Unexpected for loop behaviour

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Utkarsh Nipane 2019-7-9

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https://ww2.mathworks.cn/matlabcentral/answers/470816-unexpected-for-loop-behaviour

评论： Walter Roberson 2019-7-9

This for loop is supposed to run for 2 iterations i.e. for i=0 and i=-0.1 as variable outputOverVoltageChange_65 = -0.1. Why is it running for only i=0?

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Renato SL 2019-7-9

I assume you post this question because in the output you get, there is only 1 set of entry but you expect 2.

So, can you post a screenshot/photo of the output and highlight the fact that there is only 1 entry there?

Thank you.

Walter Roberson 2019-7-9

Your variable is not exactly what you think it is, or 0:-0.1:something does not generate exactly the values you expect. Remember that 0.1 cannot be exactly represented in binary floating point.

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Answer 1

Stephen23 2019-7-9

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编辑：Stephen23 2019-7-9

在 MATLAB Online 中打开

"Unexpected for loop behaviour"

The value -0.1 cannot be represented exactly using floating point numbers, and you could quite easily be using different values of "-0.1", even though you think that they are the same value:

>> V = -0.1
V =
   -0.1000
>> V = V+eps
V =
   -0.1000
>> fprintf('%.30f\n',V)
-0.099999999999999784000000000000
>> 0:-0.1:V
ans =
     0

What you see printed in the command window is the closest representation to 5 or 16 significant digits, depending on your current format setting. To see the "real" value download James Tursa's FEX submission:

https://www.mathworks.com/matlabcentral/fileexchange/22239-num2strexact-exact-version-of-num2str

Use James Tursa's num2strexact and you will see that none of those values really have the exact value -0.1. All you are looking at is a representation of those floating point numbers displayed in the command window, to the precision defined by your format setting. Just because you see -0.1 displayed tells you nothing about the "real" floating point number's value.

Note that you can change how the value is displayed by changing the format.

You need to learn about the limits of floating point numbers. Start by reading these:

https://www.mathworks.com/matlabcentral/answers/57444-faq-why-is-0-3-0-2-0-1-not-equal-to-zero

https://www.mathworks.com/matlabcentral/answers/316889-why-two-equal-numbers-are-not-equal

http://www.mathworks.com/matlabcentral/answers/102419-how-do-i-determine-if-the-error-in-my-answer-is-the-result-of-round-off-error-or-a-bug

http://www.mathworks.com/matlabcentral/answers/140656-about-numerical-difficulty-of-equality-comparison-operation-between-two-double-real-numbers-in-matlab

This is worth reading as well:

https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html

http://faculty.tarleton.edu/agapie/documents/cs_343_arch/papers/1991_Goldberg_FloatingPoint.pdf

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Utkarsh Nipane 2019-7-9

Thanks a lot for your prompt response. I checked the variable value in the workspace and it was actually greater than -0.1 (to be exact -0.099999999999994), that's why the loop was not working as expected. The main problem happening here is that the exact value of 0.1 is sometimes greater and sometimes smaller than 0.1, thus some loops implemented with same logic were working and some were not. At last thank you for providing me all these useful links, it will surely help me in rectifying my errors.

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