Unexpected Behaviour of the Rounding Function

General Usage
1

I am trying to round some Float64 values and don’t understand the behavior of the round() function for specific values.

If I take:

round(0.585, digits=2, RoundNearestTiesUp)

I get 0.59, as expected, since I want values to be rounded up when ending in a 5.

However, for:

round(0.575, digits=2, RoundNearestTiesUp)

I get 0.57, which I don’t understand. This goes for 0.565 as well for some reason.

My question is why does this happen, particularly when I get the expected behavior for 0.505,0.515…0.595, except for 0.565 and 0.575?

I assume this has to do with the RoundingMode, but does anyone have any suggestions as to how I can consistently get the appropriate rounding of Float64s that I’m looking for; a work around perhaps?

PS:

I’m using Julia version 1.7.1

2

Floating point numbers aren’t decimal.

julia> big(0.575)
0.5749999999999999555910790149937383830547332763671875
3 Likes
3

I think that’s because the decimal value 0.575 is not exactly represented in binary floating-point arithmetic. It’s actually the value:

julia> big(0.575)
0.5749999999999999555910790149937383830547332763671875

which is < 0.575:

julia> 0.575 < 575//1000
true

so it correctly rounds down to 0.57. See PSA: floating-point arithmetic

The surprising thing to me is that 0.585 rounds up, since we also have:

julia> big(0.585)
0.58499999999999996447286321199499070644378662109375

julia> 0.585 < 585//1000
true
3 Likes
4

I think what’s happening here is that .58 and .59 are both slightly less than their decimal values as well (but I’m not 100% sure here)

5

Note that if you want to work with decimal values exactly you can use decimal floating point:

julia> using DecFP

julia> round(d"0.575", digits=2, RoundNearestTiesUp)
0.58

julia> round(d"0.585", digits=2, RoundNearestTiesUp)
0.59

It’s rare to actually need this in a real application, but it does make human interpretation of decimal inputs and outputs easier.

2 Likes
6

Float64(0.585) is exactly halfway between Float64(0.58) and Float64(0.59). However, the same is true for Float64(0.575), which is exactly halfway between Float64(0.57) and Float64(0.58):

julia> big(0.59) - big(0.585)
0.00500000000000000444089209850062616169452667236328125

julia> big(0.585) - big(0.58)
0.00500000000000000444089209850062616169452667236328125

julia> big(0.58) - big(0.575)
0.00500000000000000444089209850062616169452667236328125

julia> big(0.575) - big(0.57)
0.00500000000000000444089209850062616169452667236328125

So, by this logic, shouldn’t 0.575 also round up to 0.58?

Probably there is an additional roundoff error in the round code…

7

Thank you for your recommendation, I’ll check out the DecFP.jl package.

8

Well, strangely, it might not be as rare in real application.

In my case, I was trying to verify a min-max normalization on a fairly large data-set of around 42000 entries. I used DataFrames’ transform function to apply the calculation and rounding of the result across each value in the DataFrame.

And since, I already have a normalized version of the data (I was just checking them), I found some inaccuracies in comparing my results and to those of the pre-normalized data.

With that said, unexpected rounding was on average extremely infrequent. Nevertheless, I could see decimal floating point having a place in data manipulations that require a great deal of accuracy.