MATLAB: Extract data from a cell array/struct

cell arraystructures

Good afternoon,

I have been having this probelm all day and I cannot fix it. I am trying to extract the the 3rd column (WaterCumulativeMass) from each of the cell arrays. The problem I am encountering is that the second cell array only contains the headers and no values (which is correct), so to keep consistancy with the other arrays I blank it. This has the effect of turning the cell array into a Struct which I then convert back into a cell array.

The code which works partially (shown) extracts the data from the third column of cell array but combines it into a single column vector where I would like it as columns like it would normally do in a matrix for anything involving a loop function with counter. If I were to use anything with a counter i.e. ABC(i) = array{i}.data(:,3) it states that the left and right sides are not equal or the dot function is not applicable to this etc.

I have tried extracting the data from the struct format aswell but this again will not work

Can someone help please?

% Partially successful cell array method
CC = mydata_EL;
CC{2} = [];
for i = 3:numel(CC); % Needs to start at 3rd column because of the blanks
    ABC = CC{i}.data(:,3)% Does extract 3rd columns. Creates a single vector 
    
    %ABC(i) = CC{i}.data(:,3) # Does not work with the counter
    
end

Best Answer

What I want to do is to extract the third column of the data field from each struct within the cell array and complile it into a matrix, whereby each extraction has its own column [..] I need in this instance 191x6.

That's a lot clearer and easily done. Except for the 191 which I have no idea where it comes from since each data matrix is 636x3. The following creates a 636x6 matrix:

%concatenate the structure in each cell into a structure array:
data_struct = [mydata_EL{3:end}];
%concatenate the data field of each element of the array into a 3D matrix
data_mat = cat(3, data_struct.data);
%extract column 3 and reshape the Mx1xN array into a MxN array
data_col3 = squeeze(data_mat(:, 3, :));

The first two lines of code use what is known as expansion of cell arrays and structure arrays into comma-separated lists. For example,

[mydata_EL{2:end}]

is automatically expanded by matlab into

[my_data_EL{3}, my_data_EL{4}, .., my_data_EL{end}]

and

cat(3, data_struct.data)

automatically expanded into

cat(3, data_struct(1).data, data_struct(2).data, .., data_struct(end).data)

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Hi Jenna, I bet this is a pre-allocation issue.

When you build a big matrix one column at a time (such as is being done for mydata_X and mydata_Y), MATLAB needs to do some juggling of memory to make the variable a little bit bigger on each iteration. If you know how big the end result will be before you start building your matrix, you can initialise the matrix to this size just once, and MATLAB no longer has the overhead of juggling memory space inside the loop.

That said, there might even be a sneakier way of avoiding the loops you're using. Let's say that after you read all the files:

%bring the data in from CSV format into a cell-array
csvFiles = dir('*.csv');         %works for the current directory only
numfiles = length(csvFiles);     %try to keep only the data you want in the dir
mydata = cell(1,numfiles);       %finds the total number of files in the dir
%save each XY data file into a cell of a cell-array called 'mydata'
for k = 1:numfiles
    mydata{k} = importdata(csvFiles(k).name); 
end

At this point, is the matrix of every cell in mydata the same size? It seems that it should be, based on the rest of your code... If it is, you can make a 3D matrix with one "sheet" for each cell in mydata like this:

myData3d = cat(3, mydata{:});

And then you can get mydata_X like this:

mydata_X = myData3d(:,1,:);

Now, mydata_X here will be an N-by-1-by-NFILES matrix, whereas I think from your code that your original mydata_X ended up as a N-by-NFILES matrix. If you want to get the same as your original, you can just use reshape or permute:

mydata_X = permute(myData3d(:,1,:), [1 3 2]);

The same can be done for mydata_Y.

Did this get you going? If it works, it will be much faster that the loop that builds matrices one column at a time.

And also, the last loop you have:

for i=1:numfiles
    arrayOfZ = 1:numfiles
    mydata_Z(i,:) = arrayOfZ(1,:) % here I assign 1 through 'numfiles' ...
end

Can be replaced by:

mydata_Z = repmat(1:numfiles, numfiles, 1);

Although I'm not quite sure what purpose this variable serves... it will be of a different size to mydata_X and mydata_Y ...

MATLAB: Extracting certain values from a cell array with nth columns containing double arrays

 mC = cellfun(@(x)max(x(:,3)),C)

Best Answer

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