MATLAB: How to sum over grouped data in a table

arraycategoricalgroupgrp2idxgrpstatsMATLABsum

Suppose I have a table like this toy example:
>> groups = { 'a'; 'a'; 'b'; 'b'; 'c'; 'c'};
>> values = [1;2;3;4;5;6];
>> exampletable = table(groups, values);
I want another column in the table that finds the sum of the values for each group. How can I do this?

Best Answer

For MATLAB R2018b and later:
If you only require a summary table containing the sum for each group, use "groupsummary".
>> G = groupsummary(exampletable,'groups','sum')
G =
  3×3 table
    groups    GroupCount    sum_values
    ______    __________    __________
     'a'          2              3    
     'b'          2              7    
     'c'          2             11
Use "grouptransform" to both perform the sum calculation by group and expand the result back to your original "exampletable".
>> exampletable = grouptransform(exampletable,'groups',@sum,'ReplaceValues',false)
exampletable =
  6×3 table
    groups    values    fun_values
    ______    ______    __________
     'a'        1            3    
     'a'        2            3    
     'b'        3            7    
     'b'        4            7    
     'c'        5           11    
     'c'        6           11
The documentation pages for "groupsummary" and "grouptransform" are below.
https://www.mathworks.com/help/matlab/ref/double.groupsummary.html
https://www.mathworks.com/help/matlab/ref/double.grouptransform.html
For MATLAB R2018a and earlier:
You can use a for loop to calculate the sums in a new, preallocated vector. Then, append this vector to the end of your original table.
y = zeros(size(groups));
for i = 1:length(groups)
    y(i) = sum(values(ismember(groups,groups(i))));
end
exampletable.sum_values = y;
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