why it will go wrong using 'cell'?

Recently, I ran two codes as follow:

a={[],[]};a{1}(a{1}==3)=[];

% it will go wrong
b=cell(1,2);b{1}(b{1}==3)=[];

[KSSV]

The difference is the space allocated for each.

a={[],[]};
b=cell(1,2);
whos a b 

  Name      Size            Bytes  Class    Attributes

  a         1x2               208  cell               
  b         1x2                16  cell               

[warnerchang]

however，what happened behind cell function? As far as I know, a = {[],[]} and b = cell(1,2) should get the same result.

Answer 1

[Jan]

I confirm, that this is an inconsistent behavior.

a = {[], []};
b = cell(1, 2);
isequal(a, b)

ans = logical
   1

a{1}([]) = []   % Working: Nothing is deleted as expected

a = 1×2 cell array
    {0×0 double}    {0×0 double}

b{1}([]) = []   % Failing:

Deletion requires an existing variable.

Both methods create a cell array of the same size, but cell() fills the elements with a NULL pointers, while {[], []} creates empty matrices. Matlab should treat NULL pointers as empty matrices, but as you see in this example, this exception has been forgotten sometimes. You cannot detect the NULL points from inside Matlab, but on the C level this is easy. KSSV's method to check the used memory is a strong hint also.

Report this bug to MathWorks. The function for deleting will be updated than to catch this exception.

[warnerchang]

thank you for reply!