How do I parse a two's complement string to a number

Question

I have binary form for -3 in two's complement form - 11111111111111111111111111111101, and use it with parseInt function:

parseInt('11111111111111111111111111111101', 2)

But it returns 4294967293, which is the integer that results if 11111111111111111111111111111101 is parsed as unsigned int. How can I parse integer as a signed one?

Related (almost duplicate): http://stackoverflow.com/a/13468626/1207195 — Adriano Repetti, May 04 '16 at 08:36
@AdrianoRepetti, I've seen that answer, and it doesn't answer my questions — Max Koretskyi, May 04 '16 at 08:36
Why not? It's for base 16 and you need base 2, that's the only difference, as far as I can see... — Adriano Repetti, May 04 '16 at 08:37
what is the number you are expecting? I think I found something, but am not sure — Tschallacka, May 04 '16 at 08:37
Yeah, to my surprise, I'm not seeing a previous "how do I parse a two's complement string" question (for JavaScript). — T.J. Crowder, May 04 '16 at 08:38
You provide a 32 Bit String to the ParseInt function. But If javascript interprets integers as 64 Bit values then, you need 64 bits. Could you try it please. I mean prepend you string with 16 x "1" — Peter Paul Kiefer, May 04 '16 at 08:38
@PeterPaulKiefer: Nowhere in the JavaScript specification does it talk about treating anything as a 64-bit integer (well, there's a brief mention of *allowing* a 64-bit int intermediate result during a time clip operation). JavaScript deals primarily with IEEE-754 double-precision floating point values, and for bit manipulation (and a couple of other things) deals with 32-bit integer values (signed or unsigned depending on what it's doing). — T.J. Crowder, May 04 '16 at 08:40
@T.J.Crowder, so there is no way to tell parseInt to treat a binary number as two's complement? — Max Koretskyi, May 04 '16 at 08:43
Why is this being downvoted? I guess initially it was a bit unfocussed, but parsing a two's complement string to a number is a reasonable thing to need to do, and doesn't appear to have been asked and answered on-site yet. — T.J. Crowder, May 04 '16 at 08:44
I looked into a ECMA Script 6 implementation (Mozilla) and saw that they treat a binary string as an unsigned string. You can not use 2-complements. But you can prepend it with a minus sign. i.e Build a bitweise complement of your string prpend it with a "-" and parse it. — Peter Paul Kiefer, May 04 '16 at 08:48

score 8 · Accepted Answer · edited Dec 11 '20 at 11:13

8

~~parseInt('11111111111111111111111111111101',2)// == -3

is what you are looking for.

Related answer ~~ vs-parseint

var x = ~~y; is a 'trick' (similar to var x = y << 0;) that (ab)uses the unary bitwise NOT operator to force the result to be in the range of a signed 32-bit integer, discarding any non-integer portion.

edited Dec 11 '20 at 11:13

FZs

16,581
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answered May 04 '16 at 08:45

Tschallacka

27,901
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2

@Maximus Forget my last comment to your question. This solution is better. – Peter Paul Kiefer May 04 '16 at 08:50

MatSch · Answer 2 · 2020-12-11T10:15:45.487

1

i had the same problem, you can use the following:

function parseInt2complement(bitstring,bitcount)
{
    value = parseInt(bitstring, 2);

    if ((value & (1<<(bitcount-1))) > 0) {
       value = value - (1<<(bitcount));
    }
    return value;
}

console.log(parseInt2complement('111111111111111111111111111101', 30))
console.log(parseInt2complement('1111111111111111111111111111101', 31))

edited Dec 11 '20 at 10:15

answered Dec 05 '20 at 15:14

MatSch

163
6

This function only work until 30 bit because of 32bit operations of javascript – MatSch Dec 11 '20 at 10:16

FZs · Answer 3 · 2020-12-11T11:14:05.217

For 32-bit numbers, it's enough to cast it to a 32-bit integer (no need for parseInt here), which can be done by applying any bitwise operation to it (and indicating that it's in binary by prefixing with 0b).

function parse(num){
  return ('0b'+num) | 0
}

console.log(parse('11111111111111111111111111111101'))

For a non-32-bit number, however, you'll need some tricks:

function parse(num, length = num.length){
  if(num[num.length-length] !== '1')
    return +('0b'+num)
  let inverse = ''
  for(const digit of num.slice(-length))
    inverse += +!+digit
  return -('0b' + inverse) - 1
}

console.log(parse('1101', 4)) //-3
console.log(parse('1111111111111101', 16)) //-3
console.log(parse('1111111111111111111111111111111111111111111111111111111111111101', 64)) //-3
console.log(parse('1111111111111111111111111111111111111111111111111111111111111101')) //-3, bit length inferred from the length of the string

How do I parse a two's complement string to a number

3 Answers3