MATLAB: Indexing one particular dimension regardless of number of dimensions

indexing

I have a structure of several dimension (7 right now) that I will be adding dimensions to in the future. In a section of my code I need to isolate the third dimension into it's three components which right now I do like

outX = in(:,:,1,:,:,:,:); outY = in(:,:,2,:,:,:,:); outZ = in(:,:,3,:,:,:,:);

Is there a way to generalize this without having to edit the number of colons in the statement?

ex:

outX = coolFunction(in,3,1); outY = coolFunction(in,3,2); outZ = coolFunction(in,3,3);

Thanks!

Best Answer

function out = coolFunction(in, dim, page)
   validateattributes(dim, {'numeric'}, {'scalar', 'positive', '<=', ndims(in)});
   validateattributes(page, {'numeric'}, {'scalar', 'positive', '<=', size(in, dim)});
   alldims = arrayfun(@(d) 1:d, size(in), 'UniformOutput', false);
   alldims{dim} = page;
   out = in(alldims{:});
end

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MATLAB: How to extract part of matrix on given dimension while this dimension is a variable

function newMatrix = extractMatrix(matrix,dim,index)
id = repmat({':'},1,ndims(matrix));
id{dim} = index;
newMatrix = matrix(id{:});

MATLAB: Nchoose2: save output in chunks

Neither nchoosek nor nchoose2 let you return a portion of the output.

You can always generate the output using a loop and break out whenever you want:

function [rowcombination, nextfirstrow, nextsecondrow] = choose2row(in, maxrows, startfirstrow, startsecondrow)
    %CHOOSE2ROW create every combination of 2 rows of a matrix/cell array
    %The function can return a portion of the output and be called again to return the next portion.
    %The function uses double loops to compute all combinations.
    %Outputs:
    %   rowcombination: matrix/cell array where each row is the concatenation of two distinct rows of the original matrix/cell array.
    %   nextfirstrow: 
    %   nextsecondrow:  parameters to pass back to a subsequent call to CHOOSE2ROW to return the next portion of row combination.
    %Inputs:
    %   in: input matrix/cell array of size [m, n].
    %   maxrows: maximum number of rows of output rowcombination. Inf for no limit. Scalar, optional. default Inf.
    %   startfirstrow: outer loop start index. Scalar, optional. default 1.
    %   startsecondrow: inner loop start index. Scalar, optional. default startfirstrow - 1.
      if nargin < 2 || maxrows == Inf
         maxrows = Inf;
      else
         validateattributes(maxrows, {'numeric'}, {'scalar', 'positive', 'integer'}, 2);
      end
      if nargin < 3
         startfirstrow = 1;
      else
         validateattributes(startfirstrow, {'numeric'}, {'scalar', 'positive', 'integer', '<', size(in, 1)}, 3);
      end
      if nargin < 4 
         startsecondrow = startfirstrow + 1;
      else
         validateattributes(startsecondrow, {'numeric'}, {'scalar', 'positive', 'integer', '<=', size(in, 1), '>', startfirstrow}, 4);
      end
      nrows = (size(in, 1) - startfirstrow + 1) * (size(in, 1) - startfirstrow) / 2 - (startsecondrow - startfirstrow - 1);   %total size of output still to generate
      rowcombination = repmat(in(1, :), min(nrows, maxrows), 2); %initialise output to required size
      rowout = 1;      
      for nextfirstrow = startfirstrow :  size(in, 1)-1
          for nextsecondrow = startsecondrow : size(in, 1)
             rowcombination(rowout, :) = [in(nextfirstrow, :), in(nextsecondrow, :)];
             rowout = rowout + 1;
             if rowout > maxrows
                nextsecondrow = nextsecondrow + 1; %#ok<FXSET> exiting the loop
                if nextsecondrow > size(in, 1)
                   nextfirstrow = nextfirstrow + 1; %#ok<FXSET>



                   nextsecondrow = nextfirstrow + 1; %#ok<FXSET>
                   if nextfirstrow == size(in, 1)
                      nextfirstrow = Inf; %#ok<FXSET>
                      nextsecondrow = Inf; %#ok<FXSET>
                   end
                end
                return
             end
          end
          startsecondrow = nextfirstrow + 2;
      end
      nextfirstrow = Inf;  
      nextsecondrow = Inf;
  end

Of course, you're trying performance for memory.

Best Answer

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MATLAB: How to extract part of matrix on given dimension while this dimension is a variable

MATLAB: Nchoose2: save output in chunks

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