MATLAB: Divide a data set into 4 parts so that the sum of each part 1/4th of the total

innovative strategyMATLABoptimizationOptimization Toolbox

I want to divide a data set into four groups such that the sum of elements of each group is approximately same.

for eg: [10, 5, 1, 20, 5, 22, 4, 15]

For the above data set: sum of all the elements = 82

So, I want this data set to be divided into 4 groups such that, the sum of elements of each group is almost same.

One such possibility is

Set 1: 10, 5, 4,1

Set 2: 20

Set 3: 22

Set 4: 15,5

How do I set up this?

Best Answer

I'd just sort them and then take the CDF and look for percentages:

c = cumsum(sort(data, 'ascend'));
c = c / c(end); % Normalize from 0 to 1
c25 = find(c>0.25, 1, 'first');
c50 = find(c>0.5, 1, 'first');
c75 = find(c>0.75, 1, 'first');

At least that's one way that might work, though it would work best for lots of data rather than just a few elements like you have.

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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;

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I figured out how to do it. basically, use the function

newstr = compose(str)

so I just added the newline character to the end of each of my lines, and used that.

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MATLAB: How to sum over grouped data in a table

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