"Math.round" is not rounding to the nearest integer correctly

July 7, 2022

Can someone please kindly explain to me why "Math.round" is not rounding to the nearest integer correctly? I have a 2D array, in which I want to find out the average of each columns.

const nums = [[ 20,10,35 ],[70,179,30],[207,223,8],[38,53,52],[234,209,251],[35,94,2]]

After transposing the each columns into rows:

[
  [ 20, 70, 207, 38, 234, 35 ],
  [ 10, 179, 223, 53, 209, 94 ],
  [ 35, 30, 8, 52, 251, 2 ]
]

I was able to return the average to be rounded off.

const avg = (arr) => arr[0].map((_,i) => arr.reduce((sum,v) => sum + Math.round(v[i]/arr.length),0))

Unfortunately, "Math.round" is not rounding the result correctly:

before  // [ 100.66666666666667, 127.99999999999999, 63.00000000000001 ]
after   // [ 101, 129, 63 ]

As you can see the 2nd element shoud have rouned off to "128" instead of "129". I’ve tried testing as followed without problem:

console.log(Math.round(127.99999999999999)); // 128

However, "Math.round" from my function is outputting incorrect result. Is this a glitch? Or is there something wrong with my function? Any feedback will be greatly appreciated, million thanks in advance 🙂

>Solution :

JavaScript math is at times inaccurate because it stores numbers in memory as binary floats, for details see Why JavaScript is Bad At Math. Anyways, if you’re looking to have an array of three averages see the example below.

const data = [
  [ 20, 70, 207, 38, 234, 35 ],
  [ 10, 179, 223, 53, 209, 94 ],
  [ 35, 30, 8, 52, 251, 2 ]
];

let output = data.map(sub => Math.round(
  sub.reduce(
    (sum, cur) => sum + cur)/sub.length)
);

console.log(output);