split a matrix according to a column with matlab.

Question

A = [1,4,2,5,10
     2,4,5,6,2
     2,1,5,6,10
     2,3,5,4,2]

And I want split it into two matrix by the last column A ->B and C

B         


        

Answer 1

Use accumarray in combination with histc:

% Example data (from Mohsen Nosratinia)
A = [...
     1     4     2     5    10
     2     4     5     6     2
     2     1     5     6    10
     2     3     5     4     2
     0     3     1     4     9
     1     3     4     5     1
     1     0     4     5     9
     1     2     4     3     1];

% Get the proper indices to the specific rows
B = sort(A(:,end)); 
[~,b] = histc(A(:,end), B([diff(B)>0;true]));

% Collect all specific rows in their specific groups
C = accumarray(b, (1:size(A,1))', [], @(r) {A(r,:)} );

Results:

>> C{:}
ans =
     1     3     4     5     1
     1     2     4     3     1
ans =
     2     3     5     4     2
     2     4     5     6     2
ans =
     0     3     1     4     9
     1     0     4     5     9
ans =
     2     1     5     6    10
     1     4     2     5    10

Note that

B = sort(A(:,end)); 
[~,b] = histc(A(:,end), B([diff(B)>0;true]));

can also be written as

[~,b] = histc(A(:,end), unique(A(:,end)));

but unique is not built-in and is therefore likely to be slower, especially when this is all used in a loop.

Note also that the order of the rows has changed w.r.t. the order they had in the original matrix. If the order matters, you'll have to throw in another sort:

C = accumarray(b, (1:size(A,1))', [], @(r) {A(sort(r),:)} );