How to convert negative integers to binary in Ruby

Question

Question 1: I cannot find a way to convert negative integers to binary in the following way. I am supposed to convert it like this.

-3 => "11111111111111111111111111111101"

I tried below:

sprintf('%b', -3) => "..101" # .. appears and does not show 111111 bit.

-3.to_s(2) => "-11" # This just adds - to the binary of the positive integer 3.

Question 2: Interestingly, if I use online converter, it tells me that binary of -3 is "00101101 00110011".

What is the difference between "11111111111111111111111111111101" and "00101101 00110011"?

You can just invert the bits of a positive number to get it's opposite — Andrew Li, Jun 29 '16 at 05:05
Also, please clarify the rules of conversion; currently I can’t think of any giving `11111111111101` for `-3`. — Aleksei Matiushkin, Jun 29 '16 at 05:08

score 11 · Answer 1 · answered Jun 29 '16 at 05:08

11

Packing then unpacking will convert -3 to 4294967293 (2³² - 3):

[-3].pack('L').unpack('L')
=> [4294967293]

sprintf('%b', [-3].pack('L').unpack('L')[0])
# => "11111111111111111111111111111101"

sprintf('%b', [3].pack('L').unpack('L')[0])
# => "11"

answered Jun 29 '16 at 05:08

falsetru

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Great, it works! but what does pack('L') and unpack('L')[0] do in this case? – Noby Fujioka Jun 29 '16 at 05:29
@NobyFujioka, Check the [documentation for `Array#pack`](http://ruby-doc.org/core/Array.html#method-i-pack) or [`String#unpack`](http://ruby-doc.org/core/String.html#method-i-unpack). – falsetru Jun 29 '16 at 05:32
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It worth to mention that the above will produce the result of length 32 on 64bit arch only. – Aleksei Matiushkin Jun 29 '16 at 05:50
@mudasobwa, According to the documentation, `L` stands for `32-bit unsigned, native endian (uint32_t)` – falsetru Jun 29 '16 at 06:48
Oh, indeed, I messed things up. I meant that probably the requested value of length 32 is OK for 64bit only. Anyway. – Aleksei Matiushkin Jun 29 '16 at 07:01

score 1 · Answer 2 · answered Jun 29 '16 at 11:28

1

Try:

> 32.downto(0).map { |n| -3[n] }.join
#=> "111111111111111111111111111111101

Note: This applies to negative number's only.

answered Jun 29 '16 at 11:28

Gagan Gami

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How to convert negative integers to binary in Ruby

2 Answers2