Extract column data of table in a struct array - matlab

Question

Summary / TLDR
How do I extract all rows of one table-column if the table is in a struct array and I want to combine alle struct-array-elements to one big matrix?
First Approch
I have a table (Values) with multiple columns and rows stored in a struct. Multiple of these structs are stored in an array (Data):
Data(1).Values = table([1; 2; 3; 4; 8], 'VariableNames', {'Rk'});
Data(2).Values = table([3; 6; 10; 8], 'VariableNames', {'Rk'});
Data(3).Values = table([2; 10; 11; 7], 'VariableNames', {'Rk'});
There are many more variables in the struct and also in the table, so it is just a simplified example. As you can see the height of the table can vary. I want to plot the columns Rk in a boxplot. So i need to create a matrix like this:
matrix = [1 1; 2 1; 3 1; 4 1; 8 1; 3 2; 6 2; 10 2; 8 2; 2 3; 10 3; 11 3; 7 3];
I use the following code to create the matrix:
matrix = zeros(0, 2);
for i=1:length(Data)
l = height(Data(1, i).Values(:, 'Rk'));
e = length(matrix) + 1;
% Reshape the data into an array
matrix((end+1):(end+l), 1) = table2array(Data(1, i).Values(:, 'Rk'));
% Creating the index of each data-row
matrix(e:end, 2) = i;
end
% Plot the boxplot
boxplot(matrix(:, 1), matrix(:, 2))
I really don't like this for-loop-version, especially because it becomes slow for big Data-Arrays and also because I don't know the size of matrix, so I can't reserve the space. Theoretically I could run through the whole data-array, counting the elements, initalizing the matrix-variable and then fill it.
Is there a more elegant version without a for-loop?
Second approach
I already tried another solution by changing the structure of the struct. Semantically, this really makes no sense, but this way I found a more elegant solution, creating the matrix-Variable without the problems of the first solution:
% Creating Data
Data(1).Values.Rk = [1; 2; 3; 4; 8];
Data(2).Values.Rk = [3; 6; 10; 8];
Data(3).Values.Rk = [2; 10; 11; 7];
% Reshape the data into an array
a = {cell2mat({Data.Values}).Rk};
b = vertcat(a{:});
% Creating the index of each data (b)-row
c = cumsum(cellfun('length', a(1, :)));
d = meshgrid(1:c(end), 1:length(c));
e = d>c';
f = sum(e);
% Plot the boxplot
boxplot(b, f);
Questions
I would apreciate a solution combining both approaches (having a table, no for-loop, no need of matrix-size-calculation) but:
I don't know how to extract the data of the table in a struct in an array.
I am asking myself if there is a more elegant solution creating the boxplot-indexes.
Whole code
%% Boxplot of Table
clear
% Creating Data
Data(1).Values = table([1; 2; 3; 4; 8], 'VariableNames', {'Rk'});
Data(2).Values = table([3; 6; 10; 8], 'VariableNames', {'Rk'});
Data(3).Values = table([2; 10; 11; 7], 'VariableNames', {'Rk'});
matrix = zeros(0, 2);
for i=1:length(Data)
l = height(Data(1, i).Values(:, 'Rk'));
e = length(matrix) + 1;
% Reshape the data into an array
matrix((end+1):(end+l), 1) = table2array(Data(1, i).Values(:, 'Rk'));
% Creating the index of each data
matrix(e:end, 2) = i;
end
boxplot(matrix(:, 1), matrix(:, 2));
%% Boxplot of Arrays
clear
% Creating Data
Data(1).Values.Rk = [1; 2; 3; 4; 8];
Data(2).Values.Rk = [3; 6; 10; 8];
Data(3).Values.Rk = [2; 10; 11; 7];
% Reshape the data into an array
a = {cell2mat({Data.Values}).Rk};
b = vertcat(a{:});
% Creating the index of each data (b)-row
c = cumsum(cellfun('length', a(1, :)));
d = meshgrid(1:c(end), 1:length(c));
e = d>c';
f = sum(e);
% Plot the boxplot
boxplot(b, f);

Answer 1

I have a partial answer if you are allowed to change the dimensions on your input table. If you transpose your input values, such that you have a row vector
Data(1).Values.Rk = [1; 2; 3; 4; 8]';
...
You can use the following commands to concatenate all the elements:
tmp = [Data.Values];
allvals = [tmp.Rk]'; %output is a column vector of your aggregated values
If you eliminate the Rk field (if it is not informative), you don't need the first step and can do the operation in one step.
If you aggregate the values into a column vector in this manner you now know the dimensions of the second column and can initialize that column before executing a for loop to populate the second column with a monotonically increasing index for each element of data.
I cannot think of a way to get the total number of elements and corresponding value for your box plot in the second column (without a for loop) unless you have the flexibility to add another field to your data structure (e.g. Data(1).nVals = 5, etc.), in which case you can get the total number of elements via sum([Data.nVals])