round rounding 2.4999999999999999 up instead of down

[Rudolf Bargholz]

Hi,

Windows 10

Elixir 1.1.1

Erlang/OTP 18 [64-bit]

Is this expected behaviour, or not?

  iex(37)> round 2.499999999999999 

  2

  iex(38)> round 2.4999999999999999

  3

Regards

Rudolf

[r...@dimpixel.com]

I'm not familiar with the reason why, but it appears to be expected behavior based on what I'm seeing in other languages. When evaluating 2.4999999999999999 in Elixir, Ruby, Go and Python 2.5 is returned, which would change your rounding results.

iex(1)> 2.4999999999999999

2.5

- Rob

[John W Higgins]

Yes, you have absolutely no guarantees when you are trying to round floating point numbers - the 2.4999999999999999 number most likely does not exist exactly as a floating point number so it ends up becoming something like 2.5 which rounds up to 3.

You cannot use floating point numbers and assume anything concerning whether or not you will have some issue with regards to the exact representation of the number.

There are multiple examples of this that exist in any language when using floating point numbers. If you require such precision then you need to use a decimal library which allows for the exact representation of the values.

John

[Jim Freeze]

My guess is that 2.499999999999999  is < 2.5

and 2.4999999999999999 is == to 2.5

iex(1)> 2.4999999999999999 == 2.5

true

iex(2)> 2.499999999999999 == 2.5

false

Jim

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[Rudolf Bargholz]

Thanks to all for the clarification. Was just a little unsure how to interpret the result.

Rudolf

[Robert Virding]

Floating point numbers often don't behave as you would expect, one major reason is that we see them as decimal while they are implemented as binary. For example 0.1 has no exact representation. Here are a couple of sights which discuss the problem:

http://floating-point-gui.de/
http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html

Robert

[Phani Mahesh]

Related: Visit http://0.30000000000000004.com. You may find this interesting and/or amusing.

[Ed W]

The other answers are also partly correct, but Elixir round function is broken and won't behave as you desire - don't use it.  I opened a bug report some months back, but it doesn't seem to have reached a conclusion I think?

If you need to accurately round a float, eg you are being naughty and doing currency calculations using a float and want to round back to actual money values at the end, then use the decimal library and it's rounding methods. 

Good luck

Ed W