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From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Re: scope and/or parameters (beginner)
Date: 1999/04/03
Message-ID: <m390cabjo0.fsf@mheaney.ni.net>#1/1
X-Deja-AN: 462127112
References: <37064309.889106243@news.dsuper.net>
NNTP-Posting-Date: Fri, 02 Apr 1999 16:58:34 PDT
Newsgroups: comp.lang.ada
Date: 1999-04-03T00:00:00+00:00
List-Id: <comp.lang.ada>

fluffy_pink@dsuper.net writes:

> Here is what I have that doesn't work:
> -----------------------------------------------------------
> PROCEDURE One_Procedure
> 	   (	Second_Variable :	IN   The_Type;
> 		Third_Variable :	OUT  The_Type ) IS
> 
> --  types & sub-types: none ===========================
> --  sub-programs ======================================
> 
> 	PROCEDURE Internal_Procedure                  
> 		   (	One_Variable :		IN OUT  The_Type ) IS
> 	BEGIN
> 		One_Variable := One_Variable + A_Constant;
> 		-- What goes on here is not important.  The point
> 		-- Is that I am (want to) modify the value of
> 		-- "One_Variable" and bring its new value to
> 		-- "One_Procedure", as an IN OUT should be used.
> 	END Internal_Procedure;              
> 
> --  variables: none ===================================
> 
> BEGIN -- One_Procedure
> 
> 	Internal_Procedure ( One_Variable => (QUESTION) );
> 	Third_Variable = Second_Variable + Another_Constant;
> 	-- More stuff goes here using "Third_Variable".
>     
> END One_Procedure;
> -----------------------------------------------------------
> 
> In my main program I have declared the following variables:
> 
> 	Var_2, Var_3 : The_Type;
> 
> In my main program I am calling "One_Proc�dure" with:
> 
>    One_Procedure
> 	(Second_Variable => Var_2, Third_Variable => Var_3);
> 
> If for "(QUESTION)" I use "Second_Variable" the compiler tells me that
> for an OUT or an IN OUT I must have an actual parameter that **is a
> variable***.  I don't understand why it tells me this.  Obviously, if
> I use "Var_2", then it says that it cannot see "Var_2"; that I think I
> understand.

The problem is that anywhere inside One_Procedure, the object
Second_Variable is a constant, because it's an in-mode parameter.

Yes, the object Var_2 is a variable, but One_Procedure doesn't know
this.  One_Procedure is looking at Var_2 via the formal parameter
Second_Variable, which is in-mode, and therefore is constant.


 
> How can I pass the value of Var_2 from my main program, into
> One_Procedure, and then from One_Procedure into Internal_Procedure,
> and then to make it come back out modified (IN OUT) so that it will
> end up in One_Proc�dure ?

Make the formal parameter Second_Variable mode in out, instead of mode
in.

Here's a little formula for formal parameter modes:

in      = object is treated as a constant
in out  = object is treated as a variable


> I don't want to use the modified value of Var_2 (modified by
> Internal_Procedure) at the main program level; I want to use it only
> in One_Proc�dure.

Then you have to make a local copy.