Why can't I find the number 1.7?

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

[Chris]

Update: I tried this in Matlab R2012b...same problem.

[Kate]

I'm running a newer version of matlab and get the same error, so it's not your version. This makes so sense to me, sorry! 1.7 exists within test.

[Jan]

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]

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.

Answer 1

[Carla]

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?"

Answer 2

[Yatin]

Hi,

It has to do with the way floating point numbers are represented in MATLAB. May be this article helps:

http://blogs.mathworks.com/loren/2006/08/23/a-glimpse-into-floating-point-accuracy/