Find the mean of nonzero elements
I am assuming that the function mean
takes a matrix and calculates its average by summing the entire element of the array and dividing it by the total number of elements.
However, I am using this function to calculate the average of my matrix. Then I come across a point where I don't want the middle function to consider 0 elements of my matrix. Specifically, my matrix is a 1x100000 array, and maybe 1/3 to 1/2 of its element is all 0. If so, can I replace element 0 with NULL
so that Matlab does not take them into account when calculating the average? What else can I do?
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Short version:
Use nonzeros
:
mean( nonzeros(M) );
Longer answer:
If you are working with an array with 100k records, a significant portion of those records are 0, you might consider working with sparse
. It might also be wise to store it as a column vector rather than a row vector.
sM = sparse(M(:)); %// sparse column
mean( nonzeros(sM) ); %// mean of only non-zeros
mean( sM ); %// mean including zeros
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As you asked, "What else can I do?" There is a different approach that does not depend on the statistics dashboard or any other toolkit.
You can compute them, therefore, yourself, by summing the values and dividing by the number of nonzero elements ( nnz()
). Since summing zeros does not affect the amount, this will give the desired result. For a 1-dimensional case, as you think, it can be done as follows:
% // 1 dimensional case
M = [1, 1, 0 4];
sum(M)/nnz(M) % // 6/3 = 2
For the two-dimensional case (or the n-dimensional case), you must specify the size over which the summation should occur
% // 2-dimensional case (or n-dimensional)
M = [1, 1, 0, 4
2, 2, 4, 0
0, 0, 0, 1];
% // column means of nonzero elements
mean_col = sum(M, 1)./sum(M~=0, 1) % // [1.5, 1.5, 4, 2.5]
% // row means of nonzero elements
mean_row = sum(M, 2)./sum(M~=0, 2) % // [2; 2.667; 1.0]
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To find the average of only non-zero elements, use boolean indexing to extract non-zero elements, and then call mean
on those:
mean(M(M~=0))
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