Accessing data in cell arrays

Question

DrKarimKecir 2015-2-16

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/178691-accessing-data-in-cell-arrays

评论： DrKarimKecir 2015-2-17

采纳的回答： Geoff Hayes

在 MATLAB Online 中打开

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

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Geoff Hayes 2015-2-16

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/178691-accessing-data-in-cell-arrays#answer_168170

编辑：Geoff Hayes 2015-2-16

在 MATLAB Online 中打开

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 个评论
显示 1更早的评论隐藏 1更早的评论

Stephen23 2015-2-16

在 MATLAB Online 中打开

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 2015-2-16

Ah, I was too late :)

请先登录，再进行评论。

Answer 2

Adam 2015-2-16

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/178691-accessing-data-in-cell-arrays#answer_168171

编辑：Adam 2015-2-16

在 MATLAB Online 中打开

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 个评论
显示 6更早的评论隐藏 6更早的评论

Stephen23 2015-2-16

编辑：Stephen23 2015-2-17

在 MATLAB Online 中打开

Each of these indexing methods is useful in different situations, so if you dismiss them as being "not important" then you cut yourself off from some very handy indexing tools. Particularly for indexing cell arrays , where sometimes you need to refer to the contents of the cells, and sometimes to the cells themselves. Both of these are useful in different situations.

As a simple example, if you have a cell array of strings, and you want to concatenate them together with commas and spaces:

>> C = {'anna','bob','cathy','doug'};
>> C(2,:) = {', '}; % note the parentheses!
>> [C{1:end-1}]
ans = 'anna, bob, cathy, doug'

This allocates a scalar cell array to multiple cells of another cell array, exactly like you can do with numeric scalars and arrays:

A = 1:6;
A(2,:) = 9;

Using x(n) = ... is one of the fastest ways to initialize and preallocate an array in MATLAB. In fact the documentation explicitly states these two methods of initializing a cell array:

C = cell(25,50);
C{25,50} = [];

Other code that utilizes this trick is to allocate an array in a loop, looping over the indices in reverse:

for k = N:-1:1
    myArray(k) = scalar_value;
end

Stephen23 2015-2-17

编辑：Stephen23 2015-2-17

在 MATLAB Online 中打开

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 2015-2-17

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

请先登录，再进行评论。

Accessing data in cell arrays

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

采纳的回答

3 个评论
显示 1更早的评论隐藏 1更早的评论

更多回答（1 个）

8 个评论
显示 6更早的评论隐藏 6更早的评论

另请参阅

类别

标签

Community Treasure Hunt

Accessing data in cell arrays

0 个评论 显示 -2更早的评论隐藏 -2更早的评论

采纳的回答

3 个评论 显示 1更早的评论隐藏 1更早的评论

更多回答（1 个）

8 个评论 显示 6更早的评论隐藏 6更早的评论

另请参阅

类别

标签

Community Treasure Hunt

WeChat

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

3 个评论
显示 1更早的评论隐藏 1更早的评论

8 个评论
显示 6更早的评论隐藏 6更早的评论