Pre-allocation of Cell Arrays

Hi

I have two questions regarding the pre-allocation of cell arrays. The online documentation says the following:

"Incrementally increasing the number of cells or the number of elements in a cell results in Out of Memory errors."

So, this is the same as for matrices, where e.g. one should pre-allocate for a for loop which fills a matrix a zero matrix of the required size. But then:

"Cell arrays do not require completely contiguous memory."

What does that mean exactly, i.e. where is the difference nevertheless?

And:

"However, each cell requires contiguous memory" - and what is that supposed to mean? I mean it is not possible to allocate fragmented memory for each cell of a cell array, right?

Thanks

Answer 1

[Matt J]

Basically, numeric matrix data types in MATLAB are allocated non-fragmented memory. As you move along columns in a matrix, you traverse adjacent memory locations.

The different elements of a cell array, however, can hold matrices or other objects of different data types (strings, structs, other cell arrays) and sizes. Because each of these objects may be non-scalar and can be of a different size/type, they cannot necessarily neighbor each other in memory.

When you pre-allocate a cell by doing

C=cell(1,N)

it is like you are creating N separate MATLAB variables except that they do not have distinct names (x,y,z, etc...) and they do not hold any data yet. If you place matrices in each cell location C{i}, then the matrix content of each C{i} individually will occupy contiguous memory, as usual. However, the data corresponding to different C{i} are like different variables and needn't neighbor each other.