Why can't I find the number 1.7?

Question

Chris 2013-10-14

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评论： John D'Errico 2013-10-14

Why does this return an empty answer for 1.7, but not 1.6 or 1.8? I am using an older version of Matlab (2006a), I have not tried this on a newer version yet.

test=0:0.1:5000;

>> find(test==1.7);

ans = Empty matrix: 1-by-0

>> find(test==1.8);

ans = 19

>> find(test==1.6)

ans = 17

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Jan 2013-10-14

This is not an error and it does not concern Matlab only, but this happens for other languages also in exactly the same way.

John D'Errico 2013-10-14

This is a problem only to the extent that you don't understand floating point numbers and how they are stored. Time to read the FAQ.

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Answer 1

Carla 2013-10-14

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https://ww2.mathworks.cn/matlabcentral/answers/90192-why-can-t-i-find-the-number-1-7#answer_99644

It's because your computer can't store decimal numbers in an exact form - there is always small but finite roundoff error in floating-point arithmetic. Have a look at this FAQ: http://matlab.wikia.com/wiki/FAQ

In particular, look at the section entitled "Why is 0.3 - 0.2 - 0.1 (or similar) not equal to zero?"