allcomb.m

function A = allcomb(varargin)

% ALLCOMB - All combinations
%    B = ALLCOMB(A1,A2,A3,...,AN) returns all combinations of the elements
%    in the arrays A1, A2, ..., and AN. B is P-by-N matrix is which P is the product
%    of the number of elements of the N inputs. This functionality is also
%    known as the Cartesian Product. The arguments can be numerical and/or
%    characters, or they can be cell arrays.
%
%    Examples:
%       allcomb([1 3 5],[-3 8],[0 1]) % numerical input:
%       % -> [ 1  -3   0
%       %      1  -3   1
%       %      1   8   0
%       %        ...
%       %      5  -3   1
%       %      5   8   1 ] ; % a 12-by-3 array
%
%       allcomb('abc','XY') % character arrays
%       % -> [ aX ; aY ; bX ; bY ; cX ; cY] % a 6-by-2 character array
%
%       allcomb('xy',[65 66]) % a combination
%       % -> ['xA' ; 'xB' ; 'yA' ; 'yB'] % a 4-by-2 character array
%
%       allcomb({'hello','Bye'},{'Joe', 10:12},{99999 []}) % all cell arrays
%       % -> {  'hello'  'Joe'        [99999]
%       %       'hello'  'Joe'             []
%       %       'hello'  [1x3 double] [99999]
%       %       'hello'  [1x3 double]      []
%       %       'Bye'    'Joe'        [99999]
%       %       'Bye'    'Joe'             []
%       %       'Bye'    [1x3 double] [99999]
%       %       'Bye'    [1x3 double]      [] } ; % a 8-by-3 cell array
%
%    ALLCOMB(..., 'matlab') causes the first column to change fastest which
%    is consistent with matlab indexing. Example: 
%      allcomb(1:2,3:4,5:6,'matlab') 
%      % -> [ 1 3 5 ; 1 4 5 ; 1 3 6 ; ... ; 2 4 6 ]
%
%    If one of the arguments is empty, ALLCOMB returns a 0-by-N empty array.
%    
%    See also NCHOOSEK, PERMS, NDGRID
%         and NCHOOSE, COMBN, KTHCOMBN (Matlab Central FEX)

% Tested in Matlab R2015a
% version 4.1 (feb 2016)
% (c) Jos van der Geest
% email: samelinoa@gmail.com

% History
% 1.1 (feb 2006), removed minor bug when entering empty cell arrays;
%     added option to let the first input run fastest (suggestion by JD)
% 1.2 (jan 2010), using ii as an index on the left-hand for the multiple
%     output by NDGRID. Thanks to Jan Simon, for showing this little trick
% 2.0 (dec 2010). Bruno Luong convinced me that an empty input should
% return an empty output.
% 2.1 (feb 2011). A cell as input argument caused the check on the last
%      argument (specifying the order) to crash.
% 2.2 (jan 2012). removed a superfluous line of code (ischar(..))
% 3.0 (may 2012) removed check for doubles so character arrays are accepted
% 4.0 (feb 2014) added support for cell arrays
% 4.1 (feb 2016) fixed error for cell array input with last argument being
%     'matlab'. Thanks to Richard for pointing this out.

narginchk(1,Inf) ;

NC = nargin ;

% check if we should flip the order
if ischar(varargin{end}) && (strcmpi(varargin{end},'matlab') || strcmpi(varargin{end},'john')),
    % based on a suggestion by JD on the FEX
    NC = NC-1 ;
    ii = 1:NC ; % now first argument will change fastest
else
    % default: enter arguments backwards, so last one (AN) is changing fastest
    ii = NC:-1:1 ;
end

args = varargin(1:NC) ;
% check for empty inputs
if any(cellfun('isempty',args)),
    warning('ALLCOMB:EmptyInput','One of more empty inputs result in an empty output.') ;
    A = zeros(0,NC) ;
elseif NC > 1
    isCellInput = cellfun(@iscell,args) ;
    if any(isCellInput)
        if ~all(isCellInput)
            error('ALLCOMB:InvalidCellInput', ...
                'For cell input, all arguments should be cell arrays.') ;
        end
        % for cell input, we use to indices to get all combinations
        ix = cellfun(@(c) 1:numel(c), args,'un',0) ;
        
        % flip using ii if last column is changing fastest
        [ix{ii}] = ndgrid(ix{ii}) ;
        
        A = cell(numel(ix{1}),NC) ; % pre-allocate the output
        for k=1:NC,
            % combine
            A(:,k) = reshape(args{k}(ix{k}),[],1) ;
        end
    else
        % non-cell input, assuming all numerical values or strings
        % flip using ii if last column is changing fastest
        [A{ii}] = ndgrid(args{ii}) ;
        % concatenate
        A = reshape(cat(NC+1,A{:}),[],NC) ;
    end
elseif NC==1,
    A = args{1}(:) ; % nothing to combine

else % NC==0, there was only the 'matlab' flag argument
    A = zeros(0,0) ; % nothing
end