Nesting arrays in MATLAB

Question

arr=[   [],[],[]  ];

for i=1:3
     arr(i)=[i,i+1];
end

Expected output: arr=[[1,2],[2,3],[3,4]];

I am trying to put multiple arrays into a single array and I have tried the above code but it does not work

How can I nest arrays?

Answer 1

What you are trying is very Pythonic, nesting lists in lists. This doesn't work in MATLAB. You have basically two easy options: a multi-dimensional array, or a cell array with arrays.

arr = zeros(3,2);  % initialise empty array
arr_cell = cell(3,1);  % initialise cell
for ii = 1:3
    arr(ii,:) = [ii, ii+1];  % store output as columns
    arr_cell{ii} = [ii, ii+1];  % store cells
end

arr =

     1     2
     2     3
     3     4

celldisp(arr_cell)
 
arr_cell{1} =
     1     2
arr_cell{2} =
     2     3
arr_cell{3} =
     3     4

Cells can take various sized (or even types of) arguments, whereas a multi-dimensional matrix has to have the same number of columns on each row. This makes a cell array more flexible, but the numerical array is a lot faster and has more numerical functions available to it.

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A small side-note, I suggest to not use i as a variable name in MATLAB, as it's a built-in function.