if statement not working

Question

Vishan Gupta 2018-9-25

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https://ww2.mathworks.cn/matlabcentral/answers/420613-if-statement-not-working

编辑： Stephen23 2018-9-26

I have a bunch of values in vector a=[0 0.3 0.55] I want to compare each of the value in a to the value of b=[0.3], if the compared value from a is greater than b, put that value in a vector a_front, if less, put in a_behind, if it is the same value, still put it in a_behind.

The problem is that when I do this, I get only 0 in a_behind, not 0 AND 0.3 like I want, it instead puts 0.3 into a_front with 0.55. Please help.

for k=1:size(a,2)
  if a(k)>b
     a_front(k)=a(k)
  elseif a(k)<b
     a_behind(k)=a(k)
  else 
     a_behind(k)=a(k)
  end
end

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KSSV 2018-9-25

The above loop works fine...re check your result.

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Answer 1

KSSV 2018-9-25

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https://ww2.mathworks.cn/matlabcentral/answers/420613-if-statement-not-working#answer_338280

编辑：KSSV 2018-9-25

在 MATLAB Online 中打开

No if statement required.....read about logical indexing.

a=[0 0.3 0.55] ;
 b = 0.3 ;
   idx1 = a>b ;
   idx2 = a<=b ;
   a_front = a(idx1) 
   a_behind = a(idx2)

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Walter Roberson 2018-9-26

在 MATLAB Online 中打开

a(2)-0.3
ans =
      5.55111512312578e-17

Your calculated 0.3 is the next representable number after your text 0.3 . This is not at all uncommon with floating point.

http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F

Stephen23 2018-9-26

编辑：Stephen23 2018-9-26

"Ok so I've come across a weird problem..."

Not weird at all, you need to learn about floating point numbers, and why it is a bad idea to compare them for equality.

Note that 0.3 cannot be exactly represented using binary floating point numbers, in exactly the same way that you cannot write 1/3 with a finite number of decimal digits. 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 0.3. 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 0.3 is 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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