MATLAB: Possibilities to read out cell arrays and properties efficiently

cell arraysMATLAB

Hi community,

I have a small question and it would be really nice, if you could help me.

Is there any efficient way to read out the porperties of objets, which are stored in a cell array? I have a nested code, where objects of a self-made class are stored in a large cell array, but I think the following short example shows the problem.

    cellArray = cell(2,1);
    obj1 = struct('A', 1, 'B', [1 2]);
    obj2 = struct('A', 2, 'B', [3 4; 5 6]);
    cellArray{1,1} = obj1;
    cellArray{2,1} = obj2;

Now I am searchching for a solution to read out the values of 'B' efficiently. The following code won't work, just to give you an idea, what I mean:

    MatrixB = cellArray{:,:}.B

My solution at the moment is not very elegant.

    [row, column] = size(cellArray);
    MatrixB = [];
    for i = 1:row
        for j = 1:column
            MatrixB = [MatrixB; cellArray{i,j}.B];
        end
    end

If there is no way to avoid the for-loops, should I count the number of rows with another loop at first and preallocate the Matix?

Thanks for your help! Best regards, lukn13

Best Answer

If the objects are structures, then the most efficient way is to store your data in a non-scalar structure, then accessing the field data is really easy:

>> obj(1) = struct('A', 1, 'B', [1 2]);
>> obj(2) = struct('A', 2, 'B', [3 4; 5 6]);
>> obj.B
ans =
   1   2
ans =
   3   4
   5   6
>> vertcat(obj.B)
ans =
   1   2
   3   4
   5   6
>> obj(2).B
ans =
   3   4
   5   6

Note that you can create a non-scalar structure with just one struct call:

 >> obj = struct('A', {1,2}, 'B', {[1 2],[3,4;5,6]});

If the objects are not structures, then most likely you will have to use a loop. You can always wrap it up with cellfun, or write your own mex function.

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S = struct('A',num2cell(A))

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How to make a 1x0 struct array

S = struct('a',cell(1,0),'b',cell(1,0), 'c', cell(1,0))
S = 

  1×0 empty struct array with fields:

    a
    b
    c

Addendum: why does this create a 1x0 struct array?

TL;DR

When defining a structure array using cell arrays, the structure array takes on the size of the cell array (see demos G and J below).

Deep dive into structure size

This can get a little confusing.

The first thing to understand is the difference between a scalar structure and a structure array. By definition, a scalar structure is a structure with size 1x1 and therefore cannot be empty. The size of a structure array is determined by its content which can result in dimensions that are conterintuitive, at least for me.

Most of the description below is based on the struct documentation.

Syntax s = struct(field,value)

The syntax s=struct(field,value) produces a scalar structure iff,

value is not a cell array
or value is a scalar cell array

% A)
S = struct('a',1);
isscalar(S) % = 1









size(S)     % = [1,1]





S = struct('a',{1:100});
isscalar(S) % = 1
size(S)     % = [1,1]

If value is a nonscalar cell array then S is a structure array with the same size as value. S(i) shows the value for element i.

% B)
S = struct('a', num2cell([1 2; 3 4; 5 6])); % 3x2 cell array
isscalar(S) % = 0







size(S)     % = [3,2]

S(3)        % struct with fields: a: 5

If value is an empty cell, then S is an empty structure array with the specified field

% C)
S = struct('a', {});  % or cell(0,0)
isscalar(S) % = 0
size(S)     % = [0,0]



isempty(S)  % = 1
S 
%  0×0 empty struct array with fields:
%    a

Here's where things get weird. When struct() contains no inputs, a scalar structure is created that has no fields and therefore may appear to be empty but has a size of 1x1.

% D)
S = struct()
isscalar(S)  % = 1
size(S)      % = [1,1]
isempty(S)   % = 0
S            % = struct with no fields.

But if the structure is defined by an empty double array then the structure is empty and is no longer scalar. This is because, by definition, a scalar value has a size 1x1 and isn't empty.

% E)
S = struct([])
isscalar(S)  % = 0
size(S)      % = [0,0]
isempty(S)   % = 1
S            % = 0×0 empty struct array with no fields.

Syntax s = struct(field1,value1,...,fieldN,valueN)

This syntax either creates a scalar structure with multiple fields or a structure array with multiple fields following the same rules as above. The result is a scalar structure iff,

all values are not cell arraya
or cell array values are scalar

% F)
S = struct('a', 'foo', 'b', [98 97 114])
isscalar(S) % = 1
size(S)     % = [1,1]
S
%  struct with fields:

%    a: 'foo'

%    b: [98 97 114]

S = struct('a', {'foo'}, 'b', {[98,97,114]})
isscalar(S) % = 1
size(S)     % = [1,1]
S
%  struct with fields:
%    a: 'foo'
%    b: [98 97 114]

If any value inputs are nonscalar cell arrays, then S is a structure array with the same size as the nonscalar cell array. All nonscalar cell arrays in value must have the same size.

% G)
S = struct('a', {'foo'}, 'b', num2cell([98,97,114]))
isscalar(S) % = 0
size(S)     % = [3,2]
S = struct('a', num2cell(1:5), 'b', num2cell(1:6))
% Error using struct
% Array dimensions of input '4' must match those of input '2', or be
% scalar.

If any value is an empty cell array then S is an empty structure array. Note the word any. Even if other fields are non-empty, S will be empty.

% H)
S = struct('a',{},'b',cell(0,0))  % showing 2 ways to create empty cell
isscalar(S)  % = 0
size(S)      % = [0,0]
isempty(S)   % = 1
S            
%   0×0 empty struct array with fields:

%     a


%     b


S = struct('a', 'foo', 'b', [98,97,114], 'c', {}) % c is empty cell
isscalar(S)  % = 0
size(S)      % = [0,0]
isempty(S)   % = 1
S             
%   0×0 empty struct array with fields:
%     a
%     b
%     c

Crazy, right? To tell Matlab hey, I wanted that field to be empty!!, define the field using an empty double array,

% I)
S = struct('a', 'foo', 'b', [98,97,114], 'c', []) % c is empty double
isscalar(S)  % = 1
size(S)      % = [1,1]
isempty(S)   % = 0
S             
%   struct with fields:
%     a: 'foo'
%     b: [98 97 114]
%     c: []

Lastly, and coming full circle, empty arrays can have different dimensions as long as one of the dimensions is 0. To create an nx0 structure array,

% J)
n =  5; 
S = struct('a', cell(n,0), 'b', (cell(n,0)));
isscalar(S)  % 0
size(S)      % [5,0]
isempty(S)   % 1
S
%   5×0 empty struct array with fields:
%     a
%     b

Best Answer

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