From: Andrew Ballantine <andrew.ballantine@...>
Date: 2005-05-05T07:14:06+09:00
Subject: Re: Fixnum's binary representation

Guy's this is very neat and it also brings back some memories.

Some years ago I did quite a lot of programming on CDC 6600 & 7600 at the
ULCC. These machines use exactly the same notation. I remember revelling in
having two zeros because I used to use one to mean null (-1) and the other
zero. Yes programming in FORTRAN and writing COMPASS assembler subroutines
when FORTRAN was stumped. Mmm about 1974-1976 I think.

Kind regards,

Andrew Ballantine
----- Original Message ----- 
From: "Robert Klemme" <bob.news@gmx.net>
Newsgroups: comp.lang.ruby
To: "ruby-talk ML" <ruby-talk@ruby-lang.org>
Sent: Wednesday, May 04, 2005 6:09 PM
Subject: Re: Fixnum's binary representation


>
> <camsight@gmail.com> schrieb im Newsbeitrag
> news:1115223668.955656.66750@o13g2000cwo.googlegroups.com...
> > Thanks, Robert!
> >
> >> For which numbers do you have problems?  I don't see any so far
> >
> > I mean...
> > If Bignum#[] is made to simulate 32-bit signed integers, it can't show
> > numbers with more than 32-bit representation.
> > Well, positive numbers will be okay.
> > But what about negative numbers?
> > When can I expect the sign bit?
>
> Well, since Bignums can be arbitrary size, you have to decide.  The values
> returned by Fixnum#[] and Bignum#[] represent bits of a two complement's
> arbitrary size binary number.  If you view it from this perspective,
you'll
> see that there is no single sign bit.  Negative numbers have *all* the
> higher bits set to 1.
>
> > I assume that numbers beyond 32-bit are not suitable for Bignum#[].
> > Do you agree?
>
> Not at all.
>
> >> n = -(1<<100)
> => -1267650600228229401496703205376
> >> 200.times{|i| print i, " ", n[i], "\n"}
> 0 0
> 1 0
> 2 0
> 3 0
> 4 0
> 5 0
> 6 0
> 7 0
> 8 0
> 9 0
> 10 0
> 11 0
> ....
> 95 0
> 96 0
> 97 0
> 98 0
> 99 0
> 100 1
> 101 1
> 102 1
> 103 1
> 104 1
> ....
> 195 1
> 196 1
> 197 1
> 198 1
> 199 1
> => 200
>
> As you clearly see, the representation is ok.
>
> Btw, you'll notice the same effect with Fixnum#[] - because these methods
do
> not represent the actual binary representation in mem but try to represent
> the general concept of signed binary numbers:
>
> >> (-1)[100]
> => 1
> >> (-1)[1<<100]
> => 1
>
> Kind regards
>
>     robert
>
>
>
>
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