How can the "find" function fail at finding elements in an array?

Question

Ion CANDEL 2016-10-29

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https://ww2.mathworks.cn/matlabcentral/answers/309787-how-can-the-find-function-fail-at-finding-elements-in-an-array

编辑： Star Strider 2016-10-29

采纳的回答： Star Strider

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Dear all,

I have used many times the "find" function with very good results... until now. Here is my VERY SIMPLE code:

a = [0.145:0.0005:0.175];
index = find(a == 0.16)
 index =
   Empty matrix: 1-by-0
   index2 = find(a == 0.15)
    index2 =
    11
   So...  both numbers (0.15 and 0.16) EXIST in the array named "a", however, ONLY 0.15 IS FOUND... 
I have checked this in R2015a and R2013b with the same results... Does anybody haves an idea of what is going on???

Best regards, Ion

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Answer 1

Star Strider 2016-10-29

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/309787-how-can-the-find-function-fail-at-finding-elements-in-an-array#answer_241270

编辑：Star Strider 2016-10-29

在 MATLAB Online 中打开

You have discovered ‘floating point approximation error’.

See: Why is 0.3 - 0.2 - 0.1 (or similar) not equal to zero? for an explanation.

EDIT —

To illustrate:

a = [0.145:0.0005:0.175];
index = find(a <= 0.16, 1, 'last')
result = a(index)
err = result - 0.16                                 % Floating Point Error
result =
         0.16
err =
  -2.7756e-17