Filtering a matrix to some small matrices

Hi everyone,

Suppose I have a matrix:

a = [12,7;12,5;2,7;23,3;23,43;23,12;3,5;76,21;76,31;4,3;4,7;4,9];

Then I want to have some sub-divide matrix like following matrixes:

A = [2,7;3,5;4,9]
B = [12,7;12,5;76,21;76,31]
C = [23,3;23,43;23,12;4,3;4,7;4,9]

If first row in matrix a repeated once (e.g. 2-3-4), then that row goes to matrix A If first row in matrix a repeated twice (e.g. 12-76), then that row goes to matrix B If first row in matrix a repeated triple (e.g. 23-4), then that row goes to matrix C ...

Repeation frequency is from 1 to 6 times.

[José-Luis]

What have you tried so far?

 doc histc

[Image Analyst]

"a" is messed up -- the code doesn't run!

[Azzi Abdelmalek]

I don't understand why A = [2,7;3,5;4,9] , it should be A = [2,7;3,5]

Answer 1

[Azzi Abdelmalek]

a = [12,7;12,5;2,7;23,3;23,43;23,12;3,5;76,21;76,31;4,3;4,7;4,9]
c1=a(:,1)
[ii,jj,kk]=unique(c1,'stable')
aa=histc(kk,1:numel(jj))
for k=1:max(aa)
  jdx=ii(ismember(aa,k))
  jj=ismember(c1,jdx)
  out{k}=a(jj,:)
end
celldisp(out)

Answer 2

[Matz Johansson Bergström]

I am sure there is a function in statistics that does this for you, but I quickly wrote a very dirty version using cells, just to handle the matrices in a nice way. You can replace the cells if you like. I assume that the highest frequency is 6, as you state.

Tally = {}; %the matrices
for i = 1:6 %assume frequency is at most 6
Tally{i} = []; %"allocate"
end

[un, in] = unique(a(:, 1));

%Instead of fixing the number of matrices, we store them in a cell and
%count the number of repeats
for i = 1:size(un, 1)
    num          = un(i); %the number we are counting on at the moment
    inds         = find( a(:, 1) == num ); %the indices where 'num' is
    freq         = length(inds);

      Tally{freq}  = [Tally{freq}; a(inds, :)]; 
      %append the new coordinate in index 'freq'
  end

So, a very quick and dirty solution. If I run it on your sample and print my cell Tally, we get

>Tally{:}
ans =
     2     7
     3     5
ans =
    12     7
    12     5
    76    21
    76    31
ans =
     4     3
     4     7
     4     9
    23     3
    23    43
    23    12
ans =
     []
ans =
     []
ans =
     []

I also noticed that [4,9] should not be in A. Hope this helps.