MATLAB: How to concatenate tables vertically

MATLAB

How can I concatenate two or more tables vertically if they have the same headers?

Best Answer

To concatenate two or more tables vertically, you can use "vertcat" either as a function or as an operator - just as you would do to vertically concatenate two or more arrays. The headers do not even need be in the same order. Here is an example:

myTable1 = array2table(eye(3));
myTable1.Properties.VariableNames = ["One","Two","Three"];
myTable2 = array2table(2*eye(3));
myTable2.Properties.VariableNames = ["Two","Three","One"];
myTable3 = array2table(3*eye(3));
myTable3.Properties.VariableNames = ["Three","One","Two"];
myTable = [ myTable1; myTable2; myTable3 ]

The documentation page for "vertcat" explains that this function accepts table inputs:

https://uk.mathworks.com/help/matlab/ref/vertcat.html#mw_f3bbc20c-c8ab-4a50-b686-3c75b5d71eb4

Related Solutions

MATLAB: How to split a table into multiple sub-tables based on the frequency of a variable

One approach to solve this issue is to use categorical variables, which will allow you to identify the frequencies of each entries and then logical index to create the subset tables. This workflow is demonstrated in the code below:

% Convert Var1 to categorical data
T = convertvars(T, "Var1", 'categorical');
% Set up variables for table generation
counts = countcats(T.Var1);
cats = categories(T.Var1);
uniqueCounts = unique(counts);
% Create tables
splitTables = cell(length(uniqueCounts));
for i = 1:length(uniqueCounts)
    currentCountCats = cats(counts == i);
    splitTables{i} = T(ismember(T.Var1, currentCountCats), :);
end

This results in the following tables:

>> splitTables{1,1}
ans =
    2×2 table
      Var1     Var2  
      ____    _______
       A      "data1"
       B      "data2"
>> splitTables{2,1}
ans =
    2×2 table
      Var1     Var2  
      ____    _______
       C      "data3"
       C      "data4"
>> splitTables{3,1}
ans =
    3×2 table
      Var1     Var2  
      ____    _______
       D      "data5"
       D      "data6"
       D      "data7"

First, you can make a variable categorical when creating the initial table by using 'categorical' or, for an existing table, you can convert the variable to categorical using 'convertvars'. Documentation for each function can be found at the links below:

https://www.mathworks.com/help/releases/R2019a/matlab/ref/categorical.html

https://www.mathworks.com/help/releases/R2019a/matlab/ref/convertvars.html

Once the entries are in a categorical variable, you can use 'categories' to get a list of the categories and 'countcats' to obtain the frequency of each category. Please see the following links for more information on these functions:

https://www.mathworks.com/help/releases/R2019a/matlab/ref/categorical.categories.html

https://www.mathworks.com/help/releases/R2019a/matlab/ref/categorical.countcats.html

Finally, you can use logical indexing to find the categories of a certain frequency and pull the relevant rows from the original table using 'ismember'. More information on logical indexing and 'ismember' can be found here:

https://www.mathworks.com/help/releases/R2019a/matlab/math/array-indexing.html#MatrixIndexingExample-3

https://www.mathworks.com/help/releases/R2019a/matlab/ref/ismember.html

Note that the same process can be followed for numeric variables as well.

MATLAB: Extract all columns from a table where the column names “start with” a string

mask = startsWith( mytable.Properties.VariableNames, 'F');
Fcols = mytable(:,mask);

Best Answer

Related Solutions

MATLAB: How to split a table into multiple sub-tables based on the frequency of a variable

MATLAB: Extract all columns from a table where the column names “start with” a string

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