Why doesn't .3 - .2 - .1 = 0

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Lucas
Lucas on 9 Nov 2012
I was bored yesterday at work and I was playing around with some decimal numbers when I stumbled across something I thought was pretty interesting.
.3 - .2 - .1 != 0
but
.2 - .1 - .1 = 0
then I began to run some other tests and I found out that only .3 and .8 are the only numbers that you use that'll not get you a 0 answer. If you take:
.4 - .2 - .1 - .1 = 0
and
.9 -.2 -.1 -.1 -.1 -.2 -.1 -.1 = 0
If you change any of the .2's to two - .1's you don't get 0. So what’s so special about those 2 decimals that make them the only ones that don't work with basic subtraction?

Accepted Answer

Matt J
Matt J on 9 Nov 2012
Edited: Matt J on 9 Nov 2012
What makes them special? Nothing. There are lots more numbers like that
>> isequal(0, .7 - .5 - .2)
ans =
0
It also depends on the operation you do. Sometimes 0.3 will work fine, e.g.,
>> isequal(0, .6 - .3 -.3)
ans =
1
  2 Comments
José-Luis
José-Luis on 9 Nov 2012
Edited: José-Luis on 9 Nov 2012
This is not Matlab specific. It has to do with the fact that computers use binary numbers and humans tend to feel more comfortable with decimals. Things gets lost in translation and these are the side effects.

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More Answers (1)

Thomas
Thomas on 9 Nov 2012
Edited: Thomas on 9 Nov 2012
This should answer your question. Look at the wiki under the section Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero? (might have to scroll on the page to locate...)
  2 Comments
Oleg Komarov
Oleg Komarov on 9 Nov 2012
@Lucas, if you already know that you wouldn't be asking the question in the first place. I recommend to read the FAQ more carefully.

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