How does one concatenate cell arrays that are part of a cell array in MATLAB?

Question

I have a cell array allData which is Nx1. Each cell contains a structure with a names property (the name is a custom object, but think of it as a cell array of strings if you like). I would like to create a single cell array that contains all of the names. For example, if N=3, so that allData is a 3x1 cell array, then the following would accomplish my goal:

A = allData{1};
B = allData{2};
C = allData{3};

allNames = [A.names B.names C.names];

The problem with this approach is that N is large and changes depending on the input, so I'm hoping that there is a clever way to do this using cellfun, but everything that I've tried fails (e.g. it doesn't work to do allNames = [cellfun(@(x) {x.names}, allData)];).

UPDATE: Thanks to the suggested solutions, I can get everything into one cell array where each cell contains a cell array. My goal here is to concatenate these. More or less, what I have is:

{A.names B.names C.names}

and what I want and cannot seem to get is

{A.names{1} A.names{2} ... A.names{end} B.names{1} ... B.names{end} ...}

SOLUTION: I needed pieces of each answer below, so here's the solution that worked for me:

from Andrew Lazarus:

allNames = arrayfun(@(x) x.name, [allData{:}], 'UniformOutput', false);

then, from gnovice:

allNames = vertcat(allNames{:});

Thanks to both!!

Are the structures scalar (i.e. 1-by-1), or can they be structure *arrays*? — gnovice, May 17 '11 at 03:10

gnovice · Accepted Answer · 2011-05-17T16:25:41.523

4

There are a few more details about your cell contents that would be needed for a more general solution, but if the names property always returns a cell array of strings, and if your structures are all scalars (i.e. 1-by-1 structure arrays), then the following solution using CELLFUN, CHAR, and CELLSTR will give you an N-by-1 cell array of strings allNames containing all of your names:

allNames = cellfun(@(x) {char(x.names)},allData);
allNames = cellstr(char(allNames{:}));

And here's an example where allData contains three different structures:

>> allData{1} = struct('names',{{'hello'}},'junk',1:3);
>> allData{2} = struct('names',{{'hi' 'yo' 'hey' 'whassup'}});
>> allData{3} = struct('names',{{'howdy';'aloha'}},'junk',4);
>> allNames = cellfun(@(x) {char(x.names)},allData);
>> allNames = cellstr(char(allNames{:}))

allNames = 

    'hello'
    'hi'
    'yo'
    'hey'
    'whassup'
    'howdy'
    'aloha'

EDIT:

Generalizing to the case where the names property returns a cell array of objects, not necessarily strings, you can try this solution which reshapes each cell array into an M-by-1 cell array, then vertically concatenates all of them into an N-by-1 cell array of objects:

allNames = cellfun(@(x) {reshape(x.names,[],1)},allData);
allNames = vertcat(allNames{:});

Or, if you would rather end up with a 1-by-N cell array of objects, you can do this:

allNames = cellfun(@(x) {reshape(x.names,1,[])},allData);
allNames = [allNames{:}];

edited May 17 '11 at 16:25

answered May 17 '11 at 05:10

gnovice

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This is great if they are cell arrays of strings, but sadly I misled you. They are cell arrays of custom objects. Still, I may be able to get some mileage out of this approach. Thanks! – PengOne May 17 '11 at 05:30
1

@PengOne: The moral of the story being "remember to provide sample data in your question". – Richie Cotton May 17 '11 at 10:55
@Richie Cotton: The object is defined over several files and many, many lines, so it only clouds the question which at the heart is simple: How can I concatenate cell arrays that themselves are cells in a cell array? – PengOne May 17 '11 at 16:04
@PengOne: I've updated my answer to deal more generally with *objects* as opposed to strings. – gnovice May 17 '11 at 16:29
JessumCrow! The `vertcat` function worked (once I transposed correctly)! Thanks for this. I did not need to reshape for my example, but please leave it in your solution in case someone else beats his head against the wall on this one. – PengOne May 17 '11 at 16:33

score 1 · Answer 2 · answered May 17 '11 at 00:04

1

Try this

 allNames = arrayfun(@(x) x.name, [allData{:}], 'UniformOutput', false)

Omit the UniformOutput vararg for straight concatenation. This gives a cell output.

answered May 17 '11 at 00:04

Andrew Lazarus

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The operation `[allData{:}]` will only work if the elements of the cell array can be horizontally concatenated into a structure array, which would mean there was no real reason to use a cell array in the first place. – gnovice May 17 '11 at 00:36
@PengOne: note the [brackets]. But I'll try for a more generic solution. – Andrew Lazarus May 17 '11 at 15:39
Tried this... result is a 1x3 cell array, each cell containing one of the cell arrays that I truly wish would just find a way to concatenate themselves! – PengOne May 17 '11 at 16:13
@PengOne, Not quite sure what you mean, but a possibility: By changing UniformOutput to true, you get array output. That is the default anyway. You may need to transpose the output to get a concatenated single string. – Andrew Lazarus May 17 '11 at 17:40

How does one concatenate cell arrays that are part of a cell array in MATLAB?

2 Answers2