Converting Bytes to a Negative Integer

ruby-talk
1

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?

~harris

···

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2

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
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

···

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

3

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

···

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
11101100

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

4

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

···

On 2/14/07, James Edward Gray II <james@grayproductions.net> 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
> 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...

5

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

  x = -123
  s = .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 = .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

···

On 2/14/07, James Edward Gray II <james@grayproductions.net> 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
> 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...

--
Austin Ziegler * halostatue@gmail.com * http://www.halostatue.ca/
               * austin@halostatue.ca * You are in a maze of twisty little passages, all alike. // halo • statue
               * austin@zieglers.ca

6

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

See my earlier post in this thread about this.

James Edward Gray II

···

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

7

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

···

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

8

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

James Edward Gray II

···

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>.

9

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 :slight_smile:

···

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:

10

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 :slight_smile:

I wasn't lording 64 bit processing over you. :wink:

James Edward Gray II

···

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