For Loop Not Executing

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Matthew Worker
Matthew Worker 2016-9-17
评论： Rena Berman 2017-2-8
Hi all!
Consider the following example:
h=(2/1000);
for i=8*h+h:h:9*h
i
end
i =
0.0180
And similarly,
>> for i=11*h+h:h:12*h
i
end
i =
0.0240
But when I do this:
>> for i=9*h+h:h:10*h
i
end
it doesn't enter the for loop! What is going on? Thanks for the help!
  2 个评论
Star Strider
Star Strider 2017-1-29
You can go as far as you did, but Rena Berman will reconstitute it in a few days.
Does Sisyphus begin to sound relevant?
Rena Berman
Rena Berman 2017-2-8
(Answers dev) Restored edit

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采纳的回答

Mischa Kim
Mischa Kim 2016-9-17
编辑：Mischa Kim 2016-9-17
Gopal, this is because
9*h+h - 10*h
ans =
3.469446951953614e-18
and
11*h+h - 12*h
ans =
0
Just because the terms above are all symbolically equal to zero, does not necessarily mean they are also numerically equal to zero. The first expression is not which is why the loop does not get executed.
Star Strider provides more detail below.
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Star Strider
Star Strider 2016-9-17
As always, my pleasure.
N/A
N/A 2016-9-17
Of course! Thanks so much I should have realized this.

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更多回答（1 个）

David Goodmanson
David Goodmanson 2016-9-17
For the floating point number h you are basically asking if n*h + h == (n+1)*h for various values of n. Well, sometimes the two sides are equal (to machine precision), sometimes not. For n=9 the left hand side is bigger than the right hand side by 3e-18, so the for loop doesn't do anything.
It's never a good idea to do exact equality tests with floating point numbers.

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