CODEWITHSUNDEEP
matlabmatrixindexingcell
P i
P i

Reputation: 30704

Difference between x(1) and x{1}

>> x = {  {'a',[1],[2]};  {'b',[3],[4]}  }
x =    
    {1x3 cell}
    {1x3 cell}

>> A1 = x(1)    
A1 =     
    {1x3 cell}

>> A2 = x{1}   
A2 =    
    'a'    [1]    [2]

Note that A1 and A2 display differently.

A1 and A2 report as being the same class and dimension:

>> info = @(x){class(x),size(x)};
>> info(A1)     
ans =    
    'cell'    [1x2 double]

>> info(A2)     
ans =     
    'cell'    [1x2 double]

yet they and are not considered equal:

>> isequal( A1, A2 )    
ans =    
     0

However, A1{:} is considered as being equal to A2:

>> isequal( A1{:}, A2 )    
ans =    
     1

Could someone explain what is going on here?

Upvotes: 1

Views: 147

Answers (3)

Adriaan
Adriaan

Reputation: 18177

() is used for function inputs and to index arrays; [] is used to denote arrays; {} is used to index cells.

A = [2 1]; % Creates and array A with elements 2 and 1
A(2) =2; % Sets the second element of A to 2
B = cell(1,2); % creates a cell array
B{2} = A; % Stores A in the second cell of B

So in conclusion: x(1) selects the first element of array x or evaluates the function x at 1; x[1] should not be possible, as square brackets can't be used to index stuff; x{1} would select the first cell in cell array x.

As to your specific problem:

A1 = x(1); % makes a copy of the element on index one, being a 1x3 cell
A2 = x{1}; % stores the content of the cell at element 1 in A2

Finally A1{:} gets the content of the cell out of the cell, ready to store as a separate variable, hence it is equal to A2, which already contains that content. (A1{:} is a comma separated list, thanks to rayryeng for pointing that out.)

Upvotes: 4

NLindros
NLindros

Reputation: 1693

Your variable x is a cell containing 2 (sub)cells.

  1. Curly brackes is used to get the content of a cell, thus,

    A2 = x{1}
ans = 
   'a'    [1]    [2]

    Which gives the same result as

    A2 = {'a',[1],[2]}

  2. Parentheses is used for indexing and therefore returns a subset of the (sub)cells

    A1 = x(1)    
A1 =     
    {1x3 cell}

    Which gives the same result as

    A1 = { {'a',[1],[2]} }

Your anonymous info function returns a cell (needs to as the content is of mixed types). The result

'cell'    [1x2 double]

tells that both A1 and A2 is cells which is true, not considering the fact that A1 is a cell containing cells and A2 is a cell containing a char and 2 numbers.

The [1x2 double] simply tells that the answer from size is itself of size [1x2]. This is true as long as you don't use higher dimensions than 2.

If you dig deeper into your info answer you will see the real size:

A1info = info(A1);
A2info = info(A2);
A1info{2}
ans =
     1     1
A2info{2}
ans =
     1     3

The part of isequal( A1, A2 ) beeing false I think you understand by now.

Also, that isequal( A1{:}, A2) is true, since {:} "de-cell" A1 and pick out its content. In this case, you could have typed isequal( A1{1}, A2) as well

Upvotes: 2

titus
titus

Reputation: 450

data{1} is to access the content of a cell data(1) is to create a cell with a copy of the first entry of data. This is the same as :

>> A1=cell(1,1);
>> A1{1}=data{1};

In your example A1 is thus a cell containing a cell, while A2 is a cell with three entries

Upvotes: 2

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