How to create a dynamic array in MATLAB - arrays

Question

Description of this array:
10 dimensions, which represent 10 clusters.
Each dimension's length is not fixed, and should be zero at first.
After clustering, one coordinate (x,y) will be assigned to the corresponding cluster.
Example:
If (1,1) belongs to cluster 10, (2,2) belongs to cluster 9, and given a sequence of coordinates
(1,1) (1,1) (1,1) (1,1) (1,1) (2,2)
then A(1) to A(8) have no element while A(9) has 1 element with value (2,2) and A(10) has 5 elements with value (1,1).
I tried to use cell array, and I "kind of" get one, here is my code.
A = cell(10,0)
%create empty cell array
A(10,end+1) = {[1,1]}
%assign (1,1) to cluster 10
So now A is:
Then suppose we have (2,2) and it should assigned to A(9)
A(9,end) = {[2,2]}
Looks fine, but then if we assign (1,1) to A(10) again,
A(10,end) = {[1,1]}
Then the length is still the same and A(1) to A(8) are not empty!
My question is, is there any other method that can help me to create a dynamic array?

Answer 1

In MATLAB, we'll often store multiple coordinates into a single array, such that p(3,:) is the 3rd point. [One of the reasons for this is that this is a much more efficient way of storing data, as each array has an overhead, and storing many points as individual arrays within a e.g. cell array therefore is very wasteful of memory.]
I would suggest you use that method to store coordinates within each cluster. For example:
A = cell(10,1); % 10 clusters, we won't change the size of A
A{10}(end+1,:) = [1,1];
A{9}(end+1,:) = [2,2];
A{10}(end+1,:) = [1,1];
Now we can see what is inside A:
>> A{1}
ans =
[]
>> A{9}
ans =
2 2
>> A{10}
ans =
1 1
1 1
Note that A{10} is the contents of the cell (in this case a numeric array), whereas A(10) is a cell array with one cell.
A{10}(1,:) is the first coordinate in cluster 10. size(A{10},1) is the number of coordinates in cluster 10.
A{10}(end+1,:) = [1,1] is one way of appending an element. end+1 is a non-existing location, meaning the array will be extended to accommodate the new data assigned there. An alternative method is A{10} = [A{10} ; 1,1]. I'm not sure if these two methods are equivalent or not. In the case of a vector, or when appending a column, then the end+1 method is much more efficient, so I always recommend that method in all cases.

Answer 2

You need to create a cell array of cells.
Use repmat to create a 10 x 1 array of empty cells:
A = repmat({{}},10,1);
In order to access nested cells you need to chain indexes:
A{10}(end+1) = {[1,1]};
A{9} (end+1) = {[2,2]};
A{10}(end+1) = {[1,1]};
Or more simple indexing:
A{10}{end+1} = [1,1];
A{9} {end+1} = [2,2];
A{10}{end+1} = [1,1];

Answer 3

You should probably take a look at matlab's map https://www.mathworks.com/help/matlab/ref/containers.map.html
E.G. For your question:
A = containers.Map('KeyType','int32','ValueType','any');
A(9) = [2 2];
A(10) = [1 1;1 1;1 1;1 1;1 1];
Now A(9) has a vector with values (2,2) and A(10) has 10 row vectors with values (1,1).
You can retrieve the values the same way that you input the values:
>> A(9)
ans =
2 2
>> A(10)
ans =
1 1
1 1
1 1
1 1
1 1
Adding vectors to already defined keys:
>> A(9) = [A(9); 2 2];
>> A(9)
ans =
2 2
2 2