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From: fluffy_doo@dsuper.net
Subject: discriminant
Date: 1999/07/11
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Date: 1999-07-11T00:00:00+00:00
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Hi,

I'm having problems with the use of a discriminant for a RECORD
that is meant to hold the value for a High Precision Real Number
(my own type defined in an ADT).  I did *not* get any compilation
time errors for any of the things I have done (described below).

In my .ADS I have the following private structure:

PRIVATE
  TYPE T_Sign_Temp IS (POSITIVE, NEGATIVE, ABSENT);
  SUBTYPE T_Sign IS T_Sign_Temp RANGE POSITIVE..NEGATIVE;

  SUBTYPE T_Digit IS integer RANGE 0..9;
  TYPE T_Mantissa IS ARRAY (1..MAX_MANT_200) OF T_Digit;
  SUBTYPE T_Len_Mantissa IS positive RANGE 1..MAX_SIG_FIG_30;

  SUBTYPE T_Power IS integer RANGE integer'first..integer'last;
  SUBTYPE T_Positive_Power IS T_Power RANGE 0..integer'last;

  TYPE T_Kind IS ( INT, REAL, NONE );

  TYPE T_Number ( kind : T_Kind := NONE ) IS
  RECORD
	Sign : T_Sign := POSITIVE;
	Mant : T_Mantisse := (OTHERS => 0);
	Len : T_Lng_Mantisse := 0;
	CASE kind IS
		WHEN REAL =>	Point : natural RANGE 0..MAX_SIG_FIG_30 := 1;
						Power_R : T_Power := 0;
		WHEN INT =>		Power_I : T_Positive_Power := 0;
		WHEN NONE =>	NULL;
		END CASE;
  END RECORD;

When I assign a value to the Power_R for a real numbrer, the "Point"
field also changes (mysteriously), and after it has changed I can no 
longer assign a new value to it.  On top of this, the new value that
"Point" takes is *outside* my subtype range and no exception is
raised !  It tends to take a value a bit over 500, while its top limit
is a constant with a value of 30 !?!?  I got the value and the
moment of the change occuring from the debugger.

I have tried a large number of variations for a subprogram whose
task is to assign the T_Number fields' values, always with the same
result (problem) I have just described.

I tried with a function and with a procedure, many versions of each.
At first I would assign the fields of my T_Number variable separately,
one after the other, as I went along through a decision tree
structure.

Then I tried putting each field value in a temporary variable and
to assign them to my T_Number variable with the
"My_Num := (REAL, its_sign, its_mantissa, ...);" method at the end
of my subprogram.

At first I had a formal parameter of the T_Kind type (INT or REAL)
for the discriminant that I was passing to the subprogram.  I then
wrote two different subprograms, one for each "kind", so I no longer
had to pass the discriminant in a parameter.  Still the problem
persisted.  I finally decided I could do without the "Point" field,
but I still need to know what I was doing wrong with respect to this
discriminant business.

If anyone can provide me with some light, I would appreciate it.

Thanks in advance.


Marc Galipeau

--
What I really am is "fluffy", no "_dong",
no "_puff", no "_woo", no  nothing, just plain fluffy.