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how to get exact sum of rounded numbers

1) Math.round( ( 100/3 + Number.EPSILON ) * 100 ) / 100  // 3.33

2) Math.round( ( 100/3 + Number.EPSILON ) * 100 ) / 100 *3 // 99.99

how to get 100 as sum if we add all the rounded numbers ?

Hithesh kumar
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  • You can't. Rounding drops or alters digits and there is no magical store that secretly keeps the original. If you need the original, then store the original. – Peter B Jul 22 '21 at 09:37
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    [How to deal with floating point number precision in JavaScript?](https://stackoverflow.com/q/1458633) – VLAZ Jul 22 '21 at 09:37
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    Some numbers just can't be represented in the binary floating point format used by JavaScript (and most other languages). Although thirds are representable neither in base 10 nor base 2 floating point, even the decimal value 0.1 cannot be precisely represented by base 2 (IEEE) floating point representation. For example, let's add `0.1` to itself 100 times... `[...Array(100)].reduce((a) => a + 0.1, 0)`. What's the result? Not 10. [Get used to it](https://floating-point-gui.de/) – spender Jul 22 '21 at 09:44
  • Here's a [followup question](https://stackoverflow.com/questions/68482867/what-tricks-does-js-employ-to-show-floating-point-numbers) related to this issue that I just asked. – spender Jul 22 '21 at 10:10

2 Answers2

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If you are rounding to display the numbers nicely, then instead of rounding, try using toFixed when you are actually rendering the numbers eg

let one = ( ( 100/3 + Number.EPSILON ) * 100 ) / 100;
let two = ( ( 100/3 + Number.EPSILON ) * 100 ) / 100 *3;
//when you print use toFixed
console.log(one, one.toFixed(2));
console.log(two, two.toFixed(2));

This way, you have your true value and a rounded value

pilchard
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PhantomSpooks
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I can see only two solutions for this maths question:

  1. Avoid rounding in the first place
  2. Round the result
IAfanasov
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