MATLAB: How to obtain total sum by group

accumarraytotal sum by group

Hi, Could someone explain how to compute the total sum by group (without using for loop)?

A example is: groupid = [1; 1; 1; 2; 2; 3; 3;3]; value=[1;2;3;4;5;6;7;8]. the desired result is: runsum = [6;6;6;9;9; 21;21;21].

I am trying to get the result without using loop. Can "accumarray" do this?

Thanks a lot

Siying

Best Answer

Siying - yes, accumarray can be used to sum elements of the same group. For example, we can do

 groupSums = accumarray(groupid,value);

which gives us

 groupSums =
     6
     9
    21

with your runsum as

 runsum = groupSums(groupid);

Related Solutions

MATLAB: How to sum over grouped data in a table

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;

MATLAB: Sum of first column based on second column and storing results in a matrix

% Create fake data for the demo
A = [rand(30,1),randi(3,30,1)+1000]; 
% Calculate group sum and store in table
[groupID, groups] = findgroups(A(:,2));
groupsum = splitapply(@sum,A(:,1),groupID); 
T = table(groups,groupsum)

Result (will vary due to random draw).

T =
  3×2 table
    groups    groupsum
    ______    ________
     1001       2.122 
     1002      6.1048 
     1003      6.8219

Best Answer

Related Solutions

MATLAB: How to sum over grouped data in a table

MATLAB: Sum of first column based on second column and storing results in a matrix

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