Concatenate every n-th row - matlab

Question

I got a data set in a matrix like the following (imported from Excel):
matrix =
Cat1 1 2 3 4
Cat2 9 10 11 12
Cat3 17 18 19 20
Cat1 5 6 7 8
Cat2 13 14 15 16
Cat3 21 22 23 24
I would like to reshape it into 3 vectors (one for every category) of the same size to do a stacked bar plot. Vectors should look like this after reshape operation (It would be nice if the vector had the name of the first column and the matrix could be of any size):
cat1 = [ 1 2 3 4 5 6 7 8]
cat2 = [ 9 10 11 12 13 14 15 16]
cat3 = [17 18 19 20 21 22 23 24]
I sincerely hope this is not duplicate. I couldn't produce a working solution with the help of the other reshape questions.

Answer 1

If your data is a matrix, you can manipulate the order of the rows when indexing, so you can do something like this:
rows = reshape(1:size(matrix, 1), n, []).';
res = reshape(matrix(rows, :).', [], n).';
The resulting matrix res is composed of the concatenated rows.
This solution holds for cell arrays as well, but you'll need an additional cell2mat to turn the result into a matrix.
Example
matrix = [1:4; 9:12; 17:20; 5:8; 13:16; 21:24];
n = 3;
rows = reshape(1:size(matrix, 1), n, []).';
res = reshape(matrix(rows, :).', [], n).';
The result is:
res =
1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16
17 18 19 20 21 22 23 24

Answer 2

EDIT:
Try the following:
%# dataset stored in a cell array
data = {
'Cat1' 1 2 3 4
'Cat2' 9 10 11 12
'Cat3' 17 18 19 20
'Cat1' 5 6 7 8
'Cat2' 13 14 15 16
'Cat3' 21 22 23 24
};
%# get all possible values of first column,
%# and map them to integer indices
[L,~,IDX] = unique(data(:,1));
%# for each possible "category"
groups = cell(max(IDX),1);
for i=1:max(IDX)
%# get the rows of numeric data matching current category
M = data(IDX==i, 2:end)';
%# flatten matrix into a vector and store in cell (row-major order)
groups{i} = [M{:}];
end
Now you can access the i-th "cat" vector as: groups{i}
>> [cat1,cat2,cat3] = deal(groups{:})
cat1 =
1 2 3 4 5 6 7 8
cat2 =
9 10 11 12 13 14 15 16
cat3 =
17 18 19 20 21 22 23 24
Note that the matching "cat" labels are stored in L{i} (the mapping keys)