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

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

Answer 1

[Azzi Abdelmalek]

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)]

[jack]

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]

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)]

Answer 2

[Sean de Wolski]

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

Answer 3

[jack]

Thank you so much for your answers! much appreciated!