CODEWITHSUNDEEP
arraysmatlabindexingcell-array
totally
totally

Reputation: 135

What is the point of cell indexing in MATLAB

The point of indexing is mainly to get the value. In MATLAB, for a cell array, there is content indexing ({}), and thus cell indexing (()) is only for selecting a subset from the cell array, right?

Is there anything other advanced usage for it? Like using it as a pointer and pass it to a function?

Upvotes: 3

Views: 119

Answers (1)

Daniel
Daniel

Reputation: 36710

There is a heavily simplified answer. {}-indexing returns you the content, ()-indexing creates a subcell with the indexed elements. Let's take a simple example:

>> a=x(2)

a = 

    [2]

>> class(a)

ans =

cell

>> b=x{2}

b =

     2

>> class(b)

ans =

double

Now continue with non-scalar elements. For the ()-indexing everything behaves as expected, you receive a subcell with the elements:

>> a=x(2:3)

a = 

    [2]    [3]

The thing really special to Matlab is using {}-indexing with non-scalar indices. It returns a Comma-Separated List with all the contents. Now what is happening here:

>> b=x{2:3}

b =

     2

The Comma-Separated List behaves similar to a function with two return arguments. You want only one value, only one value is assigned. The second value is lost. You can also use this to assign multiple elements to individual lists at once:

>> [a,b]=x{2:3} %old MATLAB versions require deal here

a =

     2


b =

     3

Now finally to a very powerful use case of comma separated lists. Assume you have some stupid function foo which requires many input arguments. In your code you could write something like:

foo(a,b,c,d,e,f)

Or, assuming you have all parameters stored in a cell:

foo(a{1},a{2},a{3},a{4},a{5},a{6})

Alternatively you can call the function using a comma separated list. Assuming a has 6 elements, this line is fully equivalent to the previous:

foo(a{:}) %The : is a short cut for 1:end, index the first to the last element

The same technique demonstrated here for input arguments can also be used for output arguments.

Regarding your final question about pointers. Matlab does not use pointers and it has no supplement for it (except handle in oop Matlab), but Matlab is very strong in optimizing the memory usage. Especially using Copy-on-write makes it unnecessary to have pointers in most cases. You typically end up with functions like

M=myMatrixOperation(M,parameter,parameter2)

Where you input your data and return it.

Upvotes: 4

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