How do concatenation and indexing differ for cells and arrays in MATLAB?

Question

I am a little confused about the usage of cells and arrays in MATLAB and would like some clarification on a few points. Here are my observations:

1. An array can dynamically adjust its own memory to allow for a dynamic number of elements, while cells seem to not act in the same way:

   a=[]; a=[a 1]; b={}; b={b 1};
   

2. Several elements can be retrieved from cells, but it doesn't seem like they can be from arrays:

   a={'1' '2'}; figure; plot(...); hold on; plot(...); legend(a{1:2});   
   b=['1' '2']; figure; plot(...); hold on; plot(...); legend(b(1:2));
   %# b(1:2) is an array, not its elements, so it is wrong with legend.
   

Are these correct? What are some other different usages between cells and array?

Answer 1

Cell arrays can be a little tricky since you can use the [], (), and {} syntaxes in various ways for creating, concatenating, and indexing them, although they each do different things. Addressing your two points:

1. To grow a cell array, you can use one of the following syntaxes:

   b = [b {1}];     % Make a cell with 1 in it, and append it to the existing
                    %   cell array b using []
   b = {b{:} 1};    % Get the contents of the cell array as a comma-separated
                    %   list, then regroup them into a cell array along with a
                    %   new value 1
   b{end+1} = 1;    % Append a new cell to the end of b using {}
   b(end+1) = {1};  % Append a new cell to the end of b using ()
   

2. When you index a cell array with (), it returns a subset of cells in a cell array. When you index a cell array with {}, it returns a comma-separated list of the cell contents. For example:

   b = {1 2 3 4 5};  % A 1-by-5 cell array
   c = b(2:4);       % A 1-by-3 cell array, equivalent to {2 3 4}
   d = [b{2:4}];     % A 1-by-3 numeric array, equivalent to [2 3 4]
   

   For d, the {} syntax extracts the contents of cells 2, 3, and 4 as a comma-separated list, then uses [] to collect these values into a numeric array. Therefore, b{2:4} is equivalent to writing b{2}, b{3}, b{4}, or 2, 3, 4.

   With respect to your call to legend, the syntax legend(a{1:2}) is equivalent to legend(a{1}, a{2}), or legend('1', '2'). Thus two arguments (two separate characters) are passed to legend. The syntax legend(b(1:2)) passes a single argument, which is a 1-by-2 string '12'.