Problem with ismember function

Dear all, Hello,

I am writing a simple function of ismember to check if an element in one matrix matches the one in another matrix, however it returns 0, although I am sure the value matches. Assume I have a array of "omega". The third element is equal to 0.1047. And I have another matrix "wt", with first element, 0.1047 and when I write

z=ismember(omega,wt,'rows')

it returns a zero at z(3). Hence, I have:

omega(3)=0.1047;
wt(1)=0.1047;

I expect:

z(3)=1;

but it gives:

z(3)=0;

I must say that it returns 1 for omega(4) which is 0.1571 and equals the wt(2).

Thank you for your help.

Cheers,

[Jan]

Test this manually:

omega(3) - wt(1)

Is this really exactly 0? Note that the limited output of numbers in Matlab's command window let 0.1047 and 0.104699999999999 appear equally, althout the numbers are different.

[Stephen23]

"Problem with ismember function"

There is no problem with ismember. You need to learn about the practical limits of floating point numbers:

http://www.mathworks.com/help/matlab/matlab_prog/floating-point-numbers.html

http://www.mathworks.com/matlabcentral/answers/69-why-does-1-2-3-1-3-not-equal-zero

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

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

http://www.mathworks.com/company/newsletters/news_notes/pdf/Fall96Cleve.pdf

http://www.mathworks.com/matlabcentral/answers/41536-why-is-2-24-10-22-4-not-equal-to-0

https://www.mathworks.com/matlabcentral/newsreader/view_thread/66553

http://www.mathworks.com/company/newsletters/news_notes/pdf/Fall96Cleve.pdf

http://blogs.mathworks.com/loren/2006/08/23/a-glimpse-into-floating-point-accuracy/

http://www.mathworks.com/help/fixedpoint/ug/floating-point-numbers.html

And some external links on this topic:

http://www.lahey.com/float.htm

http://matlabgeeks.com/tips-tutorials/floating-point-comparisons-in-matlab/

Answer 1

[Andrei Bobrov]

Use ismembertol.