Wrong code or strange math behavior? (python 3)

Question

I'm calculating with variables by multiplicating etc and I noticed this strange behavior.

If I use these calculations:

CD = 6
CDR = 0.4

CD = float(CD) - (float(CDR) * float(CD))

Theoretically that would be 6 - (6 * 0.4) = 6 - 2.4 = 3.6 if I then print(CD) it prints

3.5999999999999996

Is there a reason for this which I can avoid? Is there a way, like math.ceil to round up the number but to a certain decimal, for example to x.xxxxx (5th deciamal)?

(Let me know if I did anything wrong in this post, I find answers on this site since a while but have never posted before so I maybe did something wrong, apologies in advance.)

Answer 1

you can try the decimal module, but under the hood your answer is still "correct". It's just how floating point numbers convert to decimal representations.

Answer 2

You're running in to floating point arithmetic problems. Trying using decimal.Decimal instead of float .

Answer 3

If it's for display only (or piece of mind) you can do

x = math.ceil(x*100000.0) / 100000.0

However there's no guarantee that the will be a number that can be represented exactly in memory either (ending up with the same 3.599999999..)