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Date: 29 Sep 92 16:54:39 GMT
From: jvnc.net!darwin.sura.net!jhunix.hcf.jhu.edu!aplcen.apl.jhu.edu!ddsdx2.jhu
apl.edu!dlc@rutgers.edu  (Dave Collard x7468)
Subject: Re: Declaring Arrays of Bits (how)?
Message-ID: <1992Sep29.165439.28862@aplcen.apl.jhu.edu>
List-Id: <comp.lang.ada>

In <YOUNG.92Sep29101112@kuwait.gdfwc3> young@gdfwc3 (Mark Young) writes:


>I am trying to declare arrays of bits in Ada.  Here's my attempt:

>type Array_Range_Type is range 1..128;

>type Bit_Type is (OFF, ON);
>for  Bit_Type use (OFF => 0, ON => 1);
>for  Bit_Type'size use 1;

>type Bit_Array_Type is array (Array_Range_Type) of Bit_Type;
-- Add this line:
pragma Pack(Bit_Array_Type);
-- and you will find that Bit_Array_Type objects will be stored as 
-- you originally expected.  The compiler chose the most efficient
-- implementation of Bit_Array_Type, as it is allowed to do.  
-- However, see below for a better way to do what you are probably
-- really trying to do.

>Bit_Array : Bit_Array_Type;


-- I believe that you really want a packed array of boolean which
-- is how I often represent bits:

type Array_Range_Type is range 1..128;
type Bit_Array_Type is array(Array_Range_Type) of Boolean;
pragma pack(Bit_Array);

-- This makes it much easier to do logical bitwise operations on
-- the resulting data structure, which is usually why you want
-- arrays of bits in the first place.  Note that the packed boolean
-- array is guaranteed to take up exactly 1 bit per boolean by
-- some AI or other.


--Thor
dlc@ddsdx2.jhuapl.edu
collard@capsrv.jhuapl.edu