ACCUMARRAYSET - Overload `ACCUMARRAY`.

Description
Syntax
Inputs
Outputs
See also
Function implementation

Description

Group elements from a data vector set as they share a same index. Fast version of ACCUMARRAY(subs, val, [], @(x){x}).

Syntax

   A = accumarrayset(subs);
   [A, T] = accumarrayset(subs, val);
   [A, T] = accumarrayset(subs, val, spar);

Inputs

subs, val : the position of an element in the vector subs determines which value of the vector vals it selects for the accumulated vector; the value of an element in subs determines the position of the accumulated vector in the output; see function ACCUMARRAY; val is optional, in the case it is not passed the vector (1:numel(subs)) is used by default.

spar : (optional) logical flag defining if the output cell array A (see below) should contain empty cells; when set to true, only those indices found in subs are represented in the final output; which ones are represented is given in the vector T; default: spar=true.

Outputs

A : cell array grouping values of val with same index in subs.

T : sparse matrix giving for each (unique) value/entry found in subs the position (index) in the cell A of the set grouping the corresponding elements in val; T is output when the variable spar is set to true.

Function implementation

function [A, T] = accumarrayset(subs, val, spar)

if nargin<3,  spar = false;
    if nargin<2,  val = [];  end
end

if isempty(val),  val = (1:numel(subs))';  end
subs = mat2rc(subs,'c');  val = mat2rc(val,'c');
% tic
% A = accumarray(subs, val, [], @(x){x});
% toc

%tic
[r,c] = size(subs);
L = zeros(r,1);
L(:) = val; % (1:r);
L = L(:,ones(1,c));
[T,idx] = sort(subs(:)); % we need T for finding rows
L = L(idx);
idx = [0; find(diff(T)); numel(T)]; % run length encoding
T = T(idx(2:end));
t = numel(T);
if spar
    A = cell(t, 1);
    for ii = 1:t
        A{ii} = L(idx(ii)+1:idx(ii+1));
    end
    T = sparse(T,ones(size(T)),1:t,T(end),1);
else
    A = cell(T(end), 1);
    % create same look up table as ACCUMARRAY does
    for ii = 1:t
        A{T(ii)} = L(idx(ii)+1:idx(ii+1));
    end
end
% toc

end % end of accumarrayset

ACCUMARRAYSET - Overload ACCUMARRAY.