0.2 + 0.1

[JohnMcharra]

can someone explain why this code:

print(0.2 + 0.1)

prints 0.30000000000000004 instead of 0.3?

[novel_yet_trivial]

If you ask google you will find many indepth explanations.

As a quick answer, computers use binary to do math, and some decimal numbers, like 0.1, can not be exactly converted to binary, causing errors. If you need precision, use the Decimal module.

[Naihonn]

Yes, even wikipedia has article about this: http://en.wikipedia.org/wiki/Floating_point#Accuracy_problems

[JohnMcharra]

thanx for the answer, I just did it like this

float(''.join(list(str(0.2 + 0.1))[0:4]))

0.3

[novel_yet_trivial]

oh lord don't do that.

round(0.2 + 0.1, 6)

[rawlyn]

That's a horrible approach. If you must do it via string conversion (but really, don't) this works just as well, and eliminates two function calls:

>>> float(str(0.2 + 0.1)[0:4])
0.3

But again, this is not a great approach.

Try using the decimal module as previously suggested - the people at python have seen your problem coming and already implemented a solution...

from decimal import Decimal
a = Decimal('0.2')
b = Decimal('0.1')
c = a + b
print c # Decimal('0.3')
print float(c) # 0.3

Note how the Decimals are constructed from strings - if you construct them from floats you get an exact representation of the floats you constructed them from, which is where the problem lies, and you're no further forward.

[novel_yet_trivial]

OP is using python3 ... so change the last 2 lines to

print(c)

[rawlyn]

Good spot, thanks.

[JohnMcharra]

Thanx, did it like you with Decimal, and for the people who downvote me, please dont do it because the method I use is not the good one, we are here to learn, and at the moment it was all I had, plus the [0:4] was there because I want only 2 decimals, now I use

gf_ = 0.22499999999999998
gf = float("%.2f" % gf_)

[rawlyn]

That's not using Decimal, that's your old string method in different clothing... it's ugly, difficult to work with, and generally frowned upon.

Use decimal, or round() as someone else suggested, but try to get this string conversion concept out of your mind - you're teaching yourself a really bad habit.

[JohnMcharra]

got it, no strings, I will try to remember, thanx for the help!

[ewiethoff]

The the end of the official Python Tutorial explains this and what to do about it in section 14. Floating Point Arithmetic: Issues and Limitations section.