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comp.lang.ruby

Fixnum's binary representation

camsight

5/4/2005 4:01:00 PM

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
29 Answers

Robert Klemme

5/4/2005 4:07:00 PM

0

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.

> 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?

Yes.

> 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.

For which numbers do you have problems? I don't see any so far

>> 100.times {|sh| raise unless sh==0||(1 << sh)[0] == 0}
=> 100
>> (1<<100).class
=> Bignum

> Fixnum#[] and Bignum#[] are cleverly hiding the internal facts and are
> made to simulate 32-bit signed integers?

Yes.

Kind regards

robert

camsight

5/4/2005 4:21:00 PM

0

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?

Thanks again.

Sam

Robert Klemme

5/4/2005 4:49:00 PM

0


<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

Mark Hubbart

5/4/2005 5:06:00 PM

0

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

5/4/2005 5:42:00 PM

0

ptkwt

5/4/2005 6:29:00 PM

0

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

5/4/2005 6:46:00 PM

0

Wow, that's great.
I have to learn more...:-)

Thanks.
Sam

camsight

5/4/2005 6:47:00 PM

0

Now I understand what you mean.
Thanks a lot!

Sam

Mark Hubbart

5/4/2005 8:20:00 PM

0

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

5/4/2005 9:32:00 PM

0

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://se...
FEC2 57F1 D465 EB15 5D6E 7C11 332A 551C 796C 9F04