'splitapply' syntax to index multiple columns in a timetable

Question

Eric Escoto 2021-11-19

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1591204-splitapply-syntax-to-index-multiple-columns-in-a-timetable

评论： Kelly Kearney 2021-11-20

I would like to create a timetable using the split apply function applied to an existing timetable.

testTT = timetable(tmin, duration_min, splitapply(@mean, TT1.I, grp), ...
                                       splitapply(@mean, TT1.KE, grp), ...
                                       splitapply(@sum, TT1.num, grp), ... 
                                       splitapply(@mean, TT1.D, grp), ...
                                       splitapply(@sum, TT1{10:1033}), grp));

You can see in the last splitapply command where I'm trying to apply the 'sum' function to 1024 total columns according to the group called 'grp'. I know my syntax is incorrect. But, I also don't know if what I'm trying here is possible (at least the way I'm doing it).

Is there a better approach?

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dpb 2021-11-19

What is TT1{10:1033} intended to represent, again? The curlies will return an array, not a table which all the dot operations will, but it also lacks a 2D reference--it's only a 1D subscripting expression.

For the above to have any chance, the heights will all have to be the same.

Perhaps you're looking for

splitapply(@sum, TT1(:,10:1033)), grp)

on the last one?

Kelly Kearney 2021-11-20

Unfortunately that syntax isn't valid. You'd need to modify the call to sum to loop over multiple inputs:

% Small sample table
TT1 = array2table(rand(10,5), 'variablenames', {'one','two','three','four','five'});
grp = randi(2, 10, 1);
% One-liner with sum wrapper
testTT1 = array2table(...
    splitapply(@(varargin) cellfun(@sum, varargin), TT1, grp), ...
    'variableNames', TT1.Properties.VariableNames)
testTT1 = 2×5 table
      one       two       three      four      five 
    _______    ______    _______    ______    ______

    0.25613    1.6855    0.49839    1.1605     1.032
     5.3389    4.5527     1.9127     3.246    4.0721
% Using loops
testTT2 = table;
vname = TT1.Properties.VariableNames;
for ii = 1:length(vname)
    testTT2.(vname{ii}) = splitapply(@sum, TT1.(vname{ii}), grp);
end
testTT2
testTT2 = 2×5 table
      one       two       three      four      five 
    _______    ______    _______    ______    ______

    0.25613    1.6855    0.49839    1.1605     1.032
     5.3389    4.5527     1.9127     3.246    4.0721
% The I-wish-it-worked-this-way syntax
testTTbad = splitapply(@sum, TT1, grp);
Error using splitapply (line 132)
Applying the function 'sum' to the 1st group of data generated the following error:

Too many input arguments.

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Answer 1

Kelly Kearney 2021-11-19

1
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1591204-splitapply-syntax-to-index-multiple-columns-in-a-timetable#answer_835984

编辑：Kelly Kearney 2021-11-20

I really wish splitapply did allow you to pass a table as input and apply the same function to each column of that input. Unfortunately, it doesn't (it will accept table input, but then expects a function that treats each table column as a separate input variable). Instead, a loop is probably the easiest way to do this:

testTT = timetable(tmin, duration_min, splitapply(@mean, TT1.I, grp), ...
                                       splitapply(@mean, TT1.KE, grp), ...
                                       splitapply(@sum, TT1.num, grp), ... 
                                       splitapply(@mean, TT1.D, grp));
% Add the summed variables
vname = TT1.Properties.VariableNames(10:1033);
for ii = 1:length(vname)
    testTT.(vname{ii}) = splitapply(@sum, TT1.(vname{ii}), grp);
end