导致不正确舍入的 JS 浮点数 - Math.round() 总是有效？

Question

I have edge cases with floats after multiplying.

I have to round numbers to the nearest hundred, but in cases like that:

115000*4.35 //500249.99999999994

I'll get a wrong result:

Math.round(115000*4.35 / 100) * 100 //500200

instead of 500300

A solution:

First rounding to the nearest one decimal place:

  Math.round(500249.99999999994 * 10) / 10 //200250,

then rounding to the nearest hundred:

Math.round(500250 / 100) * 100 //200300

combined:

Math.round(Math.round(num * 10) / 1000) * 100 

Does anyone know if it's a safe way to solve it? Does it always work?

Thanks in advance!

Answer 1

Does anyone know if it's a safe way to solve it? Does it always work?

It is not safe and does not always work.

Consider 100,005 • 9.99. In real arithmetic, this is 999,049.95, and you apparently desire to round it to the nearest multiple of 100, producing 999,000.

9.99 cannot be stored in a JavaScript Number. The nearest representable value is 9.9900000000000002131628207280300557613372802734375. Multiplying this by 100.005 in JavaScript's floating-point arithmetic yields 999,049.950000000069849193096160888671875.

Your proposed solution first attempts to round that to the nearest multiple of 1/10. In this case, it succeeds, yielding 999,050. (In many cases, this fails, since the only multiples of 1/10 that can be represented are those that are multiples of 5/10.) Then using Math.round(999050 / 100) * 100) yields 999,100 (since Math.round rounds halfway cases upward).

Even if real arithmetic were used, rather than floating-point arithmetic, your proposed solution would not work. Rounding 999,049.95 to the nearest multiple of 1/10 using the same method (halfway cases up) yields 999,050, and rounding that to the nearest multiple of 100 yields 999,100.

Answer 2

Rounding to the nearest integer can be simply done by calling

Math.round(500249.99999999994)

So rounding to the nearest hundred becomes:

 const num = 115000*4.35; const roundedToNearestHundred = Math.round(Math.round(num) / 100) * 100; console.log(roundedToNearestHundred);