How do I calculate a mean value of a vector and ignore from the "0" when appears inside the vectors?

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Hello.
I need to calculate a mean value of a velocity vector. The vector contains a damaged cells which appears as "0" inside the vector. How do I calculate a mean value of a vector and ignore from the "0" when appears inside the vectors?

Accepted Answer

Star Strider
Star Strider on 24 Sep 2016
If you have a recent release (I don’t remember when the 'omitnan' option appeared) or the Statistics and Machine Learning Toolbox nanmean function you can change the zeros to NaN and use those functions:
V = randi([0 9], 5)
V(V==0)= NaN
Out_1 = nanmean(V)
Out_2 = mean(V, 'omitnan')
V =
0 4 6 0 4
1 5 1 9 3
6 2 3 7 5
7 7 6 4 5
6 1 7 4 8
V =
NaN 4 6 NaN 4
1 5 1 9 3
6 2 3 7 5
7 7 6 4 5
6 1 7 4 8
Out_1 =
5 3.8 4.6 6 5
Out_2 =
5 3.8 4.6 6 5
The problem with the logical indexing approach is that it defaults to ‘linear indexing’ because the rows and columns are no longer equal. That produces a vector argument to the mean function, and the mean of the vector.
  4 Comments

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More Answers (4)

George
George on 24 Sep 2016
Edited: George on 27 Sep 2016
Replacing the 0s with NaN and using the 'omitnan' flag should do what you want.
>> A
A =
NaN NaN 11.6780 NaN NaN NaN NaN -23.3560 -35.0340 -42.8200
NaN NaN NaN NaN NaN NaN NaN -7.7850 -7.7850 -15.5710
NaN NaN NaN NaN NaN NaN 3.8930 NaN -3.8930 -15.5710
NaN NaN NaN NaN NaN NaN -3.8930 NaN NaN 15.5710
NaN NaN NaN NaN NaN NaN NaN -3.8930 -11.6780 -15.5710
NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN
NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN
NaN NaN NaN NaN NaN NaN NaN -19.4630 -35.0340 -54.4980
NaN NaN NaN NaN NaN NaN NaN NaN -3.8930 -15.5710
NaN NaN NaN NaN NaN NaN NaN 3.8930 7.7850 11.6780
NaN NaN NaN NaN NaN NaN NaN -3.8930 -11.6780 -19.4630
NaN NaN NaN NaN NaN NaN NaN NaN -3.8930 NaN
NaN NaN NaN NaN NaN NaN NaN -3.8930 -3.8930 -11.6780
NaN NaN NaN NaN NaN NaN NaN -3.8930 -3.8930 -7.7850
NaN NaN NaN NaN NaN NaN NaN NaN -7.7850 -15.5710
NaN NaN NaN NaN NaN NaN NaN NaN NaN 7.7850
Trial>> meanCols = mean(A, 'omitnan')
meanCols =
NaN NaN 11.6780 NaN NaN NaN 0 -7.7854 -10.0562 -13.7742
>>

Image Analyst
Image Analyst on 24 Sep 2016
Edited: Image Analyst on 24 Sep 2016
Try this
meanVelocity = mean(allVelocities(allVelocities ~= 0))
  3 Comments
Image Analyst
Image Analyst on 24 Sep 2016
No it doesn't. It gives the means of only the non-zero elements of your vector. But now you've changed your question. Now you're saying that you have a matrix, NOT a vector. In that case, I'd use mean with the omitnan option after you've set 0's to nans, exactly what Star showed.
allVelocities(allVelocities == 0) = nan;
meanVelocity = mean(allVelocities, 'omitnan')
Note, in the above, allVelocities is a matrix (as in your comment), not a vector as you originally said. And meanVelocity is the column over rows (that is, going down columns) so you have one mean for every column.

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Jan
Jan on 26 Sep 2016
Edited: Jan on 26 Sep 2016
You do not have to replace the zeros by NaNs, because the zeros are neutral for the SUM already:
sum(A, 1) ./ sum(A ~= 0, 1)

Suraj Sudheer Menon
Suraj Sudheer Menon on 22 Jun 2020
All non zero elements can be stored in another location using logical indexing and mean operation can be performed.
temp=A(A~=0); %stores the non zero values in temp.
ans=sum(temp)/nnz(A) ; %nnz returns number of non zero elements.
  1 Comment
Image Analyst
Image Analyst on 22 Jun 2020
But since the sum of any number of zeros is still zero, the sum of A will be the sum of temp. So temp is not necessary, and you'd get the same thing from
ans=sum(A(:))/nnz(A) ;

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