How to sum over grouped data in a table

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MathWorks Support Team
编辑： MathWorks Support Team 2020-10-26
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?

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采纳的回答

MathWorks Support Team
编辑：MathWorks Support Team 2020-10-26
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;

更多回答（1 个）

Peter Perkins
Peter Perkins 2019-3-11
It seems more likely that you would want the sum for each group in a table with one row for each group. There are several simple ways to do that, including findgroups/splitapply, groupsummary, and this version using varfun:
>> groups = { 'a'; 'a'; 'b'; 'b'; 'c'; 'c'};
>> values = [1;2;3;4;5;6];
>> exampletable = table(groups, values);
>> exampletable.groups = categorical(exampletable.groups) % convert to categorical IN the table
>> sums = varfun(@sum,exampletable,'GroupingVariable','groups')
sums =
3×3 table
groups GroupCount sum_values
______ __________ __________
a 2 3
b 2 7
c 2 11
If broadcasting the summed values out to each row of the original table is the goal, findgroups does the trick.
>> igroup = findgroups(exampletable.groups)
>> exampletable.sums = sums.sum_values(igroup)
All of this is in base MATLAB.

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