How to create a sparse vector of class objects

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Simon 2013-8-26

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Dear all!

My aim is to write a class to handle locations in 3-d space (let's call it 'CoordinateClass'). Therefore I need to store the three coordinates (along with - at the moment - three other values) in an array. Each set of coordinate must have a unique ID. These IDs range from 0 to let's say 1e6 and are chosen by the user. The current solution is to use a sparse array. The coordinates are stored in the row corresponding to the ID.

What is the best way to get this scenario into a class? I could define a sparse array as the class property and work with that. But it is not as easy as to use a vector of objects like

C(1) = CoordinateClass([1 1 1 0 0 0]) % 3-d point at (1,1,1) with ID 1
C(100) = CoordinateClass([100 2 1 0 0 0]) % 3-d point at (100,2,1) with ID 100

The index is the user-defined ID. The problem is the memory usage. If I use large ID values all objects below are allocated as well. Is there a way to implement some kind of "sparse object" to decrease memory usage?

Thanks in advance

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Answer 1

Matt J 2013-8-26

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https://ww2.mathworks.cn/matlabcentral/answers/85609-how-to-create-a-sparse-vector-of-class-objects#answer_95093

编辑：Matt J 2013-8-26

在 MATLAB Online 中打开

You don't want to have a separate C(i) for each coordinate. That will allocate memory very inefficiently. You want a single object which holds your coordinate and other data as an array, e.g.,

    classdef CoordinateClass
     properties
       coordinates;
       IDs;
       ThreeOtherValues;
     end
    end

and now you would have

 C.coordinates=[1 1 1; 100 2 1; etc...]
 C.IDs=[1;100; etc...]
 C.ThreeOtherValues=[0 0 0; 0 0 0; etc...]

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Simon 2013-8-27

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Hi!

Thank you for your answer! This solution is of course easy to develop and efficient. But it is(in my opinion) not very user-friendly. Suppose in your example the user wants to change the coordinate with the ID 100.

ind = find(C.IDs==100);
C.coordinates(ind, 1:3) = [100 200 1];

If we had an array C(i) we could just say

C(100).coordinates = [100 200 1];

The same applies e.g. if we want to transfor the cartesian coordinates to polar ones, we might just say:

C(100).cart2pol

Or we add two coordinates (with the overloaded 'plus' function) simply with

C(200) = C(100) + C(1);

instead of

ind1 = find(C.IDs==1);
ind100 = find(C.IDs==100);
ind200 = find(C.IDs==200);
C.coordinates(ind200, 1:3) = C.coordinates(ind100, 1:3) + C.coordinates(ind1, 1:3);

(and we don't have to worry for an array C(i) if C(200) exists already as in 'find(C.IDs==200)'). At the moment I'm using a method 'sumcoordinates' that does the work. The same applies for the above example where I use something like

C = C.changecoordinate(100, [100 200 1]);

It makes things a lot easier to use C(i) in my opinion, but the memory problem ... But maybe there is another solution I don't see. Do you have any hints? Thank you very much!

Simon 2013-8-28

在 MATLAB Online 中打开

Hi Matt!

Thank you for your assistance! I understood that I can 'emulate' some kind of 'sparse behaviour' by using subsref and subsasgn, like you proposed. Anything else is not usable because a large amount of memory is used if I want to use an array of my class with - let's say - 1e7 elements. Even with pre-allocation it even takes quite some time to work with it.

Nevertheless, I don't know how to cope with my class correctly. I have problems calling the class' methods (even after reading e.g. http://www.mathworks.de/de/help/matlab/ref/subsref.html, http://www.mathworks.de/de/help/matlab/matlab_oop/indexed-reference-and-assignment.html#br09ho8, http://www.mathworks.de/matlabcentral/answers/73430, ...)

Here is the clss code

classdef CoordinateClass
      properties
          Coord;
          IDs;
          CoordNum;
      end
      methods
          function obj = CoordinateClass(val)
              if ((nargin > 0)&&  isa(val, 'numeric') && (length(val) == 1))
                  % we take the input as the maximum number of coordinates to store
                  CoordMax = val;
              else
                  % default of 10 coordinates
                  CoordMax = 10;
              end
              % pre-allocate arrays
              obj.Coord = zeros(CoordMax, 3);
              obj.IDs = zeros(CoordMax, 1);
              obj.CoordNum = 0;
          end
          function obj = cart2pol(obj, ind)
              fprintf('transforming IDs: %s\n', mat2str(obj.IDs(ind)))
              val = obj.Coord(ind, 1:3);
              [THETA,RHO,Z] = cart2pol(val(:,1), val(:,2), val(:,3));
              obj.Coord(ind, 1:3) = [THETA,RHO,Z];
          end
          function sref = subsref(obj,s)
              switch s(1).type
                  case '.'
                      % default dot notation
                      sref = builtin('subsref',obj,s);
                  case '()'
                      % index into coordinate array
                      if length(s) == 1
                          % find desired coordinate in array of IDs
                          [~, ind] = intersect(obj.IDs, s.subs{1});
                          if ~isempty(ind)
                              % found -> output as double array
                              sref = obj.Coord(ind, 1:3);
                          else
                              % not found -> output empty array
                              sref = [];
                          end
                          return
                      elseif length(s) == 2
                          if strcmp(s(2).type, '.') && strcmp(s(2).subs, 'cart2pol')
                              % we want to transform some coordinates
                              % find desired coordinate in array of IDs
                              [~, ind] = intersect(obj.IDs, s(1).subs{1});
                              if ~isempty(ind)
                                  % transform found coordinates
                                  obj = cart2pol(obj, ind);
                              end
                              sref = obj;
                              return
                          end
                      else
                          sref = builtin('subsref',obj,s);
                      end
                  otherwise
                      sref = [];
              end
          end
          function obj = subsasgn(obj,s, val)
              switch s(1).type
                  case '.'
                      % default dot notation
                      obj = builtin('subsasgn',obj,s);
                  case '()'
                      if length(s)<2
                          % find desired coordinate in array of IDs
                          [~, ind] = intersect(obj.IDs, s.subs{1});
                          if any(ind)
                              % found -> coordinate exists -> change
                              obj.Coord(ind, 1:3) = val;
                          else
                              % not found -> create new coordinate
                              obj.CoordNum = obj.CoordNum + 1;
                              obj.Coord(obj.CoordNum, 1:3) = val;
                              obj.IDs(obj.CoordNum) = s.subs{1};
                          end
                          return
                      else
                          obj = builtin('subsasgn',obj,s);
                      end
              end
          end
      end
  end

And the test script is

clear all
clc
% create object with a maximum of 10 entries
C = CoordinateClass(10);
% set coordinates
C(1) = [1 1 3];
C(100) = [100 1 3];
C(2) = [2 1 2];
% redefine coordinates
C([1 100]) = [1 2 3; 100 2 3];
% does not work -> transformed coordinates are not returned
C(1:2).cart2pol
% does not work because of missing input argument
C.cart2pol

The second problem seems to be easy to solve by improving the subsref function for all possible calls to the class' object. Is this the best solution or is there something I don't see at the moment?

But I don't know where the first problem comes from. Do you have a hint for me?

Thanks again for your help!

Matt J 2013-8-30

编辑：Matt J 2013-8-30

Why does C.cart2pol go through subsref and cart2pol(C) not?

Because C.cart2pol is an indexing expression and cart2pol(C) is not. I assume you're referring to situations when these expressions are executed outside the class. Inside a class method, neither expression goes through subsref.

Simon 2013-8-30

在 MATLAB Online 中打开

Yes, your right, I execute the command from my test script. But inside the class the expression goes through subsref if I call it directly

sref = subsref(obj, s,)

(assumed the overloaded subsref handles this correctly) and not with

sref = builtin('subsref',obj,s);

That's ok.

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How to create a sparse vector of class objects

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