A simple "bug" in finding an element in an array using find()?

Hi, I am not sure if it is a bug in matlab, but when I just simply try to find an element in an array:

list=-1:0.1:1;
find(list=0.1)

I get an strange result which is Empty matrix: 1-by-0. Instead, if I try find(list=0.6), I get the correct result 17.

Anyone have any idea?

[Stephen23]

"I am not sure if it is a bug in matlab..."

So even after millions of person-hours testing and using MATLAB, you thought that you are the first to notice this behavior, so you did not even bother to do a simple internet search, or search this forum....

"I get an strange result"

Nope, that result looks quite normal to me.

"Anyone have any idea?"

Learn about floating-point numbers.

Answer 1

[Star Strider]

Welcome to floating-point approximation error, and the way the colon operator works.

Try this:

list=-1:0.1:1;
find(list<=0.1,1,'last')

References: Why is 0.3 - 0.2 - 0.1 (or similar) not equal to zero? (link) and colon (link).

[xk l]

Thank you for the quick answer! It solved my problem.

[Star Strider]

As always, my pleasure!

Answer 2

[James Tursa]

Not a bug. Just the way floating point arithmetic works. E.g., 0.1 cannot be represented exactly in IEEE double precision, so the way the colon operator builds that list doesn't yield exactly the same number as just typing in 0.1 directly.

https://www.mathworks.com/matlabcentral/answers/11089-0-1-0-2-0-3

[xk l]

Thank you!