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From: Dennis Hoppe <dennis.hoppe@hoppinet.de>
Newsgroups: comp.lang.ada
Subject: Feature or Bug?  2#1111# shifted by 4 is 224
Date: Mon, 02 Jun 2008 17:17:19 +0200
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Date: 2008-06-02T17:17:19+02:00
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Hi,

we probably found a bug in Ada or we cannot realize, that this is indeed 
a feature ;)

In Example A, we defined a simple modular shift to the left by means of 
Item * 2**N. N is defined as an Integer. This example works well and 
Shift_Left(2#1111#, 4) returns 0.

-- Example A
type Modular_Type is mod 2**4;
function Shift_Left (Item : in Modular_Type;
                      N    : in Integer) return Modular_Type is
begin
   return Item * 2**N;
end Shift_Left;


In Example B, we limit the parameter N to Positive, because we do not 
want "negative" shifts. The result of Shift_Left(2#1111#, 4) is 224
(2#11100000#) !

-- Example B
type Modular_Type is mod 2**4;
function Shift_Left (Item : in Modular_Type;
                      N    : in Positive) return Modular_Type is
begin
   return Item * 2**N;
end Shift_Left;



Why does this happen? Positive is defined as all Integer values starting 
at 1 through the highest value of Integer, so the result should also be 
0. Especially, because the returning value is additionally limited by 
mod 2**4.

Thanks,
   Dennis Hoppe