For Loop Not Executing
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Matthew Worker
il 17 Set 2016
Commentato: Rena Berman
il 8 Feb 2017
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 Commenti
Star Strider
il 29 Gen 2017
You can go as far as you did, but Rena Berman will reconstitute it in a few days.
Does Sisyphus begin to sound relevant?
Risposta accettata
Mischa Kim
il 17 Set 2016
Modificato: Mischa Kim
il 17 Set 2016
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 Commenti
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David Goodmanson
il 17 Set 2016
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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