"find" function - precision problem?

Hi Folks,

I'm trying to match values in two double precision matrices but it's not finding the line numbers where the values obviously match. Is MATLAB hiding some decimal places that get factored into the find function?

Time(1) = 25.9947

T =

     25.9117
     25.9281
     25.9484
     25.9666
     25.9802
     25.9947
     26.0116
     26.0280
     26.0486
     26.0617
     26.0802

>> find(T==Time(1))

ans =

     Empty matrix: 0-by-1

I'm using the following code to build these two matrices in the first place (the underlying numerical data is in .txt files).

fid=fopen('explorationmaster.txt');
Str=textscan(fid,'%s','delimiter',',');
Time=Str(2:2:end);
Time=str2double(Time);

In the original text files, Time(1) looks like this:

25.994734107267824

So it appears MATLAB is chopping it down to 25.9947, which is good because it then matches an entry in T exactly... so why doesn't find pick up on the fact that these two values match?

Thanks! Jon

Answer 1

[Sean de Wolski]

no, it's still the full double precision, you're just not seeing it; show the full thing using format long.

format long
Time(1)

To reduce the tolerance use an absolute difference with a tolerance:

find(abs(T-Time(1))<10^-6) %right to 10^-6

[Jon Ericson]

thanks very much, the tolerance worked very well!

Answer 2

[Walter Roberson]

http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F