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如何访问这个变量?

ada variables

yCalleecharan · 技术社区 · 16 年前

我有一个修改过的教科书示例如下(这本书是由Bray和Pokrass撰写的《理解Ada——一种软件工程方法》):

PACKAGE SOLAR_SYSTEM IS

 TYPE PLANET IS (MERCURY, VENUS, MARS, JUPITER, SATURN, NEPTUNE);

 SUBTYPE TERRESTRIAL_PLANET IS PLANET RANGE MERCURY..MARS;
 SUBTYPE JOVIAN_PLANET IS PLANET RANGE JUPITER..NEPTUNE;

 TYPE MILES IS DIGITS 5 RANGE 0.0..5.0E9;
 TYPE PLANET_FACTS IS ARRAY (PLANET) OF MILES;

 DISTANCE_TO_SUN : CONSTANT PLANET_FACTS :=
  (MERCURY => 36.0E6, VENUS => 67.2E6, MARS => 141.7E6,
  JUPITER => 484.0E6, SATURN => 887.0E6, NEPTUNE => 2797.0E6);

 NUMBER_OF_MOONS: CONSTANT ARRAY (PLANET) OF NATURAL :=
  (MERCURY => 0, VENUS => 0, MARS => 2,
  JUPITER => 12, SATURN => 10, NEPTUNE => 2);

END SOLAR_SYSTEM;

现在要访问一些变量及其内容,

WITH Ada.Float_Text_IO;
WITH Ada.Text_IO;
WITH Ada.Integer_Text_IO;
WITH SOLAR_SYSTEM;
USE SOLAR_SYSTEM;


PROCEDURE TEST2 IS

BEGIN -- main program
  Ada.Integer_Text_IO.Put(Item => NUMBER_OF_MOONS(Saturn));
  Ada.Text_IO.New_Line;
  Ada.Float_Text_IO.Put(Item => DISTANCE_TO_SUN(Saturn));
END TEST2;

距离太阳(土星)。如何访问它?

谢谢。。。

这就是我对这句话的理解

Ada.Float_Text_IO.Put(Item => DISTANCE_TO_SUN(Saturn));

1. WITH Ada.Float_Text_IO;
            |
    >>> warning: no entities of "FLOAT_TEXT_IO" are referenced

 2. WITH Ada.Text_IO;
 3. WITH Ada.Integer_Text_IO;
 4. WITH SOLAR_SYSTEM;
 5. USE SOLAR_SYSTEM;
 6.
 7.
 8. PROCEDURE TEST2 IS
 9.
10.    BEGIN -- main program
11.     Ada.Integer_Text_IO.Put(Item => NUMBER_OF_MOONS(Saturn));
12.     Ada.Text_IO.New_Line;
13.     Ada.Float_Text_IO.Put(Item => DISTANCE_TO_SUN(Saturn));
                         1            5
    >>> no candidate interpretations match the actuals:
    >>> missing argument for parameter "To" in call to "PUT" declared at a-tiflio.ads:81, instance at a-flteio.ads:20
    >>> missing argument for parameter "File" in call to "PUT" declared at a-tiflio.ads:63, instance at a-flteio.ads:20
    >>> possible missing instantiation of Text_IO.Float_IO
    >>> expected type "Standard.Float"
    >>> found type "MILES" defined at solar_system.ads:8
    >>>   ==> in call to "Put" at a-tiflio.ads:70, instance at a-flteio.ads:20

14.    END TEST2;

问题是到太阳(土星)的距离是MILES类型,这是浮点类型之一。所以仅仅使用Ada.Float\u Text\u IO.Put是行不通的。

1 回复 | 直到 16 年前

Marc C 16 年前

问题是,你已经宣布了一个全新的类型,英里,这是“浮动式”,但它是不预定义的浮点类型。因此,当您尝试放置该类型的项目时,即距离太阳(土星),没有可用于该类型的放置过程。

您有三个主要选择:

 package Miles_IO is new Ada.Text_IO.Float_IO(Solar_System.Miles);

然后使用Miles\u IO输出值:

 Miles_IO.Put(DISTANCE_TO_SUN(Saturn));

2) 将英里的定义更改为Float的受约束子类型:

SUBTYPE MILES IS FlOAT RANGE 0.0..5.0E9;

那么原来的Float\u Text\u IO.Put就可以正常工作了。

3) 由于Miles是一种浮点类型,因此您可以在输入之前对其进行转换:

 Ada.Float_Text_IO.Put(Item => Float(DISTANCE_TO_SUN(Saturn)));