How to group by matrix by row numbers

Assume matrix A as follows:

A = [
12  4  5  4  3  4  2
12  3  1  6  1  8  10
12  5  10  3  9  1  9
14  6  5  4  5  6  5
14  8  4  7  3  7  10
16  1  5  3  6  3  6
16  6  4  9  8  5  6
98  4  10  1  5  7  9
98  8  8  4  6  1  9
98  5  3  6  10  3  3
98  3  2  6  5  5  9
];

I want to group by matrix A based on the unique ID (first column) and row numbers. For example, looking at first column of matrix A, there are four unique IDs (12,14,16,98). So, for output matrix A1, I want every first row from these unique ID to add in matrix A1. For output A2, the second row, and so on.

Answer 1

[Stephen23]

One way to split into those groups, although it does not preserve the row order, is to use accumarray:

>> [~,~,idx] = unique(A(:,1));
>> C = accumarray(idx,ones(size(idx)),[],@(v){cumsum(v)});
>> D = accumarray(vertcat(C{:}),(1:size(A,1)).',[],@(r){A(r,:)});
>> D = cellfun(@sortrows,D,'uni',0);
>> D{:}
ans =
    12     4     5     4     3     4     2
    14     6     5     4     5     6     5
    16     1     5     3     6     3     6
    98     4    10     1     5     7     9
ans =
    12     3     1     6     1     8    10
    14     8     4     7     3     7    10
    16     6     4     9     8     5     6
    98     8     8     4     6     1     9
ans =
    12     5    10     3     9     1     9
    98     5     3     6    10     3     3
ans =
    98     3     2     6     5     5     9