For Loop Not Executing
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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
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
2017-2-8
(Answers dev) Restored edit
采纳的回答
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.
4 个评论
Star Strider
2016-9-17
Almost.
In the top two, the end equals the beginning, so the loops in them execute once. The loop is satisfied after the first iteration.
In the third one, the difference between the end and the beginning is negative, so the loop is satisfied at the first iteration and never executes:
h=(2/1000);
d = [ 9*h - (8*h+h); 12*h - (11*h+h); 10*h - (9*h+h)]
d =
0
0
-3.4694e-18
Because of the negative direction:
for i=9*h+h:-h:10*h
i
end
i =
0.02
executes once with a step of ‘-h’.
Mischa Kim
2016-9-17
Absolutely. Thanks.
Star Strider
2016-9-17
As always, my pleasure.
N/A
2016-9-17
Of course! Thanks so much I should have realized this.
更多回答（1 个）
David Goodmanson
2016-9-17
0 个投票
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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