MATLAB: How to use cell2mat for specific elements of cell array

cell2mat

Hi everyone,

I am trying to create a double from specific elements of a cell array. If I have a cell array B, of dimensions 6×5, of which each element contains a double 10×4, such as:

for i = 1 : 6
for j = 1 : 5
A = randn(10,4)
B{i,j} = A;
end
end

I would like to create a big double out of all the rows and first 3 columns of B, and the second and third column of each interior double A (on this 6×3 cell array). Hence, the expected double C that should be a 60×6 matrix. I am using

C = cell2mat(B{:,1:3}(:,[2,3]));

But I get an error. What am I missing here?

Best Answer

C = cell2mat(B(:,1:3));
C = C(:,2:3);

"What am I missing here?"

Several things: firstly, that the B{...} syntax creates a comma-separated list from the contents of the cell array, which in general is not a valid input for cell2mat:

https://www.mathworks.com/help/matlab/matlab_prog/comma-separated-lists.html

https://www.mathworks.com/matlabcentral/answers/320713-how-to-operate-on-comma-separated-lists

Secondly, that in MATLAB indexing cannot be arbitrarily added on to the end of any other expression, so you will need to use a temporary variable.

Related Solutions

MATLAB: Concatenate arrays within a loop,

Easy, given any number of arrays in a cell array C. You could use a loop like this, although it is inefficient because the output array is not preallocated:

C = {[1,2],[3,4],...} % cell array of arrays.
A = C{1};
for k = 2:numel(C)
    A = [A;C{k}];
end

Simpler and more efficient would be to concatenate them all at once, without a loop, e.g.:

A = vertcat(C{:})

Whatever you do, do NOT try to access variable names dynamically in a loop! Read this to know why:

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

Using an array (e.g. a cell array) and indexing is much simpler, neater, much more efficient, less buggy, and easier to debug.

MATLAB: Divide a cell arrays with a part of another cell array

Try this:

C = num2cell(bsxfun(@rdivide,cell2mat(A),cell2mat(B(1,2:end))));

Note that B{1,2:end} using the curly braces will be a comma-separated-list of the contents of B, whereas B(1,2:end) using parentheses will simply be another cell array.

Best Answer

Related Solutions

MATLAB: Concatenate arrays within a loop,

MATLAB: Divide a cell arrays with a part of another cell array

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