MATLAB: Basic indexing question Find ismember

Question

Hi World,

I would like to create an index of a matrix based on a vector to resize this array by extracting the indexed rows:

 X=(0.5:0.5:17)';
 F=(1/52:1/52:30+1/52)';
 Q=find(ismember(F,X));

But there are missing values from Q while I can see them in the workspace.

Can you help?

Answer 1

You need to learn about the practical limits of floating point numbers.

Basically the numbers that you see are only decimal approximations of the real binary values stored in the computer's memory. With calculations involving decimal fractions these values can differ by a small amount even though they look they same when you look at them in the command window or variable viewer. However they will fail an equality test, and so are not considered to be equal, thus ismember does not match them.

You could use ismembertol instead.

Information on 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/