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Is There a Way to Execute splitapply Functionality on Subtables of Master Table?

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Paul
Paul on 9 May 2020
Commented: Peng Li on 11 May 2020
Suppose I have a function that operates on a table and returns a row vector:
function rowvec = myfunc(Table)
Suppose I have a master table, call it T with one of its variables being Name. I'd like to do something like the following to group by Name and concatenate rowvec computed from each subgroup:
G = findgroups(T.Name);
R = splitapply(@myfunc,T,G);
This won't work because splitapply sends the group of each variable in T to myfunc and not the subtable of T defined by G.
Is there already a function that does what I'm trying to do?
Or do I have to use the code here: https://www.mathworks.com/matlabcentral/answers/457422-separate-table-data-in-to-sub-tables to generate the cell array of subtables, loop over the subtables with a call to myfunc, and then concatenate the rows myself? Or maybe use cellfun on the cell array of subtables?
  2 Comments
Paul
Paul on 10 May 2020
>> x1=rand(4,1);x2 = rand(4,1);colors=["red"; "green"; "red"; "green"]; % create some data
>> T=table(colors,x1,x2); % create a table
>> myfunc = @(T) ([sum(T.x1) sum(T.x2)]); % function that returns the sums of the data columns as row vector
>> myfunc(T) % show that it works
ans =
2.28 2.44
>> [G,Vars]=findgroups(T.colors); % group by color
>> splitapply(myfunc,T,G) % try to sum the data for each color
Error using splitapply (line 132)
Applying the function '@(T)([sum(T.x1),sum(T.x2)])' to the 1st group of data generated the following error:
Too many input arguments.
According to the doc, this workflow should not work because myfunc is expecting one input, but splitapply is trying to pass all three variables in T to myfunc. So no complaints there. I was hoping that there is a built-in that would give me the desired output:
>> [myfunc(T([1 3],:)) ; myfunc(T([2 4],:))] % desired result
ans =
1.22 1.45
1.06 1.00
I was able accomplish this by using Walter Roberson's code in the link in the OP to generate a cell array of subtables grouped by color, and then using cellfun(myfunc, etc) on the resulting cell array. So it wasn't too bad; I was just looking for something built-in.

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Answers (1)

Peng Li
Peng Li on 10 May 2020
you'd better do this way:
splitapply(@(x) sum(x, 1), T{:, 2:3}, G);
  5 Comments
Peng Li
Peng Li on 11 May 2020
It is not clear why you want to subtable your table before splitapply. splitapply is supposed to finish the job for you. Anyway, I'm happy that you found out the solution for your problem!

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