standard deviation of same values in a matrix

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Fabi Boro
Fabi Boro 2015-11-30
评论： the cyclist 2015-12-1
Hello,
I couldn't find anything which could help me with my problem.
I have a matrix (n x 2) like shown below:
I want to calculate the standard deviation of column 2 for all records which have the same valaue in column 1. So I get this:
Is there a smart way to do this? Maybe is several steps?
Thanks!

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回答（2 个）

the cyclist
the cyclist 2015-11-30
编辑：the cyclist 2015-11-30
The accumarray function is designed for this:
M = [5 0.30;
5 0.35;
7 0.50;
7 0.51;
9 0.98;
9 0.87;
9 0.71];
[uniqueM,idxToUnique,idxFromUniqueBackToAll] = unique(M(:,1));
S = accumarray(idxFromUniqueBackToAll,M(:,2),[],@std)
stdDevM = [uniqueM,S]
  2 个评论
the cyclist
the cyclist 2015-12-1
The best form of thanks is accepting the solution, which rewards the person who helped you, and also points future users to good answers.

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John D'Errico
John D'Errico 2015-11-30
编辑：John D'Errico 2015-11-30
Download consolidator , from the file exchange. One line then.

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