A loop that is I told to increase by .1 is returning a value of 0.30000000000000004

Question

Basically I thought I created a loop to just increase .1 every iteration. What I have got is these numbers below like 0.30000000000000004,0.7999999999999999, 3.0000000000000013. Here is my code and the results. Why is it not .1, .2. .3, etc. and/or why is it 0.30000000000000004, 0.4, 0.5, 0.6, 0.7, 0.7999999999999999, etc. Basically why are their unexpected, for me, decimals.

>>> tph_bin = []
>>> bin_num = 0
>>> while bin_num <= 3.5:
    tph_bin.append(bin_num)
    bin_num = bin_num + .1


>>> tph_bin
[0, 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, 3.1000000000000014, 3.2000000000000015, 3.3000000000000016, 3.4000000000000017]

Bonus Question: Is there a better way to create a list of numbers increasing by .1?

If you want precision, use the decimal lib, https://docs.python.org/3/library/decimal.html — Padraic Cunningham, Jul 14 '16 at 20:58
Sorry. I thought I was clear but re reading it I wasn't. I was just wondering why it would round up to 0.30000000000000004. — Don Quixote, Jul 14 '16 at 21:00
@DonQuixote, rounding up to what? Did you read the duplicate? — Padraic Cunningham, Jul 14 '16 at 21:02
I didn't understand that .1 in binary was different than .1 in decimal. PressingOnAlways answered my question. And thanks for linking the duplicate. — Don Quixote, Jul 14 '16 at 21:13

PressingOnAlways · Accepted Answer · 2016-07-15T01:46:45.990

2

This is a floating point precision limitation. Please refer to:

https://docs.python.org/2/tutorial/floatingpoint.html

0.1 is actually stored as the binary fraction:

0.00011001100110011001100110011001100110011001100110011010

As you can see, that can lead to binary rounding errors as numbers are added.

Try using Decimal as an alternative if all you care is 1 decimal place precision:

from decimal import *
value = Decimal("0.1")+Decimal("0.1")+Decimal("0.1")
print value
# 0.3
if Decimal('0.3') == value
  print 'This works!'

edited Jul 15 '16 at 01:46

answered Jul 14 '16 at 20:56

PressingOnAlways

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Thanks! Here's is a video that explains why this is in case anyone is curious in the future. https://www.youtube.com/watch?v=KMcUWgIAu1c – Don Quixote Jul 14 '16 at 21:08
@DonQuixote - no problem. If this answer works for you, please accept it. Thanks. – PressingOnAlways Jul 15 '16 at 01:47

score -1 · Answer 2 · answered Jul 14 '16 at 20:58

-1

Answer to Bonus Question: Is there a better way to create a list of numbers increasing by .1?

with numpy

numpy.arange(0,3.5,0.1)

answered Jul 14 '16 at 20:58

karakfa

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I don't understand the downvotes here. – C8H10N4O2 Jan 19 '17 at 22:14

score -1 · Answer 3 · answered Jul 14 '16 at 21:02

-1

Its basically the disadvantages of using float numbers. The number 0.2 plus 0.1 just got overflowed and the result wont fit inside a memory block reserved to float types so it cuts of the part overflowed and rounds it around that value. If you want to work with numbers that way, you should operate with integers and then divide the result by 10 or just round up the values, but never try to compare floats by value because the outcome could be surprising.

answered Jul 14 '16 at 21:02

Tacsiazuma

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Thanks. I would have never thought of that. I changed by code to the below and it worked! I really appreciate it. for num in range(35): bin_num = (num / 10) tph_bin.append(bin_num) num =+ 1. I can't get the formatting to work but you're a smart guy. Appreciate it. – Don Quixote Jul 14 '16 at 21:23

A loop that is I told to increase by .1 is returning a value of 0.30000000000000004

3 Answers3