Using "find" for finding decimal values
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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
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채택된 답변
Matt Fig
2012년 12월 13일
편집: Matt Fig
2012년 12월 13일
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.
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Matt Fig
2012년 12월 14일
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.
추가 답변 (4개)
Azzi Abdelmalek
2012년 12월 13일
Try
idx = find(abs(X(:,1 )-17.9203)<eps)
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Jan
2012년 12월 13일
편집: Jan
2012년 12월 13일
Substracting two numbers in the magnitude of 20 can have a roundoff error of eps(20). Then this is better:
idx = find(abs(X(:,1 ) - 17.9203) <= eps(17.9203)) % EDITED: was "<"
Even 10*eps(17.9203) would be a reliable limit.
[EDITED] Thanks, Kye. I cannot test this currently. But I assume that even "<=" can fail, when the values are at the limits of 2^n. Does eps(16 + eps(16)) reply eps(32)? Then a factor of 2 would be obligatory.
Jan
2012년 12월 13일
편집: Jan
2012년 12월 13일
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.
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MiauMiau
2012년 12월 13일
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Azzi Abdelmalek
2012년 12월 13일
Yes, I said Matlab because we are working with Matlab. I think, representing real numbers for numerical programming is almost similar to what an ADC (analogic-digital converter) do. There is a quantification of a real number, which means there is an error of quantification which depends on the number of used bits and method.
Jan
2012년 12월 13일
@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.
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