Vector comparison by value produces strange results

Hello,

I have two vectors and I think they are the same, but MATLAB says all the elements are different.

>> whos x
  Name         Size            Bytes  Class     Attributes

    x         1138x1              9104  double              

>> whos x2
  Name         Size            Bytes  Class     Attributes

    x2        1138x1              9104  double              

>> x(1) == x2(1)

ans =

       0

>> x(1)

ans =

      0.6557

>> x2(1)

ans =

      0.6557

Why is this? Thank you.

[Stephen23]

Never use exact equivalence to test if decimal values are the same: always compare the difference against a tolerance:

abs(A-B)<tol

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/

[Bence Meszaros]

Hi Stephen, Really great comment and resources, thank you very much.

Answer 1

[jonas]

try to type

format long

and compare again

[Bence Meszaros]

Thanks. Is there a way to compare only the first 4 digit for example?

[Stephen23]

"Is there a way to compare only the first 4 digit for example?"

This topic has been discussed many times on this forum: you should compare absolute difference against a tolerance, as I showed in my comment to your original question. Note that using round is popular with beginners, but it introduces artifacts into your data that do not actually exist. Read this to know more:

https://www.mathworks.com/matlabcentral/answers/321709-strange-behaviour-of-matlab-find-command#comment_444258