Adding 0.02 to number in javascript results in incorrect values

Question

I'm trying to get an array of numbers from some min to max by a decimal interval, in this case, 0.02, for example: 1.00, 1.02, 1.04, 1.06

The code below starts to break at the number 1.14:

function getOptions(min, max, interval) {
  var options = [];

  for (var i = min; i <= max; i += interval) {
    options.push(i);
  }

  return options;
}

var options = getOptions(1.0, 1.4, 0.02);

At this point, options is like:

[
    1, 
    1.02, 
    ...
    1.12, 
    1.1400000000000001, 
    1.1600000000000001
    ...
]

What causes this and how can I fix it?

try - options.push(parseFloat(i.toFixed(2)));, where toFixed(n) converts the input to string keeping n number of decimals. — BatScream
– BatScream, Commented May 21, 2015 at 16:57
@BatScream, that's my answer. In fact, I was using options.push(i.toFixed(2) prior to posting the question, but my loop wasn't working because it wasn't inclusive on i <= max -- using i.toFixed(2) <= max is what I needed. — Tom
– Tom, Commented May 21, 2015 at 17:00

Community · Accepted Answer · 2020-06-20 09:12:55Z

3

As per What Every Programmer Should Know About Floating-Point Arithmetic:

internally, computers use a format (binary floating-point) that cannot accurately represent a number like 0.1, 0.2 or 0.3 at all.

When the code is compiled or interpreted, your “0.1” is already rounded to the nearest number in that format, which results in a small rounding error even before the calculation happens.

For Javascript specifically:

JavaScript numbers are really just floating points as specified by IEEE-754. Due to inadequecies when representing numbers in base-2, as well as a finite machine, we are left with a format that is filled with rounding errors. This article explains those rounding errors and why errors occur. Always use a good library for numbers instead of building your own

I have had some success with this BigDecimal port and the smaller, faster big.js.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered May 21, 2015 at 16:55

Andy

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