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 camsight@gmail.com 05-04-2005 04:00 PM

Fixnum's binary representation

Hi, people!

As far as I know, Ruby's Fixnum is 30-bit signed integer.
One bit is used as a flag for whether it's a direct value.
Another bit is used for non-Fixnum direct values.
I wondered how Fixnum#[] works and tested it.

def show_binary(i)
result = ''
32.times do |b|
result = i[b].to_s + result
if (b % 8 == 7) and (b != 31)
result = " " + result
end
end
puts result + ": " + i.to_s
end

show_binary(1)
show_binary(-1)
show_binary(2 ** 31 - 1)
show_binary((2 ** 31 - 1) * (-1))

Result:

00000000 00000000 00000000 00000001: 1
11111111 11111111 11111111 11111111: -1
01111111 11111111 11111111 11111111: 2147483647
10000000 00000000 00000000 00000001: -2147483647

This is exactly how 32-bit signed integer's binary representations look
like.
My guess is that Fixnum#[] works as if it's 32-bit signed integer.
It's not showing its real binary representation.
Is my guess true?

My another question is that even if (2 ** 31 - 1) is not a Fixnum, the
above code works for it (Actually Bignum#[]).
If the number is bigger than that, [] doesn't work.
Fixnum#[] and Bignum#[] are cleverly hiding the internal facts and are
made to simulate 32-bit signed integers?

Thanks.
Sam

 camsight@gmail.com 05-04-2005 04:21 PM

Re: Fixnum's binary representation

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.
When can I expect the sign bit?
I assume that numbers beyond 32-bit are not suitable for Bignum#[].
Do you agree?

Thanks again.

Sam

 Robert Klemme 05-04-2005 04:49 PM

Re: Fixnum's binary representation

<camsight@gmail.com> schrieb im Newsbeitrag
> 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

 Mark Hubbart 05-04-2005 05:06 PM

Re: Fixnum's binary representation

On 5/4/05, camsight@gmail.com <camsight@gmail.com> wrote:
> 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?
> I assume that numbers beyond 32-bit are not suitable for Bignum#[].
> Do you agree?

IIUC, it's not precisely a "sign bit"; it's more like an entire
bit-flip. -1 is zero, bit-flipped. So if you are trying to use
Integer#[] to get determine the sign of a number, the question is
simply: how high do you want to go? integer[512] will correctly
determine sign for numbers in the range of (-(2**512-1)..2**512) (that
is, a 512 bit integer). There no way (that I can think of) to use
Integer#[] to return the correct sign on *any* integer usable in Ruby.

hth,
Mark

 Ara.T.Howard@noaa.gov 05-04-2005 05:42 PM

Re: Fixnum's binary representation

On Wed, 4 May 2005 camsight@gmail.com wrote:

> Hi, people!
>
> As far as I know, Ruby's Fixnum is 30-bit signed integer.
> One bit is used as a flag for whether it's a direct value.
> Another bit is used for non-Fixnum direct values.
> I wondered how Fixnum#[] works and tested it.
>
> def show_binary(i)
> result = ''
> 32.times do |b|
> result = i[b].to_s + result
> if (b % 8 == 7) and (b != 31)
> result = " " + result
> end
> end
> puts result + ": " + i.to_s
> end
>
> show_binary(1)
> show_binary(-1)
> show_binary(2 ** 31 - 1)
> show_binary((2 ** 31 - 1) * (-1))
>
> Result:
>
> 00000000 00000000 00000000 00000001: 1
> 11111111 11111111 11111111 11111111: -1
> 01111111 11111111 11111111 11111111: 2147483647
> 10000000 00000000 00000000 00000001: -2147483647

don't deny yourself the joys of printf ;-)

harp:~ > cat a.rb
[ (1), (-1), (2 ** 31 - 1), ((2 ** 31 - 1) * (-1)) ].each{|n| printf "%32.32b\n", n}

harp:~ > ruby a.rb
00000000000000000000000000000001
11111111111111111111111111111111
01111111111111111111111111111111
10000000000000000000000000000001

cheers.

-a
--
================================================== =============================
| email :: ara [dot] t [dot] howard [at] noaa [dot] gov
| phone :: 303.497.6469
| renunciation is not getting rid of the things of this world, but accepting
| that they pass away. --aitken roshi
================================================== =============================

 Phil Tomson 05-04-2005 06:29 PM

Re: Fixnum's binary representation

In article <de63abca05050410051035e0c2@mail.gmail.com>,
Mark Hubbart <discordantus@gmail.com> wrote:
>On 5/4/05, camsight@gmail.com <camsight@gmail.com> wrote:
>> 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?
>> I assume that numbers beyond 32-bit are not suitable for Bignum#[].
>> Do you agree?

>
>IIUC, it's not precisely a "sign bit"; it's more like an entire
>bit-flip. -1 is zero, bit-flipped.

It's 2's complement, isn't it?

Phil

 camsight@gmail.com 05-04-2005 06:45 PM

Re: Fixnum's binary representation

Wow, that's great.

Thanks.
Sam

 camsight@gmail.com 05-04-2005 06:46 PM

Re: Fixnum's binary representation

Now I understand what you mean.
Thanks a lot!

Sam

 Mark Hubbart 05-04-2005 08:19 PM

Re: Fixnum's binary representation

On 5/4/05, Phil Tomson <ptkwt@aracnet.com> wrote:
> In article <de63abca05050410051035e0c2@mail.gmail.com>,
> Mark Hubbart <discordantus@gmail.com> wrote:
> >On 5/4/05, camsight@gmail.com <camsight@gmail.com> wrote:
> >> 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?
> >> I assume that numbers beyond 32-bit are not suitable for Bignum#[].
> >> Do you agree?

> >
> >IIUC, it's not precisely a "sign bit"; it's more like an entire
> >bit-flip. -1 is zero, bit-flipped.

>
> It's 2's complement, isn't it?

(looking it up)

yeah! that's what it is. IANACSM (I am not a CS major) and I still
have terminology to learn. I am woefully ignorant of it, and really
should read some programming theory. When I get the time. :)

cheers,
Mark

 Eric Hodel 05-04-2005 09:32 PM

Re: Fixnum's binary representation

On 04 May 2005, at 09:04, camsight@gmail.com wrote:

> Hi, people!
>
> As far as I know, Ruby's Fixnum is 30-bit signed integer.
> One bit is used as a flag for whether it's a direct value.
> Another bit is used for non-Fixnum direct values.
>
> My guess is that Fixnum#[] works as if it's 32-bit signed integer.
> It's not showing its real binary representation.
> Is my guess true?

\$ ruby
puts (-10..10).map { |i| "#{i}: #{i.object_id}" }.join("\n")
-5: -9
-4: -7
-3: -5
-2: -3
-1: -1
0: 1
1: 3
2: 5
3: 7
4: 9
5: 11

A Fixnum's object_id is 2N+1 its value, so if you want a Fixnum's
binary representation, use its object_id. (So long as the object_id is
a Fixnum.)

#define FIXNUM_FLAG 0x01
#define INT2FIX(i) ((VALUE)(((long)(i))<<1 | FIXNUM_FLAG))

Also, a Fixnum always has an odd object_id, while any other VALUE has
an even object_id.

--
Eric Hodel - drbrain@segment7.net - http://segment7.net
FEC2 57F1 D465 EB15 5D6E 7C11 332A 551C 796C 9F04

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