if statement not working

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

[KSSV]

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

Answer 1

[KSSV]

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)

[Vishan Gupta]

Thank you very much

[KSSV]

Thanks is accepting and voting the answer. :)

[Vishan Gupta]

Ok so I've come across a weird problem, the code you have given me works perfectly. However, I have an Excel file which I have some data in, I load that data, do some calculations to get the value of vector a and the code stops working. I.e it doesn't put 0.3 in a_behind but puts it into a_front. pls help

 h=xlsread('UDL.xlsx')
 b=[0.3]
 for k=1:size(h,1)
      a(k)=(h(k,2)/2)+h(k,4)
 end 
 idx1 = a>b ;
 idx2 = a<=b ;
 a_front = a(idx1) 
 a_behind = a(idx2) 
end

[Walter Roberson]

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]

"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