Re: How do you bitwise operations in Ada '83 and '95

["Jeffrey R. Carter" <spam.not.jrcarter@acm.not.spam.org>]

jimmaureenrogers@worldnet.att.net wrote:
> 
> Actually it does not work exactly the same. The algorithm is
> exactly the same, and the same numbers are produced, but the
> packed array of bits executes faster on my machine by a factor
> of 3 and uses significantly less space (by a factor of 8).

"The algorithm is the same and the same numbers are produced" = "works 
the same". Performance may differ, but behavior is the same. Since 
performance is a function of many things in addition to the SW, only 
behavior is a meaningful comparison. No doubt there is a 
platform/OS/compiler combination such that the unpacked version would 
run faster than the packed version does on your machine.

> Your definition of bit-wise appears to me to be based upon the
> desire to use a low level abstraction to access the bits. My
> definition includes all abstractions that allow access to
> individual bits.  When I only programmed in C, I only
> thought of bit-wise access in low level abstractions. Since I
> have programmed in Ada I think of bit-wise operations in
> many abstraction levels.

"Bitwise operations" is an inherently low-level concept. At a high 
level, your abstraction is a set. If we treat that as a high-level 
abstraction, then we hide the implementation, and neither bits nor 
Booleans appear in the handling of sets in the main program. An array of 
Booleans, packed or not, is only one possible implementation of a set. 
We could rewrite your program using the ordered-set package from the 
Ada-0X data-structures library and get the same behavior. It would not 
involve any bit-level access, though.

However, you raise an important point, and that is that we should ask 
the OP what he is trying to achieve. It may be that low-level concepts 
such as bitwise operations are an inappropriate way to approach the 
problem, and instead of showing how to do low-level things, we should be 
advising on appropriate high-level abstractions.

-- 
Jeff Carter
"I'm a lumberjack and I'm OK."
Monty Python's Flying Circus
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