5.5 Loop Statements
1
[A loop_statement includes
a sequence_of_statements that is to be executed
repeatedly, zero or more times.]
Syntax
2
loop_statement ::=
[
loop_statement_identifier:]
[
iteration_scheme]
loop
sequence_of_statements
end loop [
loop_identifier];
3
iteration_scheme ::= while condition
|
for loop_parameter_specification
4
loop_parameter_specification ::=
defining_identifier in [
reverse]
discrete_subtype_definition
5
If a loop_statement
has a loop_statement_identifier, then
the identifier shall be repeated after the
end loop; otherwise, there shall not be an identifier
after the end loop.
Static Semantics
6
{loop parameter}
A
loop_parameter_specification
declares a
loop parameter, which is an object whose subtype is
that defined by the
discrete_subtype_definition.
{parameter: See also loop parameter}
Dynamic Semantics
7
{execution (loop_statement)
[partial]} For the execution of a
loop_statement,
the
sequence_of_statements is executed repeatedly,
zero or more times, until the
loop_statement
is complete. The
loop_statement is complete
when a transfer of control occurs that transfers control out of the loop,
or, in the case of an
iteration_scheme, as
specified below.
8
{execution (loop_statement
with a while iteration_scheme) [partial]} For
the execution of a
loop_statement with a
while
iteration_scheme, the condition is evaluated
before each execution of the
sequence_of_statements;
if the value of the
condition is True, the
sequence_of_statements is executed; if False,
the execution of the
loop_statement is complete.
9
{execution (loop_statement
with a for iteration_scheme) [partial]} {elaboration
(loop_parameter_specification) [partial]} For
the execution of a
loop_statement with a
for
iteration_scheme, the
loop_parameter_specification
is first elaborated. This elaboration creates the loop parameter and
elaborates the
discrete_subtype_definition.
If the
discrete_subtype_definition defines
a subtype with a null range, the execution of the
loop_statement
is complete. Otherwise, the
sequence_of_statements
is executed once for each value of the discrete subtype defined by the
discrete_subtype_definition (or until the
loop is left as a consequence of a transfer of control).
{assignment
operation (during execution of a for loop)} 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
reverse is present, in which case
the values are assigned in decreasing order.
9.a
Ramification: The order of creating the
loop parameter and evaluating the discrete_subtype_definition
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).
10
6 A loop parameter is a constant; it cannot
be updated within the
sequence_of_statements
of the loop (see
3.3).
11
7 An object_declaration
should not be given for a loop parameter, since the loop parameter is
automatically declared by the loop_parameter_specification.
The scope of a loop parameter extends from the loop_parameter_specification
to the end of the loop_statement, and the
visibility rules are such that a loop parameter is only visible within
the sequence_of_statements of the loop.
11.a
Implementation Note: An implementation
could give a warning if a variable is hidden by a loop_parameter_specification.
12
8 The discrete_subtype_definition
of a for loop is elaborated just once. Use of the reserved word reverse
does not alter the discrete subtype defined, so that the following iteration_schemes
are not equivalent; the first has a null range.
13
for J in reverse 1 .. 0
for J in 0 .. 1
13.a
Ramification: If a loop_parameter_specification
has a static discrete range, the subtype of the loop parameter is static.
Examples
14
Example of a loop
statement without an iteration scheme:
15
loop
Get(Current_Character);
exit when Current_Character = '*';
end loop;
16
Example of a loop
statement with a while iteration scheme:
17
while Bid(N).Price < Cut_Off.Price loop
Record_Bid(Bid(N).Price);
N := N + 1;
end loop;
18
Example of a loop
statement with a for iteration scheme:
19
for J in Buffer'Range loop -- works even with a null range
if Buffer(J) /= Space then
Put(Buffer(J));
end if;
end loop;
20
Example of a loop
statement with a name:
21
Summation:
while Next /= Head
loop --
see 3.10.1
Sum := Sum + Next.Value;
Next := Next.Succ;
end loop Summation;
Wording Changes from Ada 83
21.a
Sponsored by Ada-Europe