Standard deviation of all matrix elements (single value)

Question

Joachim Schmid 2015-3-2

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https://ww2.mathworks.cn/matlabcentral/answers/181056-standard-deviation-of-all-matrix-elements-single-value

评论： Adam 2015-3-2

I have a random matrix X and I am looking for the standard deviation considering all elements; however I am asked (by a text book, learning Matlab) not to use loops. The result should be one single value.

std(X) gives a matrix of standard deviation of all columns which is not what I am looking for. (something corresponding to m=mean(mean(X)) it is not working for the standard deviation)

PS: sorry for asking this simple question; I did some search on this topic but couldn't find useful hints.

Thank you for your help!

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Answer 1

Adam 2015-3-2

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编辑：Adam 2015-3-2

在 MATLAB Online 中打开

std( X(:) )

should give you the standard deviation of all elements in a matrix, X, of any shape.

This is also generally better for mean since it works for any dimensionality rather than mean( mean( X ) ) which is hard-coded as 2d.

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Adam 2015-3-2

在 MATLAB Online 中打开

Please add a comment to an answer rather than a new answer (it is a mistake many make though, not just you!).

The syntax

X(:)

simply collapses your array into a column so that the std command (or any other similar command) will produce a single answer as it would on a normal column array. For statistics where the 2d or n-d structure of the data is irrelevant this works well. Obviously it needs more manipulation for functions where the 2d structure matters.

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