Concatenating cell arrays in matlab

Hello,

I have three cell arrays:

a =

1×3 cell array

{37720×155 double} {37720×155 double} {37720×155 double}

b =

1×3 cell array

{13706×155 double} {13706×155 double} {13706×155 double}

c =

1×3 cell array

{10169×155 double} {10169×155 double} {10169×155 double}

I want to concatenate these vertically so I use:

for j = 1:3

z{j} = cat(1,a{j},b{j},c{j})

end

This does not work and I get the error: Cell contents assignment to a non-cell array object.

Please can someone help me combine these datasets so:

z =

1×3 cell array

{51426×155 double} {51426×155 double} {51426×155 double}

Many thanks,

Phil

Answer 1

[John D'Errico]

Easy peasy?

a
a =
  1×3 cell array
    {37720×155 double}    {37720×155 double}    {37720×155 double}

b
b =
  1×3 cell array
    {13706×155 double}    {13706×155 double}    {13706×155 double}
c
c =
  1×3 cell array
    {10169×155 double}    {10169×155 double}    {10169×155 double}
    
abc = cell(1,3);
for i = 1:3
  abc{i} = [a{i};b{i};c{i}];
end
  
abc
abc =
  1×3 cell array
    {61595×155 double}    {61595×155 double}    {61595×155 double}

I think you were mistaken in the final expected size of z. As well, I think perhaps the array z already existed as an array of doubles, so when you tried to stuff elements into it as a cell array, you then got the error message that you did. Or, something like that.

Finally, I have a funny feeling that I can write the entire thing without a loop, but the result would not be terribly readable, and why bother? A simple loop is not a costly thing, and preoptimizing code that works just fine is silly IMHO.

[Phil Roberts]

Thank you very much for this. it works perfectly! Yes, you were correct about the expected output. Must be a dodgy calculator!

Many thanks,

Phil

[Guillaume]

I have a funny feeling that I can write the entire thing without a loop, but the result would not be terribly readable, and why bother?

In my opinion, it's much clearer and readable without the loop, but, to each their own:

z = cellfun(@vertcat, a, b, c, 'UniformOutput', false);

It also does not require any change if the numbers of elements in a,b,c changes. It may be slightly slower than the loop however.

[John D'Errico]

Yes, that works superbly, and is indeed quite readable, though I think readable either way. My rule is not to bother looking to preoptimize code where I don't see an immediate better solution, and if I know the solution I have is already efficient. I'd give Guillaume a +1 for his response.

[Phil Roberts]

Hello again,

Now I have the output as:

olddata =

{61595×155 double} {61595×155 double} {61595×155 double}

How do I ge these into individual matricies?

I have tried:

for i = 1:3

mydata(i) = cell2mat(olddata(i))

end

The above does not work but the following does, for one of the cells anyway:

mydata = cell2mat(olddata(1))

Any help is greatly appreciated.

Many thanks,

Phil

[Guillaume]

How do I ge these into individual matricies?

You don't. Keep them as a single cell array and use indexing. It's probably the best. Instead of a, b, c, use mydata{1}, mydata{2} and mydata{3}.

If you really, really, really want to split it again into separe variables:

[a, b, c] = olddata{:};

Important, if mydata is a matrix, then mydata(i) is a single element of that matrix, one number. You cannot assign a whole matrix to a single element of a matrix, so

mydata(i) = olddata{i};

is never going to work. Furthermore, you need to learn how to access the elements of a cell array.

mydata = cell2mat(olddata(i));

is something that somebody that doesn't understand cell array indexing would write. The proper way of accessing the content of the ith element of a cell array is:

mydata = olddata{i};  %note the use of {} vs ().