0.48 and 0.72 not detected in the array

Hi,

I have attached varmat.mat. In this array, I can find any other elements in the array using find (SessionData.CoherenceTypes == 0.08) for example.

whenever I want to look for 0.48 or 0.72 using SessionData.CoherenceTypes == 0.48 or SessionData.CoherenceTypes == 0.72, the vector returned is empty although 0.48 and 0.72 are in the row vector. I tried in multiple computers and those two values are not detected.

Please suggest what was the problem. thanks.

[Stephen23]

"Please suggest what was the problem."

This is a completely expected result with binary floating point numbers:

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 highly recommended reading material 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

Answer 1

[the cyclist]

Due to floating point precision, the values are not exactly 0.48. For example,

load("varmat.mat","SessionData")
SessionData.CoherenceTypes(256) - 0.48

ans = 5.5511e-17

When doing comparison for floating point numbers, you need to check for equality within a tolerance.

See, for example, this documentation for details about floating point numbers and how to deal with them.