Aggregate Data in a large matrix using unique identifiers.

Hello Matlab world, I hope everyone is doing really well. I have a large matrix and I need to aggregate the data in based on unique identifiers from the matrix's first column ( grouping the data by unique identifiers) . Example:(5*4) Matrix A=

A=[5 2 1 3; 
   4 3 7 5; 
   5 1 1 1; 
   3 1 2 7; 
   4 1 5 8]

So the desired matrix should be (3*3) and that is by aggregating rows based on common identifiers from the first column.

B=[3 1 2 7; 
   4 4 12 13;
   5 3 2 4];

I also need to know the frequency of each unique identifier (i.e., 4 and 5 both appear twice) so I can get the average in each row. I hope I was clear, Thank you so much

Below is a code that I used earlier:

[a,~,c] = unique(MAVGS_S(:,1));  %extract unique values of the indicator column
out(:,1)=a; % assign the unique values extracted to the first column of the output table
% loop for all the columns to compute summary statistics according to the unique  values 
    by_col = [a, accumarray(c,MAVGS_S(:,2))]; % extract (unique values in first column and summary statistics in the second column for each column
    out(:,2)=by_col(:,2); % extract the summary statistics for i column to the output table;

Acc_COST=[out]; %The matrix containing the aggregated data at the CBG level. (758*102)
%Count how many 1km*1km Pixels in each census block group.
[x,y]=hist(MAVGS_S(:,1),unique(MAVGS_S(:,1)));
Pixels_Count=x';

Answer 1

[Guillaume]

A=[5 2 1 3; 4 3 7 5; 5 1 1 1; 3 1 2 7; 4 1 5 8];
[ids, ~, subs] = unique(A(:, 1));
B = [ids, cell2mat(accumarray(subs, 1:size(A, 1), [], @(rows) {sum(A(rows, 2:end), 1)}))]
idcount = accumarray(subs, 1)

is how I would do it.

[Amine Ben Ayara]

Hey Guillaume, Thanks for the answer so much. I did try your code and I have this error: Error using accumarray When SUBS is a column vector, the third input SZ must be of the form [N 1].

[Guillaume]

Oops, sorry. I thought I tested the code. It was missing the empty array for size. Fixed now.