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I am using IronPython 2.7 integrated into Rhinoceros 5 application. I am having a strange int() function result:

import math

angle = 45.0
tangent = math.tan(math.radians(angle))
n = 12*tangent
print "angle: ", angle
print "tangent: ", tangent
print "n: ", n
print "int(n): ", int(n)

Results in:

angle:  45.0
tangent:  1.0
n:  12.0
int(n):  11

So as written above, the int() function returns 11 not 12. I know there are some floating point number issues, but this should not be related with integers? Does anyone know why is this happening?

Thank you for the reply.

EDIT: I added the code, on how I got the "12.0"

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  • 1
    Works fine for me on 2.7.3 Commented May 19, 2015 at 11:15
  • 1
    Works fine for me using Python 2.7 Commented May 19, 2015 at 11:16
  • 1
    Works fine in Python 2.7.8 Commented May 19, 2015 at 11:19
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    If you type those two lines into a Python interactive shell, does it print 11? Or is your figure of 12.0 actually the result of some other floating point operation that should yield 12.0? Can you edit your code into the above question, please. Commented May 19, 2015 at 11:44
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    @marco, I reopened the question, you need to add the code that you are using that causes the issue so it can be replicated. Commented May 19, 2015 at 11:45

2 Answers 2

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4

You got 11.9999999999999982 !

If you add the following line to your code, you'll get an amazing result

print "%.16f" % n

Output

11.9999999999999982

How to solve

You may want to try this:

from decimal import Decimal

k=Decimal(12.0)
print int(k) #prints 12

This will convert 12.0 (a float) to a Decimal value. This approach can solve some issues related to floats.

Why it happened

You could read this and see that, for example, 2.675 can be represented as 2.67499999999999982236431605997495353221893310546875 because of some limitations.

In your case you've got a number that's close to 12. That's why int rounded it to 11. For example:

>>> int(11.9999999999)==11
True
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12 Comments

I didn't downvote, but you didn't answer any of the question
under no circumstances should int(12.0) become 11
Your edits still don't answer the question. The OP is not experiencing floating point errors, the issue is int(12.0) becomes 11. Also what OS would represent 12 as 11.999999?
@ForceBru: No, you've shown that int(11.99999999) will give 11 not that 12.0 could be represented as 11.99999999
then it isn't an answer but a speculation. What part of an OS' representation of a number is used by python? -> This is the part where the bug is. And relates in this specific case most probably to the way python has been linked to a bugged library while being embedded into the application
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In your case, the error is occurring in the tan operation, and your confusion is arising because of the way that print tries to make the output more readable:

>>> math.tan(math.radians(angle))
0.9999999999999999
>>> print(math.tan(math.radians(angle)))
1.0

The easiest solution is to use round instead of int:

import math

angle = 45.0
tangent = math.tan(math.radians(angle))
n = 12*tangent
print "angle: ", angle
print "tangent: ", tangent
print "n: ", n
print "round(n,0): ", round(n,0)

Yields:

angle:  45.0
tangent:  1.0
n:  12.0
round(n,0):  12.0

As ForceBru mentioned, there is a Decimal module to help with floating-point operations, but I've found to be quite cumbersome, and rounding at the end of the calculation is normally sufficient.

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