Add different rounding method

[yunghzz35]

There is no way to use other rounding method like banker's rounding, which is common in financial applications. Adding this feature would be great I think since this library has so many math functions that others libraries do not have. For now, I can only rely on another library like big.js. But functionalites are seems to be duplicate with this library.

[dvd101x]

You could try to extend the functionality according to your needs. Maybe something like this.

function bankersRounding(value) {
  const roundedValue = math.round(value);
  const remainder = value - math.floor(value);
  if (math.equal(remainder, 0.5)) {
    return math.floor(value) % 2 === 0 ? math.floor(value) : math.ceil(value);
  }
  return roundedValue;
}

math.import({ bankersRounding : bankersRounding})

[josdejong]

If I'm not mistaken, the only difference with bankers rounding is rounding 0.5 to 0 instead of to 1 right?

When you configure mathjs to use BigNumbers, you can utilize the rich rounding options of the library used for the bignumbers: decimal.js.

math.config({ number: 'BigNumber'}) 
math.BigNumber.set({ rounding: math.BigNumber.ROUND_HALF_EVEN })

console.log(math.format(math.evaluate('round(0.5)')))      // 0
console.log(math.format(math.evaluate('round(0.225, 2)'))) // 0.22
console.log(math.format(math.evaluate('round(0.235, 2)'))) // 0.24

[dvd101x]

I'm not familiar with this topic, but searched online some references saying:

numbers which are equidistant from the two nearest integers are rounded to the nearest even integer. Thus, 0.5 rounds down to 0; 1.5 rounds up to 2.

https://wiki.c2.com/?BankersRounding

[josdejong]

Ah, so then we must not use ROUND_HALF_DOWN but ROUND_HALF_EVEN I think. I'll update my example, thanks for looking this up David!

ROUND_HALF_EVEN Rounds towards nearest neighbour. If equidistant, rounds towards even neighbour
https://mikemcl.github.io/decimal.js/#modes