Accessing data in cell arrays

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DrKarimKecir
DrKarimKecir on 16 Feb 2015
Commented: DrKarimKecir on 17 Feb 2015
Hi,
I have a basic question to which I can't find an answer.
Suppose we have a cell matrix:
C = cell(2,2);
What is the difference between
C{1,2} = 2;
and
C{1}{2} = 2;
?
Thanks in advance,
KK

Accepted Answer

Geoff Hayes
Geoff Hayes on 16 Feb 2015
Edited: Geoff Hayes on 16 Feb 2015
Karim - what do you observe if you try the above two statements? The first
C{1,2} = 2;
initializes the cell array in the first row and second column to 2. So if
>> C = cell(2,2)
C =
[] []
[] []
then
>> C{1,2} = 2
C =
[] [2]
[] []
As for the second statement, it initializes the first cell array element of C to be a cell array of two elements with the second set to 2. So
>> C{1}{2} = 2
C =
{1x2 cell} [2]
[] []
where
>> C{1}
ans =
[] [2]
Remember that arrays (cell or otherwise) can be accessed using the row column pair (as in your first example) or using a linear index (as in your second example). For more details on linear indexing see http://www.mathworks.com/help/matlab/math/matrix-indexing.html#f1-85511.
  3 Comments
Stephen23
Stephen23 on 16 Feb 2015
Great answer, I think it only needs a mention of that they also use different indexing styles:
C{1,2} % one instance of subscript indexing (row,col)
C{1}{2} % two examples of linear indexing (element #)
Adam
Adam on 16 Feb 2015
Ah, I was too late :)

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More Answers (1)

Adam
Adam on 16 Feb 2015
Edited: Adam on 16 Feb 2015
Simplest way to learn these things is to try it on the command line and see what the result is.
C = cell(2,2)
C =
[] []
[] []
>> C{1,2} = 2
C =
[] [2]
[] []
C2 = cell(2,2)
C2 =
[] []
[] []
>> C2{1}{2} = 2
C2 =
{1x2 cell} []
[] []
The first syntax is the standard one which does exactly what you expect, i.e. it puts the value 2 into the cell indexed at row 1, column 2. Thus:
C{1,2}
will return the value 2 as a double and
C(1,2)
would return a 1*1 cell array containing the single value 2.
The second syntax is one I never use as it is rather less intuitive and not useful for me. It will index into the first cell of the cell array using 1d indexing and then further use 1d indexing to create a cell array of 1x2, the second element of which is the 2 that you specify.
  8 Comments
Stephen23
Stephen23 on 17 Feb 2015
Edited: Stephen23 on 17 Feb 2015
To get the best performance out of MATLAB you want to not keep resizing your variables. For example this code enlarges the vector on every iteration:
A = [];
for k = 1:1e4;
A(k) = sin(k);
end
Whereas iterating in reverse will create the whole vector on the first iteration, after which it does not change size again:
A = [];
for k = 1e4:-1:1
A(k) = sin(k);
end
Alternatively you can preallocate the vector and iterate in the usual direction:
A = nan(1,1e4);
for k = 1:1e4
A(k) = sin(k);
end
Have a play with tic and toc, you might be surprised how much faster well written code can be in MATLAB.
DrKarimKecir
DrKarimKecir on 17 Feb 2015
Yes, that's exactly what I said : " not to change the size of…".
Thank you for the tricks, Stephen, and have a nice day!
KK

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