MATLAB Answers

Using "find" for finding decimal values

114 views (last 30 days)
MiauMiau
MiauMiau on 13 Dec 2012
Hi
I use the matlab command importdata:
X = importdata('filename.csv');
to read in a csv file with three columns.
Now, for finding a specific values in the matrix X, I simple use the find command as follows:
idx = find(X(:,1 ) == 17)
But, the same seems not to be possible for decimal numbers. This for instance would not work:
idx = find(X(:,1 ) == 17.9203)
even though 17.9203 is to be found in the original csv file. What is the problem and what can I do?
Thanks

Accepted Answer

Matt Fig
Matt Fig on 13 Dec 2012
Edited: Matt Fig on 13 Dec 2012
MiauMiau, the numbers you are looking for are obviously different from the short format you see. Do this:
[~,idx] = min(abs(X(:,1) - 6.0018));
Y = X(idx,1);
[~,idx] = min(abs(X(:,1) - 17.9203));
fprintf('%15.15f %15.15f\n',Y,X(idx,1))
And show us the ouput in a comment on this answer, don't add another answer!
I have a feeling you will need to set your tolerance much higher, like 10^-4.
  4 Comments
Matt Fig
Matt Fig on 14 Dec 2012
I found out by looking at how close your guess value was to the actual value. You may have to do some tweaking but you get the idea now.

Sign in to comment.

More Answers (4)

Azzi Abdelmalek
Azzi Abdelmalek on 13 Dec 2012
Try
idx = find(abs(X(:,1 )-17.9203)<eps)
  2 Comments

Sign in to comment.


Jan
Jan on 13 Dec 2012
Edited: Jan on 13 Dec 2012
There is no exact representation of decimal floating point numbers in binary format for all values. You find a lot of corresponding discussion in this forum:
0.1 + 0.2 - 0.3 == 0
>> false
This is no bug, but the expected behaviour, when floating point numbers are represented with a limited precision.
A consequence is, that you cannot compare numbers like 17.9203 and 17.92029999999999999999 sufficiently and even the display in the command window can be rather confusing.

MiauMiau
MiauMiau on 13 Dec 2012
I was not aware of that, that is pretty horrible! Anyway, both of your suggestions did not work, so also for:
idx = find(abs(X(:,1 ) - 17.9203) < eps(17.9203))
I did get an empty matrix returned - although the value is to be found in X. Also, when I changed the number a bit, I then got too many answers returned. But what strikes me most, is that in the first case, I just did not get any answer at all!..?!
  7 Comments
Jan
Jan on 13 Dec 2012
@Azzi: I've stressed this detail, because the OP seems to be confused about this topic already. I did not assume, that you struggle with floating point arithmetics.

Sign in to comment.


MiauMiau
MiauMiau on 13 Dec 2012
I am really sorry for my use of the word "horrible".
If you meant the following command though ( I have used another nummerical value):
idx = find(abs(X(:,1 ) - 6.0018) <= eps(6.0018))
I - again - got an empty 0,1-matrix.
Also, I was not criticising the obviously necessary restriction on measurement and computation precision but I was amazed that there is not a simpler command to handle that. Anyway. What shall I try next?
  1 Comment
Jan
Jan on 13 Dec 2012
Why do you assume that 6.0018 is an element of X?

Sign in to comment.

Tags

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by