First attribute on Enumerated types

First attribute on Enumerated types

[David Starner]

I'm trying to compile a program that refers to Color'First, and GNAT
gives an error that 
tree.ads:12:91: prefix for "First" attribute must be array
According to the Ada book at hand, First is a valid attribute for
enumerated types. What am I doing wrong? How do I get the first item in
an enumerated list?
-- 
The irony is, there ARE drugs that ENHANCE it. This says something
profound about either science or medicine or people or muppets or all
four. - S. John Ross

Re: First attribute on Enumerated types

[David Starner]

"James S. Rogers" wrote:
> 
> David,
> 
> I suspect that, in your case, Color is a variable, and not a subtype name.
> The 'First attribute works for an enumeration type, not a variable of an
> enumeration type.
> 
> Jim Rogers
> Colorado Springs, Colorado
> 
> -----Original Message-----
> From: David Starner <dstarner98@aasaa.ofe.org>> 
> >I'm trying to compile a program that refers to Color'First, and GNAT
> >gives an error that
> >tree.ads:12:91: prefix for "First" attribute must be array
> >According to the Ada book at hand, First is a valid attribute for
> >enumerated types. What am I doing wrong? How do I get the first item in
> >an enumerated list?

Since I quickly composed the first message, here's the actual code.
Color is definitely a type. It should be enumerated - is there any
way to tell the compiler that? In retrospect, that's probably my error.

with Lists;
generic 
    type Node_type is private;
    type Color is private; 
package Tree is
    type Tree is limited private;
    type Node_Pointer is access Node_type;
    type TreePointer is access Tree;
    
    package Tree_List is new Lists(TreePointer, Natural); 
    
    procedure SetRoot(BaseTree: in out Tree; Item: in Node_Pointer;
NewColor: in Color := Color'First);
    procedure AddChild(BaseTree: in out Tree; Child: in TreePointer);
    procedure RemoveChild(BaseTree: in out Tree; Child: in integer);
--  procedure RemoveChild(BaseTree: in out Tree; Child: in out
Tree_List.Pointer);   
    function GetRootData (BaseTree: in Tree) return Node_type;
    procedure SetColor (BaseTree: in out Tree; NewColor: in Color);
private
    type Tree is 
        record 
            Data: Node_Pointer;
	    NodeColor: Color;
            Children: Tree_List.List;
        end record;
end Tree;

-- 
The irony is, there ARE drugs that ENHANCE it. This says something
profound about either science or medicine or people or muppets or all
four. - S. John Ross

Re: First attribute on Enumerated types

[Matthew Heaney]

David Starner <dstarner98@aasaa.ofe.org> writes:

> Since I quickly composed the first message, here's the actual code.
> Color is definitely a type. It should be enumerated - is there any
> way to tell the compiler that? In retrospect, that's probably my error.
> 
> with Lists;
> generic 
>     type Node_type is private;
>     type Color is private; 
> package Tree is

Say this instead:

generic 
    type Node_type is private;
    type Color is (<>);
package Tree is

Read the section of the RM (or your fav textbook) that describes
"generic formal types."

Re: First attribute on Enumerated types

[John English]

David Starner wrote:
>     type Color is private;

... but not necessarily an enumeration; it might be a record type, or a Float...

>     procedure SetRoot(BaseTree: in out Tree; Item: in Node_Pointer;
> NewColor: in Color := Color'First);

... but private types (or the record types that might be used in an instantiation)
don't have a 'First attribute...

Try this:
  type Color is (<>);
or this:
  type Color is private;
  Color_First : Color;    -- supply your desired "first" value here and use
                          -- Color_First instead of Color'First

> The irony is, there ARE drugs that ENHANCE it. This says something
> profound about either science or medicine or people or muppets or all
> four. - S. John Ross

Enhance what??? (just curious...;-)

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Re: First attribute on Enumerated types

[Mark A Biggar]

David Starner wrote:
> Since I quickly composed the first message, here's the actual code.
> Color is definitely a type. It should be enumerated - is there any
> way to tell the compiler that? In retrospect, that's probably my error.
> 
> with Lists;
> generic
>     type Node_type is private;
>     type Color is private;

change this to:

       type Color is (<>);

That will restrict Color to either an emumeration or an integer range.  Now
if you write the body code so that it only uses 'first, 'last, 'succ, 'pred,
etc. it should work correctly given any discrete type.

--
Mark Biggar
mark.a.biggar@lmco.com