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I have sets of data in column vector form (around 70000 elements).

To find the maximum element I used "max(x)" and the value comes to around 0.0305.

I want to know the indices of this element (number of the max value element). I am using find command as

find(x==0.0305)

But I get answer as

"Empty matrix: 0-by-1".

It works when I use

find(x<0.0305)

and displays all indices with value less than 0.0305. Can someone explain what I am doing wrong?

Doug Eastman
on 28 Jan 2011

The issue here is that the value is not exactly 0.0305, it gets truncated when displayed in the command window. You need to save the maximum value in a variable and then use that variable to find the indices:

myMax = max(x);

i = find(x==myMax);

jiro
on 28 Jan 2011

I recommend this answer. By saving the actual max value, you will get the full precision. Using the

[C,I] = max(...)

syntax for a vector will only give you the index of the first occurrence, not all occurrences.

Oleg Komarov
on 28 Jan 2011

Look at second output:

[C,I] = max(...)

Oleg

Michelle Hirsch
on 28 Jan 2011

Todd Flanagan
on 28 Jan 2011

You are bumping into issues with floating point accuracy. Loren has a nice blog post about accuracy with floating point numbers.

You can get the result directly from the MAX command using the form that returns both the numerical and index results:

[C, I] = max(x);

Note that with [C,I] = max(...) if there are several identical maximum values, the index of the first one found is returned.

If you want to use find or need to use find because you expect more than 1 occurrence of the maximum, you'll need to take some approach that either brackets the number you are looking for or programatically gives you the exact floating point representation you are looking for.

For example:

a = [1 2 3 .3501/10 4];

myMin = min(a)

find(a == myMin)

or for other sorts of problems you may want to bracket like:

find(a<.0351 & a>.0350)

Image Analyst
on 22 Oct 2013

Image Analyst
on 9 Dec 2013

omar kammouh
on 15 Apr 2018

Image Analyst
on 16 Apr 2018

The function you should use now to compare floating point numbers is called ismembertol().

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