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From: "Jeffrey D. Cherry" <jdcherry@utech.net>
Subject: Re: Exception Handling
Date: 2000/06/02
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To: "David C. Hoos, Sr." <david.c.hoos.sr@ada95.com>
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Date: 2000-06-02T00:00:00+00:00
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"David C. Hoos, Sr." wrote:
> 
> I'm curious as to why you wouldn't simply use the 'Value
> attribute of the specific type to attempt the conversion.
> This avoids the expense of an instance of Text_IO.Float_IO
> which might be needed for no other purpose, and makes use
> of the compiler vendor's parsing routine.

The reason is that the 'Value attribute raises Constraint_Error if there is 
something wrong with the syntax of the input string while the Get procedure
from Ada.Float_Text_IO raises Data_Error.  Constraint_Error can be suppressed 
while Data_Error cannot.

One other thing, while I was looking at the Get_Float procedure I posted I 
noticed an error.  The assignment statement following the call to Get is
incorrect.  The corrected procedure is:

procedure Get_Float(St : in string; Value : out float; Is_Valid : out boolean)
is
   Last : positive;
begin -- Get_Float
   Ada.Float_Text_IO.Get(St, Value, Last);
   Is_Valid := Last = St'last;
exception
   when others =>
      Is_Valid := false;
end Get_Float;

The reason the previous version was wrong is that parameter Last will contain
the index of the last character read during the conversion.  For the example 
string of "2.3W-4", Last would be set to 3, Is_Valid would have erroneously 
been set to true, and Value would be 2.3 rather than 0.00023.  In the corrected
version (above), Is_Valid is set to true only if the entire input string is 
consumed during the conversion.

-- 
Regards,
Jeffrey D. Cherry
Senior IV&V Analyst
Logicon Space and Information Operations
Logicon Inc.
a Northrop Grumman company