My if x ~= y statement is not working

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Nils Norlander
Nils Norlander 2017-3-2
评论： dpb 2017-3-2
Dear all,
I have a really simple annoying problem which I can't wrap my head around.
In my code:
for k = time
if E_load2(k) ~= Allowed_winter
disp(k)
overload = overload+1;
else
overload = overload+0;
end
end
I can see that the if loop returns the logical true even though I know that E_load2(k) == to Allowed_winter at that the certain k, and thus "overload" becomes larger than it should.
Allowed_winter = -0.5 if that helps.
Please help!

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采纳的回答

dpb
dpb 2017-3-2
== is precisely that to the LSB; undoubtedly the values in E_load2 in question are close but not identically equal. See the <FAQ> for the "why" in some detail.
For the k in question try
delt=E_load2-Allowed_winter
and observe the result; you'll find it will be something very small but not zero.
Use a tolerance on the comparison or later releases have the function ismembertol that makes writing the expression a little easier.
overload=sum(~ismembertol(E_load2,Allowed_winter));
should do the trick (note no loop needed here).
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Nils Norlander
Nils Norlander 2017-3-2
Sorry, I'm pretty new to this community as I learned Matlab for my masters thesis.
dpb
dpb 2017-3-2
No problem; just wondered about the inconsistency if ismembertol did work out (as I figured it would)...

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更多回答（2 个）

Adam
Adam 2017-3-2
编辑：Adam 2017-3-2
Never compare floating point numbers with == or ~=
Floats are not 100% accurate so you will run into these kinds of confusions. If you get a value that is 0.50000000000001 then it will return false even though it may have been the result of a floating point rounding error.
http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F
John D'Errico
John D'Errico 2017-3-2
Why are you bothering with this line?
overload = overload+0;
The last time I checked, adding zero to something does nothing but waste CPU cycles. I sincerely doubt that your goal is code that runs more slowly than necessary.
Anyway, it is high time to learn how to use MATLAB.
overload = sum(E_Load2 ~= Allowed_winter);
No loop required.
Or, if time is just the indices of a subset of the elements of E_load2, then use
overload = sum(E_Load2(time) ~= Allowed_winter);
Finally, your problem with the test. This is likely due to the fact that E_load2 or Allowed_winter is not exactly -0.5 in some cases.
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Nils Norlander
Nils Norlander 2017-3-2
Thank you for your input! I knew the overload = overload+0; was unnecessary, and was simply there in a desperate way to bugtest

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