2973"></A>
::= </FONT><BR>
<A NAME="I2974"></A><FONT FACE="Arial, Helvetica">defining_identifier</FONT> <B>in</B> [<B>reverse</B>] <A NAME="I2975"></A><FONT FACE="Arial, Helvetica">discrete_subtype_definition</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="SyntaxIndented">If a <FONT FACE="Arial, Helvetica">loop_statement</FONT>
has a <I>loop_</I><FONT FACE="Arial, Helvetica">statement_identifier</FONT>,
then the <FONT FACE="Arial, Helvetica">identifier</FONT> shall be repeated
after the <B>end loop</B>; otherwise, there shall not be an <FONT FACE="Arial, Helvetica">identifier</FONT>
after the <B>end loop</B>. </DIV>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Normal"> <A NAME="I2976"></A>A <FONT FACE="Arial, Helvetica">loop_parameter_specification</FONT>
declares a <I>loop parameter</I>, which is an object whose subtype is
that defined by the <FONT FACE="Arial, Helvetica">discrete_subtype_definition</FONT>.
<A NAME="I2977"></A></DIV>
<H4 ALIGN=CENTER>Dynamic Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Normal"> <A NAME="I2978"></A>For the execution of a <FONT FACE="Arial, Helvetica">loop_statement</FONT>,
the <FONT FACE="Arial, Helvetica">sequence_of_statements</FONT> is executed
repeatedly, zero or more times, until the <FONT FACE="Arial, Helvetica">loop_statement</FONT>
is complete. The <FONT FACE="Arial, Helvetica">loop_statement</FONT>
is complete when a transfer of control occurs that transfers control
out of the loop, or, in the case of an <FONT FACE="Arial, Helvetica">iteration_scheme</FONT>,
as specified below.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Normal"> <A NAME="I2979"></A>For the execution of a <FONT FACE="Arial, Helvetica">loop_statement</FONT>
with a <B>while</B> <FONT FACE="Arial, Helvetica">iteration_scheme</FONT>,
the condition is evaluated before each execution of the <FONT FACE="Arial, Helvetica">sequence_of_statements</FONT>;
if the value of the <FONT FACE="Arial, Helvetica">condition</FONT> is
True, the <FONT FACE="Arial, Helvetica">sequence_of_statements</FONT>
is executed; if False, the execution of the <FONT FACE="Arial, Helvetica">loop_statement</FONT>
is complete.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="Normal"> <A NAME="I2980"></A><A NAME="I2981"></A>For the
execution of a <FONT FACE="Arial, Helvetica">loop_statement</FONT> with
a <B>for</B> <FONT FACE="Arial, Helvetica">iteration_scheme</FONT>, the
<FONT FACE="Arial, Helvetica">loop_parameter_specification</FONT> is
first elaborated. This elaboration creates the loop parameter and elaborates
the <FONT FACE="Arial, Helvetica">discrete_subtype_definition</FONT>.
If the <FONT FACE="Arial, Helvetica">discrete_subtype_definition</FONT>
defines a subtype with a null range, the execution of the <FONT FACE="Arial, Helvetica">loop_statement</FONT>
is complete. Otherwise, the <FONT FACE="Arial, Helvetica">sequence_of_statements</FONT>
is executed once for each value of the discrete subtype defined by the
<FONT FACE="Arial, Helvetica">discrete_subtype_definition</FONT> (or
until the loop is left as a consequence of a transfer of control). <A NAME="I2982"></A>Prior
to each such iteration, the corresponding value of the discrete subtype
is assigned to the loop parameter. These values are assigned in increasing
order unless the reserved word <B>reverse</B> is present, in which case
the values are assigned in decreasing order. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>The order
of creating the loop parameter and evaluating the <FONT FACE="Arial, Helvetica">discrete_subtype_definition</FONT>
doesn't matter, since the creation of the loop parameter has no side
effects (other than possibly raising Storage_Error, but anything can
do that). </FONT></DIV>
<DIV Class="NotesHeader"><FONT SIZE=-1>NOTES</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>6 A loop parameter is a constant;
it cannot be updated within the <FONT FACE="Arial, Helvetica">sequence_of_statements</FONT>
of the loop (see <A HREF="AA-3-3.html">3.3</A>).</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>7 An <FONT FACE="Arial, Helvetica">object_declaration</FONT>
should not be given for a loop parameter, since the loop parameter is
automatically declared by the <FONT FACE="Arial, Helvetica">loop_parameter_specification</FONT>.
The scope of a loop parameter extends from the <FONT FACE="Arial, Helvetica">loop_parameter_specification</FONT>
to the end of the <FONT FACE="Arial, Helvetica">loop_statement</FONT>,
and the visibility rules are such that a loop parameter is only visible
within the <FONT FACE="Arial, Helvetica">sequence_of_statements</FONT>
of the loop. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Implementation Note: </B>An
implementation could give a warning if a variable is hidden by a <FONT FACE="Arial, Helvetica">loop_parameter_specification</FONT>.
</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>8 The <FONT FACE="Arial, Helvetica">discrete_subtype_definition</FONT>
of a for loop is elaborated just once. Use of the reserved word <B>reverse</B>
does not alter the discrete subtype defined, so that the following <FONT FACE="Arial, Helvetica">iteration_scheme</FONT>s
are not equivalent; the first has a null range. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<DIV Class="Examples"><TT><B>for</B> J <B>in</B> <B>reverse</B> 1 .. 0<BR>
<B>for</B> J <B>in</B> 0 .. 1</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>If a <FONT FACE="Arial, Helvetica">loop_parameter_specification</FONT>
has a static discrete range, the subtype of the loop parameter is static.
</FONT></DIV>
<H4 ALIGN=CENTER>Examples</H4>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <I>Example of a
loop statement without an iteration scheme:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15</FONT></DIV>
<DIV Class="Examples"><TT><B>loop</B><BR>
Get(Current_Character);<BR>
<B>exit</B> <B>when</B> Current_Character = '*';<BR>
<B>end</B> <B>loop</B>;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>16</FONT></DIV>
<DIV Class="Wide" Style="margin-bottom: 0.4em"> <I>Example of a loop
statement with a <B>while</B> iteration scheme:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>17</FONT></DIV>
<DIV Class="Examples"><TT><B>while</B> Bid(N).Price < Cut_Off.Price <B>loop</B><BR>
Record_Bid(Bid(N).Price);<BR>
N := N + 1;<BR>
<B>end</B> <B>loop</B>;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>18</FONT></DIV>
<DIV Class="Wide" Style="margin-bottom: 0.4em"> <I>Example of a loop
statement with a <B>for</B> iteration scheme:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>19</FONT></DIV>
<DIV Class="Examples"><TT><B>for</B> J <B>in</B> Buffer'Range <B>loop</B> --<I> works even with a null range</I><BR>
<B>if</B> Buffer(J) /= Space <B>then</B><BR>
Put(Buffer(J));<BR>
<B>end</B> <B>if</B>;<BR>
<B>end</B> <B>loop</B>;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20</FONT></DIV>
<DIV Class="Wide" Style="margin-bottom: 0.4em"> <I>Example of a loop
statement with a name:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>21</FONT></DIV>
<DIV Class="Examples"><TT>Summation:<BR>
<B>while</B> Next /= Head <B>loop</B> --<I> see <A HREF="AA-3-10-1.html">3.10.1</A></I><BR>
Sum := Sum + Next.Value;<BR>
Next := Next.Succ;<BR>
<B>end</B> <B>loop</B> Summation;</TT></DIV>
<H4 ALIGN=CENTER>Wording Changes from Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>21.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>The constant-ness of loop parameters
is specified in <A HREF="AA-3-3.html">3.3</A>, ``<A HREF="AA-3-3.html">Objects
and Named Numbers</A>''. </FONT></DIV>
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