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Date: Wed, 16 Apr 2014 04:27:09 +1000
Subject: Re: Simple question
From: Chris Angelico <rosuav@gmail.com>
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Xref: csiph.com comp.lang.python:70279

On Wed, Apr 16, 2014 at 4:18 AM, Phil Dobbin <phildobbin@gmail.com> wrote:
> '>>> 0.1 + 0.1 + 0.1 - 0.3
> 5.55111.....'
>
> What I'm wondering is why the first calculation that arrives at
> '5.55111...' is so far out?

There's something you're not seeing here. In full, the output I see is:

>>> 0.1 + 0.1 + 0.1 - 0.3
5.551115123125783e-17

See that bit at the end? "e-17" means "times ten to the -17th power" -
that is, move the decimal point 17 places to the left. So the actual
value is:

>>> "%50.50f"%(0.1 + 0.1 + 0.1 - 0.3)
'0.00000000000000005551115123125782702118158340454102'

It's not actually as far out as you think; in fact, that's incredibly
close to zero.

A full explanation of why floating point arithmetic does this can be
found in many places on the internet, but the main thing to note is
that it really isn't showing 5.5.

ChrisA