How to average certain elements in a matrix using for loop.

Question

jack 2014-3-3

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https://ww2.mathworks.cn/matlabcentral/answers/119986-how-to-average-certain-elements-in-a-matrix-using-for-loop

评论： Azzi Abdelmalek 2014-3-3

Hello,

I am trying to operate on a matrix that has the following columns:

M=[x_coordinates y_coordinates ID] I would like to find the average x and y coordinates for same ID, in other words to group all the points that has the same ID. Can you please advice me on how to do it with Matlab.

Best regards, Jack

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Answer 1

Azzi Abdelmalek 2014-3-3

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https://ww2.mathworks.cn/matlabcentral/answers/119986-how-to-average-certain-elements-in-a-matrix-using-for-loop#answer_126921

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A=[ 1 2 10;5 8 10;4 3 20]
c1=A(:,1);
c2=A(:,2);
[ii,jj,kk]=unique(A(:,3));
cc1=[ii accumarray(kk,c1,[],@mean)]
cc2=[ii accumarray(kk,c2,[],@mean)]

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jack 2014-3-3

Thank you so much, this worked great. Is it possible to use a customized function instead of mean? For example I might need to use a weighted mean by using the associated elements in another column of the matrix, for instance intensity if M=[x_coordinates y_coordinates intensity ID].

thanks.

Azzi Abdelmalek 2014-3-3

在 MATLAB Online 中打开

A=[ 1 2 10;5 8 10;4 3 20]
weight=[10;54;87]
c1=A(:,1);
[ii,jj,kk]=unique(A(:,3));
cc1=[ii accumarray(kk,c1.*weight,[],@mean)]

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Answer 2

Sean de Wolski 2014-3-3

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https://ww2.mathworks.cn/matlabcentral/answers/119986-how-to-average-certain-elements-in-a-matrix-using-for-loop#answer_126920

在 MATLAB Online 中打开

doc accumarray

And an example:

%%sample data
n = 50;
id = randi(5,n,1);
x = rand(n,1);
y = rand(n,1);
%%engine
[uid,~,idx] = unique(id);  % unique ids and the index of occurrences
mx = accumarray(id,x,[],@mean); % mean of x by id
my = accumarray(id,y,[],@mean); % mean of y by id
results = [uid mx my]  % display