standard deviation of non-zero elements of columns of a matrix

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osminbas
osminbas 2012-3-1
回答： David Freese 2013-10-11
Hello,
How can I find the standard deviation of each column of a matrix ignoring the zero values? I must add that I do not have the stats package so I cannot use nanstd! Thank you!

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采纳的回答

the cyclist
the cyclist 2012-3-1
Here is a straightforward way.
% Some input data
x = rand(7,6);
x(1,1) = 0; % Put in a couple zeros by hand
x(6,5) = 0;
% Preallocate the array that will hold the standard deviations
sd = zeros(1,6);
% Calculate and store the standard deviations of the non-zero elements.
for nc = 1:6
indexToNonZero = x(:,nc)~=0;
sd(nc) = std(x(indexToNonZero,nc));
end

更多回答（2 个）

Walter Roberson
Walter Roberson 2012-3-1
There is a MATLAB File Exchange contribution to provide these functions.
David Freese
David Freese 2013-10-11
A way to go about it using just matrix operations:
% Some input data
x = rand(7,6);
x(1,1) = 0; % Put in a couple zeros by hand
x(6,5) = 0;
% Calculate and store the standard deviations of the non-zero elements.
% using the identity std^2 = Var(x) = mean(x.^2,2) - mean(x,2).^2
avg = sum(x,2)./sum(x~=0,2);
sd = sqrt(sum(x.^2,2)./sum(x~=0,2) - avg.^2);
% Zero out any of the columns that were all zeros
sd(isnan(sd)) = 0;

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