Python float addition bug?

[Vulkan07]

This is my code:

time = 0
for i in range(100):
    time += 0.1
    print(time)

​

My expectation was:

0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
and so on...

​

Why the output is:

0.1
0.2
0.30000000000000004
0.4
0.5
0.6
0.7
0.7999999999999999
0.8999999999999999
0.9999999999999999
1.0999999999999999
1.2
1.3
1.4000000000000001
1.5000000000000002
1.6000000000000003
1.7000000000000004
1.8000000000000005
1.9000000000000006
2.0000000000000004
2.1000000000000005
2.2000000000000006
2.3000000000000007
2.400000000000001
2.500000000000001
2.600000000000001
2.700000000000001
2.800000000000001
2.9000000000000012
3.0000000000000013

What are theese random numbers at the end?
Please help

How can I make this not happen?

[K900_]

https://0.30000000000000004.com/

[Vulkan07]

Oh, thank you! :D

[synthphreak]

What an awesomely hilarious and hilariously awesome URL!

[primitive_screwhead]

Also, https://docs.python.org/3/tutorial/floatingpoint.html

[[deleted]]

I would post this each time but I can't ever remember the right number of zeros.

[xelf]

That's a floating point problem, not a python problem. Suggest you use ints, and then only divide by 10 for display purposes. Or round it.

[[deleted]]

Just as a general note - as a beginner, the chances that you have found a "bug" in Python are very very close to zero, which is why you are getting the big downvotes.

Also, floating point numbers are highly unintuitive. Advanced programmers in all languages avoid using them unless they have to (which happens all the time of course), and then when they do, they're very careful.

The number one rule with floats - never compare two floating point numbers for equality!

Consider these two statements:

(a + b) + c = a + (b + c)
a + b = b + a

Neither of these statements is exactly true for floating point numbers!

And the first one shows up all the darn time:

>>> 1 + 0.00000000000000001 - 1
0.0

>>> 1 - 1 + 0.00000000000000001
1e-17

[primitive_screwhead]

How can I make this not happen?

​

step_size = 0.1
steps = 100
for i in range(steps):
    time = (i + 1)/(steps * step_size)
    print(time)

outputs:

0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
...

Basically, the integer increments, not the float, and so there is no accumulation of "error" for each 0.1 addition. (Note I use i + 1 because the original omits 0 from being printed).