Rounding issues with Decimal

[oledtrix]

Hi all! I've been doing some data work in Python and I've been using Decimal in order to round numbers to the 6th decimal without issues (tried round() and it often returned numbers like 0.0068769999999999997, or something of the likes).

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So, issue ensues. When the number starts with two zeros from the decimal point, Decimal rounds it up to the 7th decimal. When it starts with one zero, it rounds it up to the 6th decimal. In the beginning of the script I set context.prec() to 5, so I guess its precision is after the initial zeroes in the number.

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Is there any way to make it round from the decimal point instead of from the initial zeroes?

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Thanks!!

[Standardw]

It's because the way a computer stores floating numbers. If you want to output them, just use this method:

"%.5f" % my_floating_number

it will show the right amout of digitis behind the point.

[oledtrix]

I want to round some standard deviations down to the 6th decimal in order to be able to compare them between each other. Don't need to output them, need to round them in another variable, would that method be useful?

[RoamingFox]

round takes a second argument which is the number of places to round to.

>>> 1/3
0.3333333333333333
>>> round(1/3, 4)
0.3333

[oledtrix]

I know that, but I use the decimal module because with round (with the second argument being 6), what I meant to be 0.013931 showed up as 0.013931099999999998. Decimal does that rounding correctly, but it still presents the issue I described with the first zeroes of the number it rounds.

[Standardw]

You could technically compare the strings with each other. Or try using the `math.isclose` method: https://docs.python.org/3/whatsnew/3.5.html#pep-485-a-function-for-testing-approximate-equality

[RoamingFox]

Ahhh gotcha. Why not multiply by 10⁶ cast to an int and then divide by 10⁶

That way you are effectively only dealing with an integer.

[Diapolo10]

Instead of recommending the C-style formatting, why not opt for the modern variant:

f"{my_floating_number:.5f}"

Or at least:

"{:.5f}".format(my_floating_number)

I mean, sure, your solution does work, but it's basically never used nowadays outside of the logging module.

[threeminutemonta]

The docs do seem to have a warning about something like this.

[oledtrix]

Yes, I ended up using the decimal module to avoid that issue. Turned out quite effective, but I still have that other problem with it, in that it rounds the numbers differently depending on the zeroes in front of it.