Getting 2 digits floating number isn't working as expected

Question

I have this number : -0.0166667 and I would like to get only two floating digits, so the number should be -0.01

I tried this

-0.0166667.toFixed(2)  # gives : -0.02

and also this

Math.round(-0.0166667 * 100) / 100 # also gives : -0.02

But both of them transform the number to -0.02 instead of -0.01

What's wrong here ?

Can't get jsPerf to work right now, but I don't see why substring() wouldn't work here if you're just truncating — colonelsanders, Mar 22 '16 at 13:22

Nina Scholz · Accepted Answer · 2016-03-22T13:49:11.397

3

Use | bitwise or for truncate the fraction part.

var x = (-0.0166667 * 100 | 0) / 100,
    y = (0.0166667 * 100 | 0) / 100;
document.write(x + '<br>' + y);

A better solution which saves the sign, applies flooring and puts the sign back.

function toFixed(x, digits) {
    return (x < 0 ? - 1 : 1) * Math.floor(Math.abs(x) * Math.pow(10, digits)) / Math.pow(10, digits);
}

document.write(toFixed(-0.0166667, 2) + '<br>');
document.write(toFixed(0.0166667, 2) + '<br>');

document.write(toFixed(-0.0166667, 3) + '<br>');
document.write(toFixed(0.0166667, 3) + '<br>');

edited Mar 22 '16 at 13:49

answered Mar 22 '16 at 13:16

Nina Scholz

376,160
25
347
392

1

The bitwise operation will work, but it potentially loses information. – Pointy Mar 22 '16 at 13:20
Could this be used for 3 precision digits? (curious) – phenxd Mar 22 '16 at 13:23
If Math.sign was supported in ie and safari, the second one would be even shorter. – Tamas Hegedus Mar 22 '16 at 13:27

Adnan Umer · Answer 2 · 2016-03-22T13:26:35.377

1

For numbers between less then zero, Math.ceil truncates without rounding and for numbers greater then zero Math.floor works fine. As your number is less then zer so use Math.ceil instead

Math.ceil(-0.0166667 * 100) / 100

EDIT

Write a custom helper method to truncate floating point

function truncate(num) {
    return num > 0 ? Math.floor(num) : Math.ceil(num);
}

and to round number off by 2 digits use

truncate(num * 100) / 100

edited Mar 22 '16 at 13:26

answered Mar 22 '16 at 13:19

Adnan Umer

3,669
2
18
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This will make positive numbers round upwards as well. Truncation is "rounding to zero". – Tamas Hegedus Mar 22 '16 at 13:20

score 1 · Answer 3 · answered Mar 22 '16 at 13:24

Math.round will round the number to the nearest integer, so the output of your code is actually correct. What you need is truncation instead of rounding. Here is the simplest way to do that:

function toZero(x) {
  return x > 0 ? Math.floor(x) : Math.ceil(x);
}

function round2digits(x) {
  return toZero(x * 100) / 100;
}

document.write(round2digits(-0.0166)); // prints `-0.01`

Getting 2 digits floating number isn't working as expected

3 Answers3