How to convert two's complement binary to decimal in ruby?

Question

What is the easiest way to convert a two's complement binary to decimal? For example, if I normally convert a string such as"1001" to decimal I'll get a 9. But I'm actually trying to get a simple -7. What do you guys suggest?,

Two's Complement only makes sense if the highest bit is set to 1, and that's not the case here unless for some strange reason you're encoding this in 4 bits. — tadman
If you have four-bit two's-complement, "1001" would be -1, not -7. Therefore, would just doing if val > 8 then val = 8 - val work here? — Ken Y-N
@tadman not everyone can afford a 64-bit computer. This is mine.. — Cary Swoveland
@CarySwoveland Glad you got the memory upgrade so you can use lower-case letters. — tadman

Roan Roan · Accepted Answer · 2017-07-28T06:01:58

From your question it looks like you are using a 4 bit system.
This might work for you, I got the results you were asking for.
Here are two functions, one for 4 bit and one for 16 bit twos' complement.

  # For 4 bit
  def convert_4bit_to_signed_binary(binary)
    binary_int = binary.to_i(2)
    if binary_int >= 2**3 # for 4 bit
      return binary_int - 2**4
    else
      return binary_int
    end
  end

  # For 16 bit
  def convert_16bit_to_signed_binary(binary)
    binary_int = binary.to_i(2)
    if binary_int >= 2**15 # for 4 bit
      return binary_int - 2**16
    else
      return binary_int
    end
  end

  i = convert_4bit_to_signed_binary('1001')   # will give -7
  j = convert_16bit_to_signed_binary('1001')  # will give 9

  puts i
  puts j

Let me know if this works for you?

How to convert two's complement binary to decimal in ruby?

1 Answers