Velocity Reviews > Ruby > Converting Bytes to a Negative Integer

# Converting Bytes to a Negative Integer

Harris Reynolds
Guest
Posts: n/a

 02-14-2007
Let's say I have 4 bytes:=0A=0A255 255 255 236=0A=0AThe binary representati=
on of this is:=0A=0A11111111 =0A11111111 =0A11111111 =0A11101100=0A=0AThe=
decimal number should be -20. Does anyone know how to convert these 4 byt=
es into an Integer?=0A=0A~harris=0A=0A=0A =0A______________________________=
__________________________________________________ ____=0AAny questions? Get=

James Edward Gray II
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Posts: n/a

 02-14-2007
On Feb 14, 2007, at 2:27 PM, Harris Reynolds wrote:

> Let's say I have 4 bytes:
>
> 255 255 255 236

>> bytes = [255, 255, 255, 236].map { |b| b.chr }.join

=> "\377\377\377\354"

> The binary representation of this is:
>
> 11111111
> 11111111
> 11111111
> 11101100

>> bytes.unpack("B*").first.scan(/[01]{8}/)

=> ["11111111", "11111111", "11111111", "11101100"]

> The decimal number should be -20. Does anyone know how to convert
> these 4 bytes into an Integer?

This is where I had trouble. I found the following:

>> bytes.reverse.unpack("l*").first

=> -20

This is tied to my processor though. I had to reverse the bytes
because they are not in the order my machine expects.

unpack() has directives for when the order is known, but all of those
seemed to be unsigned.

I'm anxious to see the cross-platform solution for this...

James Edward Gray II

Tim Pease
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Posts: n/a

 02-14-2007
On 2/14/07, James Edward Gray II <(E-Mail Removed)> wrote:
> On Feb 14, 2007, at 2:27 PM, Harris Reynolds wrote:
>
> > Let's say I have 4 bytes:
> >
> > 255 255 255 236

>
> >> bytes = [255, 255, 255, 236].map { |b| b.chr }.join

> => "\377\377\377\354"
>
> > The binary representation of this is:
> >
> > 11111111
> > 11111111
> > 11111111
> > 11101100

>
> >> bytes.unpack("B*").first.scan(/[01]{8}/)

> => ["11111111", "11111111", "11111111", "11101100"]
>
> > The decimal number should be -20. Does anyone know how to convert
> > these 4 bytes into an Integer?

>
> This is where I had trouble. I found the following:
>
> >> bytes.reverse.unpack("l*").first

> => -20
>
> This is tied to my processor though. I had to reverse the bytes
> because they are not in the order my machine expects.
>
> unpack() has directives for when the order is known, but all of those
> seemed to be unsigned.
>
> I'm anxious to see the cross-platform solution for this...
>

LITTLE_ENDIAN = [42].pack('i')[0] == 42

bytes = [255, 255, 255, 236]
str = bytes.pack('C*')
str.reverse! if LITTLE_ENDIAN
str.unpack('i')[0]

Blessings,
TwP

Austin Ziegler
Guest
Posts: n/a

 02-14-2007
On 2/14/07, James Edward Gray II <(E-Mail Removed)> wrote:
> On Feb 14, 2007, at 2:27 PM, Harris Reynolds wrote:
> > Let's say I have 4 bytes:
> >
> > 255 255 255 236

>
> >> bytes = [255, 255, 255, 236].map { |b| b.chr }.join

> => "\377\377\377\354"
>
> > The binary representation of this is:
> >
> > 11111111
> > 11111111
> > 11111111
> > 11101100

>
> >> bytes.unpack("B*").first.scan(/[01]{8}/)

> => ["11111111", "11111111", "11111111", "11101100"]
>
> > The decimal number should be -20. Does anyone know how to convert
> > these 4 bytes into an Integer?

>
> This is where I had trouble. I found the following:
>
> >> bytes.reverse.unpack("l*").first

> => -20
>
> This is tied to my processor though. I had to reverse the bytes
> because they are not in the order my machine expects.
>
> unpack() has directives for when the order is known, but all of those
> seemed to be unsigned.
>
> I'm anxious to see the cross-platform solution for this...

Joel VanDerWerf had something that I'll be adapting if I need it elsewhere.

x = -123
s = [x].pack("N")

bits = 32
max_unsigned = 2 ** bits
max_signed = 2 ** (bits - 1)
to_signed = proc { |n| (n >= max_signed) ? n - max_unsigned : n }

puts to_signed[s.unpack("N").first]

I need it for unsigned->signed words, so:

x = -123
s = [x].pack("n")

bits = 16
max_unsigned = 2 ** bits
max_signed = 2 ** (bits - 1)
to_signed = proc { |n| (n >= max_signed) ? n - max_unsigned : n }

puts to_signed[s.unpack("n").first]

-austin
--
Austin Ziegler * http://www.velocityreviews.com/forums/(E-Mail Removed) * http://www.halostatue.ca/
* (E-Mail Removed) * http://www.halostatue.ca/feed/
* (E-Mail Removed)

Morton Goldberg
Guest
Posts: n/a

 02-14-2007

On Feb 14, 2007, at 3:27 PM, Harris Reynolds wrote:

> Let's say I have 4 bytes:
>
> 255 255 255 236
>
> The binary representation of this is:
>
> 11111111
> 11111111
> 11111111
> 11101100
>
> The decimal number should be -20. Does anyone know how to convert
> these 4 bytes into an Integer?

Maybe this will help:

data = ""
[255, 255, 255, 236].each { |e| data << e }
data # => "\377\377\377\354"
data.unpack('l').first # => -20

Regards, Morton

James Edward Gray II
Guest
Posts: n/a

 02-14-2007
On Feb 14, 2007, at 5:02 PM, Morton Goldberg wrote:

> data = ""
> [255, 255, 255, 236].each { |e| data << e }
> data # => "\377\377\377\354"
> data.unpack('l').first # => -20

Watch what happens when I run the same code:

>> data = ""

=> ""
>> [255, 255, 255, 236].each { |e| data << e }

=> [255, 255, 255, 236]
>> data

=> "\377\377\377\354"
>> data.unpack('l').first

=> -318767105

James Edward Gray II

Morton Goldberg
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Posts: n/a

 02-15-2007
On Feb 14, 2007, at 6:37 PM, James Edward Gray II wrote:

> On Feb 14, 2007, at 5:02 PM, Morton Goldberg wrote:
>
>> data = ""
>> [255, 255, 255, 236].each { |e| data << e }
>> data # => "\377\377\377\354"
>> data.unpack('l').first # => -20

>
> Watch what happens when I run the same code:
>
> >> data = ""

> => ""
> >> [255, 255, 255, 236].each { |e| data << e }

> => [255, 255, 255, 236]
> >> data

> => "\377\377\377\354"
> >> data.unpack('l').first

> => -318767105

I see your point. Many new computers have 64-bit CPUs and we should
remember that. Also, you want to be sure I, lowly 32-bit CPU owner,
envy you for having one of these <just kidding>.

Regards, Morton

James Edward Gray II
Guest
Posts: n/a

 02-15-2007
On Feb 15, 2007, at 9:47 AM, Morton Goldberg wrote:

> On Feb 14, 2007, at 6:37 PM, James Edward Gray II wrote:
>
>> On Feb 14, 2007, at 5:02 PM, Morton Goldberg wrote:
>>
>>> data = ""
>>> [255, 255, 255, 236].each { |e| data << e }
>>> data # => "\377\377\377\354"
>>> data.unpack('l').first # => -20

>>
>> Watch what happens when I run the same code:
>>
>> >> data = ""

>> => ""
>> >> [255, 255, 255, 236].each { |e| data << e }

>> => [255, 255, 255, 236]
>> >> data

>> => "\377\377\377\354"
>> >> data.unpack('l').first

>> => -318767105

>
> I see your point. Many new computers have 64-bit CPUs and we should
> remember that. Also, you want to be sure I, lowly 32-bit CPU owner,
> envy you for having one of these <just kidding>.

I'll prop you back up then, this isn't a 32 to 64 bit (size) issue.
It's an endian (order) issue:

http://en.wikipedia.org/wiki/Endianness

James Edward Gray II

Morton Goldberg
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Posts: n/a

 02-15-2007
On Feb 15, 2007, at 11:04 AM, James Edward Gray II wrote:

> On Feb 15, 2007, at 9:47 AM, Morton Goldberg wrote:
>
>> On Feb 14, 2007, at 6:37 PM, James Edward Gray II wrote:
>>
>>> On Feb 14, 2007, at 5:02 PM, Morton Goldberg wrote:
>>>
>>>> data = ""
>>>> [255, 255, 255, 236].each { |e| data << e }
>>>> data # => "\377\377\377\354"
>>>> data.unpack('l').first # => -20
>>>
>>> Watch what happens when I run the same code:
>>>
>>> >> data = ""
>>> => ""
>>> >> [255, 255, 255, 236].each { |e| data << e }
>>> => [255, 255, 255, 236]
>>> >> data
>>> => "\377\377\377\354"
>>> >> data.unpack('l').first
>>> => -318767105

>>
>> I see your point. Many new computers have 64-bit CPUs and we
>> should remember that. Also, you want to be sure I, lowly 32-bit
>> CPU owner, envy you for having one of these <just kidding>.

>
> I'll prop you back up then, this isn't a 32 to 64 bit (size)
> issue. It's an endian (order) issue:

Mea culpa -- I didn't see your point at all. Let's see if I've got it
now.

1. Your result couldn't the result of conversion into a 64-bit
integer with big-endian byte order, because if that were the case we
would see either -20 or a large positive number (if sign extension
didn't occur).

2. On my computer (iMac G5), I get

[-20].pack('i') # => "\377\377\377\354"
[-318767105].pack('i') # => "\354\377\377\377"

and

[-20].pack('N') # => "\377\377\377\354"
[-318767105].pack('N') # => "\354\377\377\377"

which shows it's big-endian because both unpack directives produce
the same result.

3. On your computer (which I think is also a Macintosh), you would get

[-20].pack('i') # => "\354\377\377\377"
[-318767105].pack('i') # => "\377\377\377\354"

and

[-20].pack('N') # => "\377\377\377\354"
[-318767105].pack('N') # => "\354\377\377\377"

which would show it's little-endian and, presumably, some kind of
Intel-based Mac.

Do I have it right now?

Regards, Morton

P.S. But I don't see how this props me back up

James Edward Gray II
Guest
Posts: n/a

 02-16-2007
On Feb 15, 2007, at 4:58 PM, Morton Goldberg wrote:

> 2. On my computer (iMac G5), I get
>
> [-20].pack('i') # => "\377\377\377\354"
> [-318767105].pack('i') # => "\354\377\377\377"
>
> and
>
> [-20].pack('N') # => "\377\377\377\354"
> [-318767105].pack('N') # => "\354\377\377\377"
>
> which shows it's big-endian because both unpack directives produce
> the same result.
>
> 3. On your computer (which I think is also a Macintosh), you would get

Yes, it's an Intel Core Duo Mac.

> [-20].pack('i') # => "\354\377\377\377"
> [-318767105].pack('i') # => "\377\377\377\354"
>
> and
>
> [-20].pack('N') # => "\377\377\377\354"
> [-318767105].pack('N') # => "\354\377\377\377"
>
> which would show it's little-endian and, presumably, some kind of
> Intel-based Mac.

Exactly.

> Do I have it right now?

Sure do.

> P.S. But I don't see how this props me back up

I wasn't lording 64 bit processing over you.

James Edward Gray II