Parsing of binary twos-complement number to decimal?

Question

Well I'm confused this isn't easier than I thought... I want to convert a twos complement binary string to the appropriate number:

-5 = '11111011' according to Wikipedia

So I would think the following would return negative 5, but it doesn't. After reading Wikipedia it seems all I have to do is subtract one then invert the bits:

~(parseInt('11111011', 2) - 1)

But it returns -251. Confusing...

Please note that I will be dealing with many odd bit lengths and all won't be 8 bits.

Is this useful? https://stackoverflow.com/questions/12337360/javascript-tilde-twos-complement — mplungjan, Jul 06 '18 at 07:11
Or this? https://stackoverflow.com/questions/16155592/negative-numbers-to-binary-string-in-javascript — mplungjan, Jul 06 '18 at 07:12
I looked at every single question I could find regarding it and still couldn't get it to work... — wayofthefuture, Jul 06 '18 at 07:12
Why does this not work for you? `var binary = (-5 >>> 0).toString(2); // coerced to uint32` — mplungjan, Jul 06 '18 at 07:16
I'm trying to convert from the string to the number, I don't know the number. — wayofthefuture, Jul 06 '18 at 07:18
This one? https://stackoverflow.com/questions/27911677/how-to-convert-a-binary-number-into-a-signed-decimal-number — mplungjan, Jul 06 '18 at 07:20

mplungjan · Answer 1 · 2021-09-24T11:31:56.813

This will take an unknown bit pattern <= 32 bits and if it is < 8 bits convert to positive int and if >= 8 with a leading 1, a negative signed int.

The trick is to pad to 32 before parseInt

const getSigned = binStr => parseInt(binStr.length >= 8 && binStr[0] === "1" ?
  binStr.padStart(32, "1") : binStr.padStart(32, "0"), 2) >> 0;

const signed = [
  "11111011",     // -5
  "01111011",     // 123
  "1111111111000" // -8
].map(getSigned);

console.log(signed);

score 1 · Answer 2 · answered Jul 06 '18 at 07:56

After some time working on it I created this solution... if you are experienced with binary and bitwise operations please let me know your thoughts:

function getSignedInteger(bits) {
    let negative = (bits[0] === '1');
    if (negative) {
        let inverse = '';
        for (let i = 0; i < bits.length; i++) {
            inverse += (bits[i] === '0' ? '1' : '0');
        }
        return (parseInt(inverse, 2) + 1) * -1;
    } else {
        return parseInt(bits, 2);
    }
}

getSignedInteger('11111011') returns -5

score 0 · Answer 3 · answered Jul 06 '18 at 08:07

I feel your confusion. The wikipedia article is showing you examples with 8 bit numbers, but you aren't generally dealing with 8 bit numbers in javascript. If you want want to explore binary number math in javascript you will probably be using 32 bits and forcing conversation to 32bit ints with bitwise operators.

So translating the wikipedia article would give you this:

-5 would look more like 11111111111111111111111111111011

All those extra zeros get flipped. With that in mind this makes a bit more sense:

let str = "11111111111111111111111111111011"
console.log(str.length, 'bits')
console.log((parseInt(str, 2) >> 0))

It should make more sense now that:

let five = '00000000000000000000000000000101'
let neg6 = '11111111111111111111111111111010'

console.log('5: ', parseInt(five, 2) >> 0)
console.log('-6: ', parseInt(neg6, 2) >> 0)

// two complement
let neg6Plus1 = '11111111111111111111111111111011'

console.log('-5: ', parseInt(neg6Plus1, 2) >> 0)

what do you think about this? parseInt('11111011'.padStart(32,'1'), 2) >> 0 — wayofthefuture, Jul 06 '18 at 08:17
@wayofthefuture - yeah, it helped to make all 32 bits. Two great minds ;) — mplungjan, Jul 06 '18 at 08:26

Parsing of binary twos-complement number to decimal?

3 Answers3